Exploratory behavior in blind infants: how to improve touch

Exploratory behavior in blind infants: How toimprove touch?

Ad W. Smitsman, Roelof Schellingerhout

Department of Psychology, University of Nijmegen, Nijmegen, The Netherlands

Received 20 July 2000; received in revised form 5 December 2000; accepted 9 January 2001

Abstract

A key issue for our understanding of exploration and how it evolves concerns the information thatis gathered or the structure that emerges in flows of energy for sensory systems. It allows a state ofawareness to be maintained or transformed into another state. The very essence of exploration mustbe the emergent property that information forms, and changes in this property when exploration isorganized differently. We took the stance that information involves cooperative activity of differentsenses, which is promoted by the way flows of energy become structured in relation to how theexploratory system is organized. Moreover, we assumed that detrimental effects of sensory handicapsare not as much the result of the sense that is missing as they are of the cooperation between the sensesthat cannot take place. Guided by this assumption, we constructed surfaces that contained texturegradients to evoke haptic exploration and way finding in near space in congenitally blind youngchildren on manual encounters with the surface. The surfaces extended in front of the child. Wehypothesized that cooperation of the two submodalities of touch, i.e., kinesthesis and the cutaneoussense, afforded by the texture gradients would allow the child to gather an environmental framecoupled to the body frame. Results indeed showed enhanced exploration in 8- to 20-month-oldcongenitally blind infants and an evolving dynamic landscape of exploratory behavior that becameadapted to the texture gradients. Way finding was studied in 4-year-old congenitally blind children.Results showed that way finding improved for the texture gradients compared to a surface thatcontained homogeneously distributed texture elements. © 2000 Elsevier Science Inc. All rightsreserved.

E-mail address: [email protected] (A.W. Smitsman).

Infant Behavior & Development 23 (2000) 485–511

0163-6383/00/$ – see front matter © 2000 Elsevier Science Inc. All rights reserved.PII: S0163-6383(01)00056-X

1. Introduction

Exploration is unquestionably of great importance for every developing infant. It enablesthe infant to discover the affordances in its surroundings. Affordances are the opportunitiesfor acting and exploration, which includes acting, in the animate and inanimate environment.These opportunities are relative to an agent’s bodily (sensory, neuromuscular and link-segmental) resources for exploration and performance (Gibson, 1979; Gibson, 1989; Reed,1996). Therefore, new opportunities become available when bodily resources change due togrowth of the body. The significance of exploration has since long been acknowledged bydevelopmental psychologists. Over the years, different interests and perspectives havemotivated the study of exploration. These include perception, learning and motivation, andpersonality development (see Keller et al., 1994). However, irrespective of what the per-spectives and specific interests are, the information gathering activities that constituteexploration, and the underlying bodily and environmental resources that motivate theseactivities, remain of central significance for our understanding of exploration and how itcontributes to development.

The aim of the present article is to gain more insight in how underlying bodily andenvironmental resources motivate exploration. To study this question we looked at thedevelopment of exploratory activity in young infants. Specifically, we were interested in howinfants learn to use the sensory system of touch in order to explore surfaces. We have chosento study this by looking at the way congenitally blind infants learn to make use of theresources of their bodies in relation to the environment. For these infants, the possibilities forexploration are seriously hampered by the absence of the visual system, as observations andtime lags in several domains underscore (Warren, 1994). Touch is of particular significancefor the blind. However, congenitally blind infants are known to be hampered in using theirsense of touch. It is therefore of importance to know how exploration by touch can beenhanced in congenitally blind infants. Before we discuss this issue, we first address thequestion of exploration itself, its goal and how this goal is obtained. Subsequently we discussresults of earlier studies that show how tactual exploration is enhanced and evolves incongenitally blind infants by presenting them surfaces that contain texture gradients. Finally,we will present data that show that such texture gradients also provide a frame that allowsthe child to orient movements and to recover earlier locations on the surface. Although theseresults do not directly prove the correctness of the theoretical position that guided ourresearch they are highly consistent with it, as we will show.

1.1. Search for information

Exploration is sustained by activities and is goal directed. However the goal of explorationis not a particular performance, like grasping an object or maintaining a particular posture forreaching. Rather, it is reaching a state of awareness about the self and its environment. Thisstate is ascertained by activities that involve the gathering of information. Goals such asorienting, reaching, grasping are embedded in the goal of the search for information.Searching for information enables the organism to become or stay aware of itself in relationto the environment. Therefore, to understand what exploration entails, we first need to

486 A.W. Smitsman, R. Schellingerhout / Infant Behavior & Development 23 (2000) 485–511

address the issue of how awareness is regulated. How does information about the relationshipbetween the ‘self’ and the environment arise? Most importantly we need to understand thatinformation, like exploration, is dynamic. This means that the organism never has informa-tion, but instead always is in the process of gathering information.

Gibson, 1979 defined information as sensory stimulation that is specific to its source.These sources are surfaces, substances, and events involving those surfaces. Informationwith respect to those sources is of importance because of the affordances that they furnish theorganism. Information for the perceiving those affordances arises due to the way surfacesspecifically structure energy such as light, sound and pressure waves. This structure can beseen as specific to the source it originated from. Thus, the mere stimulation of the senses doesnot give information for perceiving the environment, but instead how the environmentstructures this stimulation determines the information that becomes available. However inGibson’s view, gathering information requires also organs that are sensitive to this speci-ficity, and that can be adapted to the way energy fields become structured to the activeperceiver. Therefore, specificity is never solely determined by the environment or perceiver,but lies in the interaction of both transmitter and receiver.

Although specificity is of importance for perceiving, it is not something that a perceiveris aware of in the process of exploration. One needs knowledge of a source and thestimulation originating from that source to be able to determine whether there is specificitywith respect to that source. All that is available to the perceiver are energy flows that becomedifferentiated through the environment and the exploratory activities. In organizing theseactivities, the human body provides enormous flexibility in how to assemble the exploratorysystem. One can look at surfaces, touch and smell them, and even taste them in some cases.Moreover, activities of looking, touching, smelling and tasting may be performed in a varietyof ways. By exploiting this flexibility, surfaces and events not discriminated in advance canbecome distinct. This means that a state of not knowing is transformed into a state ofknowing. This shows that awareness of the self and the environment (and therefore alsoinformation) is not given, but is dynamic. Information is not out there in the environmentwaiting to be found. Instead, it is a property that emerges from the active engagement of anorganism, e.g. a child, with her surroundings.

1.2. Cooperation of the senses and sensory handicaps

The discovery of what the surrounding layout affords is not a passive and automatic enterprise,but depends on how the organism organizes the exploratory system. In the activity of exploring,several sensory modalities mostly cooperate. The perceiver may interchange visual exploration ofan object, by mouthing and wielding to discover what its affordances are. Stimulation that isambiguous at the level of one modality can be turned into information that is specific throughcooperative activity of the senses. In discovering an object’s affordances, the perceiver experi-ences the object, and not the visual, acoustic or haptic experience of the object. The dynamiclandscape for discrimination evolves from the way a detection apparatus is configured and theenvironment is structured. Objects and events will be noticed depending on the componentsinvolved for exploration and their coupling over time in the gathering of information. Therefore,an even more basic question is what promotes the cooperation of the senses.

487A.W. Smitsman, R. Schellingerhout / Infant Behavior & Development 23 (2000) 485–511

In our view, the cooperation between different sensory modalities is promoted by the wayenergy flows become structured. For instance, in case of a rolling or bouncing ball, orbreaking glass, periodicity of the sound wave produced by the motion of the ball or breakingof the glass corresponds highly with that of reflected light. Therefore, the cooperation ofsensory systems may be supported by the way the environment structures energy for thedifferent sensory modalities. Consequently, it may make more sense for studying explora-tion, to address the information gathering system that is afforded by the environment than toconsider the different senses as separate entities. In other words, perception may be anactivity that is done by the body as a whole, instead of constituted of separate activities ofdifferent senses see (Stoffregen & Bardy, in press, for a similar view).

Many studies have been done on infants’ and neonates’ a-modal or intermodal perception(Bahrick & Pickens, 1994; Bahrick, 2000). The results of these studies indicate that infantspick up a-modal information. However, these studies also indicate that infants’ and neonates’sensory systems are sensitive to the way ambient arrays of energy are structured by eventsand that information may arise because of this sensitivity. In studying exploration in infancy,it would be of interest to investigate how the different sensory systems interact in thegathering of information and how their cooperation is supported by the spatial and temporalcharacteristics of the sensory stimulation to the different senses.

Cooperation between sensory modalities involves intact sensory and motor systems. Innormal infants, visual perception is coupled to tactual perception, and depending on thesurroundings that are explored also accompanied by other sensory modalities such ashearing, smelling and tasting. Handicaps for one of those senses diminish the flexibility ofthe infant to organize an exploratory system that can take advantage of the way theenvironment structures different energy sources. When an important sense such as the visualsystem fails at birth, gathering of information for guidance of reaching, grasping, locomotionand postural control in executing these actions may be seriously perturbed.

The above problems in the guidance of actions occur in congenitally blind infants and lessso in infants with poor vision. We don’t know of cases of infants who fail touch, but thetime-lags of congenitally blind infants in the acquisition of motor milestones and otherdomains, such as space perception (O’Donnell & Livingston, 1991; Reynell, 1978) under-score the significance of the cooperation of vision and touch. The fact that these time lagslargely disappear later on (Warren, 1994) indicates that there is still flexibility left in the wayexploration can be constituted to guide actions and to explore the surroundings. Note that inour view it is not vision per se that is the cause of these detrimental developmental outcomes,but the coupling of vision to other senses, especially touch, that is perturbed. The infant mayno longer be able to assemble an exploratory system that enables her to orient and guidemovements and postures at a particular stage of development of the movement apparatus.

Control of posture and movements of the body takes place with respect to surroundingsurfaces and requires regulation of the relationship with respect to those surfaces (Goldfield,1995). Discovery of the environment as a place to act and to orient postures and movementsto in action, emerges from the information that is gathered. The relationship with theenvironment is multiply determined. The information for guiding the actor originates fromdifferent sources. For instance, in body sway there is a change in orientation with respect tothe direction of gravity, or the line of balance (Riccio & Stoffregen, 1990; Riccio et al.,


1992), but at the same time also with respect to surrounding surfaces. The direction of gravitymay form a frame to orient movements with respect to the body. However, to make thosemovements goal directed, orientation with respect to surrounding surfaces is also needed(Goldfield, 1995). Orientation is usually made possible by the visual system. The couplingof vision with touch ensures that sense of a loss of balance is coupled to information aboutthe orientation of the body to surrounding surfaces, including the ground. This brief analysissuggests that when there is no visual system the relation with the environment may bedifferent. It may become difficult for an infant, for example, to discover the relation betweena loss of balance and a change in position of its body with respect to its surroundings.

The above analysis is consistent with observations of congenitally blind infants’ behavior.Compared to sighted infants and even visually handicapped infants with a limited degree ofresidual vision, congenitally blind infants show little outer directness (O’Donnell et al., 1991;Carolan, 1973). They exhibit little or no spontaneous reaching out and touching nearbysurfaces compared to nonvisually handicapped infants, even though manual explorationwould be very helpful to them. As a consequence, these infants are described as passive.

If the above analysis is correct, exploration and outer directness can be improved in congen-itally blind infants by providing them an environmental frame for orienting movements of limbs,for instance the hands, with respect to a surface or arrangements of surfaces such as solid objects.Such a frame could enable the infant to discover how to move and to orient movements. Althoughuse of the visual system is impossible to those infants, cooperation between different senses is stillpossible, given that the energy for those senses can be structured in a way that enables orientationof postures and movements. Such occurs in case of infants provided with echolocation as researchshows (Bower, 1977; Farmer & Smith, 1997; Humphrey et al., 1988).

To provide congenitally blind infants with the opportunity of obtaining a frame fororientation and way finding, we investigated a novel possibility. We enabled the infants totake advantage of the cooperation of different subsystems that compose touch: kinesthesisand tactile perception see (Loomis & Lederman, 1986). Kinesthesis or proprioceptionregisters the static position and movements of limbs, the head and trunk. The combinedactivities of muscle spindles, tendons and joint receptors provide kinesthetic stimulation. Thetactile submodality reacts to cutaneous stimulation, which arises on deformation of the skin.Mechanical contact with a surface activates the different types of mechano-receptors in theskin. When one rubs with the palm of the hand or finger pads over a surface, cutaneous aswell as kinesthetic stimulation arises. The pattern of cutaneous stimulation will vary de-pending on whether a smooth, or textured surface is touched, and how texture elements overthe surface are distributed. When a surface is smooth and flat, such as a tabletop made ofplastic or glass, the pattern of cutaneous stimulation will stay the same into every directionthe hand moves across the surface and on every location the surface is touched. The sameoccurs when there is texture, but texture elements are homogeneously or randomly scatteredover the surface. The presence of texture elements makes it perhaps easier to registermovement, but the absence of structure makes it still difficult to orient the movement withrespect to the surface in terms of directions of movements and locations touched. Of course,the person could orient a trajectory or distance with respect to the body. There is kinestheticinformation to do so, as linear positioning tasks show (Smyth, 1984; Schellingerhout et al.,1998). However, it may be doubted whether a blindfolded person, using solely cutaneous


stimulation, could repeat an original trajectory on the surface if the surface were orienteddifferently with respect to that person.

This thought experiment demonstrates that smooth or homogeneously textured surfaces donot afford a frame for orientation with respect to this miniscale environment. There is textureand there are other properties that can be discovered such as its temperature and rigidity, butit provides no structure for way finding. Consequently, it will be difficult for a blind orblindfolded perceiver to orient hand movements to such surfaces. To put this in another way;when we conceive orientation to an environment as a necessary ingredient of goal-directedactivities, goal-directedness will become problematic if the environment does not provide aframe for orientation (see also Goldfield, 1995). In this respect, it may be interesting to noticethat surfaces that surround an immobile blind infant, for instance in laying on a hairy carpetor cloth, often consist of texture elements that are homogeneously distributed.

Information about direction and location could be gathered without the need of a visualsystem if across a surface, texture elements would systematically vary in density or someother dimension. If in addition, this variation were continuous, direction would be specifiedat any moment in time and place on the surface. A way to vary texture continuously is tospread texture elements over a surface according to a gradient of, for instance, changingdensity. Normally texture gradients arise as a result of natural perspective to the visualsystem. Gradients have been used to make surfaces represent visual stimulation with respectto slant (Holmes et al., 1998). However, the relationship of gradients and exploration may bemore basic. Any texture gradient provides a spatial frame for a haptic system, which is ableto discern the change in texture. Through such an environmental frame, awareness of spatiallayout may emerge as a result of the interaction between the action system and its immediatesurroundings. When rubbing, for instance, an increase in density of texture elements specifiesa different direction to move for the hand than a decreasing density. When density increasesfrom all directions, a location becomes discernable at the intersection of those directions. Thelocation forms a hilltop, metaphorically spoken. When the opposite occurs, a well is specifiedat the intersection of all possible directions of decreasing density. The intersection of directionsof increasing density can also form ridges or valleys, in case of decreasing density, that run overthe surface. Ridges and valleys are formed when density does not change from all possibledirections (see Fig. 1). For a rubbing hand or finger tip, going uphill would in principle bedistinguishable from going downhill by the way texture deforms the skin on rubbing.

A surface structured in the above way might provide a landscape for exploration andorientation of hand movements, which combines with the frame that the body provides.When we consider kinesthesis as important for the body frame, and the cutaneous sense asimportant for the environmental frame, cooperative activity of both systems would enable aninfant to gather information that grounds manual exploratory activities into the reality of thesurface and make them goal directed.

1.3. Tactile exploration of texture gradients in congenitally blind infants

To investigate how exploration can be enhanced and may develop in blind 8- to 21-month-old infants, we offered them the opportunity to haptically explore a surface containinga texture gradient (Schellingerhout et al., 1997). The surface was a flat rectangular surface


of 80 � 40 � 0.8 cm in diameter and made of silicon rubber that extended in front of theinfant. Lumps of equal height were scattered over the surface according to a gradient ofincreasing density and decreasing diameter into depth and to the sides from the midline (seeFig. 2). The smallest distance between the summits of the lumps was 2 mm; the largestdistance was 15 mm.

Because of the gradient structure three directions could be discerned on the surface: onefrom proximal to distal and two opposing directions from the midline to the sides (taking thelocation where the child was seated into account). When we consider the edges of the surfaceon the two opposite sides as hills, the midline formed a valley in the landscape. Althoughthere were no specific locations discernable, there were regions that differed from oneanother. For instance, lumps were most densely distributed at the far most left and rightcorners and least at the midline just in front of the infant.

The above texture gradient was presented for exploration at a 2 weeks interval to 8-, 13-,17- and 21-month-old children, making 5 subsequent sessions in total. There were 2 childrenof each age forming the total population of congenitally blind infants in the Netherlands and

Fig. 1. A density distribution of texture elements may be described as a landscape.

Fig. 2. The gradient texture. Arrows on the texture and the sine-wave beside the texture indicate directions ofchange.


Belgium at the time of the study. On the fifth session two additional surfaces, of the samediameter and material were presented for comparison with the texture gradient: one surfacewas completely smooth without any lumps, the other surface consisted of lumps, but nowequally distributed over the surface at a constant density.

If a gradient surface would afford goal directed activity of the infant, one of the first thingsone would expect to see is sustained exploratory activity over the sessions. This was exactlywhat we found. The amount of exploratory activity continued to stay high and increasedsomewhat after the first sessions for most infants, which is remarkable given the little amountof exploration that is often reported in blind infants (Carolan, 1973; O’Donnell & Livingston,1991; Reynell, 1978). To investigate how exploration developed over the ages and sessions,Schellingerhout et al. (1997) distinguished 7 different exploratory categories defined after theexploratory procedures (EP’s) suggested by (Lederman & Klatzky, 1987), (Ruff, 1984) and(Bushnell & Boudreau, 1993). Exploratory procedures are different categories of manipu-latory activities, constrained to maximize discrimination of particular properties, such asrubbing to discriminate texture, and static contact to register temperature. Consonant withour earlier definition of exploration we would conceive such EP’s as possible stable states foran information gathering system. By registering how these states evolve over time (sessionsand ages) we may obtain information about the dynamic landscape that evolves for theinformation gathering system the infant configures in exploring the environment. For thetexture gradient, one might expect to see EP’s emerging that exploit the gradients fordiscovering trajectories and regions on the surface. This means that for the informationgathering system, rubbing and fingering would be the stable behavioral categories to discernthe landscape. Besides, the surface may also afford other EP’s that are less relevant to thegradient, but instead are adapted to material properties such as its temperature and softness,and the fact that it is spatial. This means that other categories such as hitting, mouthing andstatic touching may also occur. However, if the gradient structure is of importance to theinfant, we may expect that the dynamic landscape for the exploratory system will evolve intothe direction of categories such as rubbing and fingering. This means that those latercategories will become prevalent above other categories when occasions to explore thesurface increase.

The evolution of a dynamic landscape for exploration not only depends on the way thesmall-scale-environment is structured but also on the infant’s developing movement appa-ratus. Configuring an information gathering system that suits the environment involves thecoupling of different action components over time such as motor systems and sensorymodalities and submodalities. We know that there are considerable time lags in the devel-opment of the movement apparatus for blind infants compared to sighted infants. But we donot know yet how these time lags will affect the emergence of categories as rubbing andfingering. We know that postural control is of importance to the development of independentfinger and hand movements. In sighted infants this develops by the end of the first year oflife (Bertenthal & von Hofsten, 1998), but in blind infants postural control is considerablydelayed. Therefore, we expected rubbing and fingering to show up mainly in the olderchildren. Fig. 3 shows how the dynamic landscape evolved for the four ages. The scoresrepresent the frequency for the 7 categories of exploratory behavior Schellingerhout et al.


Fig. 3. Dynamic landscape of the exploratory activities for the four age groups across the four gradient textureonly sessions.


observed during the last 4 sessions each lasting 10 min at most. Frequency is weighted bytime (frequency x duration/session time).

We divided rubbing and fingering each into 2 subcategories: a more undifferentiated anda more differentiated version. In rubbing 1, the hand palm and fingers pads make contact withthe surface and move in synchrony, in rubbing 2 there is the same movement, but only oneor more finger pads make contact with the surface. Fingering consists of scratching move-ments of solely the digits. In fingering 1 the digits move together and in fingering 2,movements of one or more digits are shown and those movements become differentiated.Hitting was defined as a repeatedly and forceful banging on the surface with the hands,touching as static contact, and mouthing as using the mouth to explore the surface.

As can be seen in Fig. 3, both types of rubbing show up over the sessions especially at theage of 20 months, and hitting disappears. There is also individuated fingering, but thisoscillates more over the sessions. When we consider what happens over ages for instance onthe last session, we see static contact, mouthing and hitting disappearing at the benefit ofindividuated fingering and rubbing. The over age and sessions evolving dynamic landscapeshows a gradually emerging information gathering system adapted to the structure of thesmall-scale environment presented. The emergence of this system is presumably constrainedby the bodily resources that become available to assemble the system given that the relevantexploratory categories appeared only in the oldest children.

To test whether the exploratory categories, rubbing and fingering that were evolving in theoldest infants were specific to the gradient structure, Schellingerhout et al. (1997) presentedon the last session two additional surfaces to the infants, both of the same size and materialas the gradient surface. One of these surfaces was with lumps of the same height as thetexture gradient, but now of constant diameter, which made texture elements homogeneouslydistributed. The other surface was completely smooth and without any lumps. Although thesesurfaces were new for the infants and might have elicited heightened exploration, resultsnevertheless showed more rubbing and fingering for the texture gradient in the oldest infants.

These latter findings add to the conclusion that the information gathering system infantsgradually configured became more adapted to the structure of the surface they explored.Results together show that exploration is not a single competence owned by the child thatmay be triggered by particular situations. Instead, it involves a gradually evolving dynamiclandscape of behavioral categories to gather information, which depends as much on the waysurrounding sources of energy are structured as the infant’s flexibility to configure behavioralorganizations that can take advantage of those opportunities.

1.4. Detection of locations

The tactual system is often considered as less suited to spatial perception than the visualsystem (Barber & Lederman, 1988). Moreover, congenitally blind children’s lower perfor-mance in spatial tasks than adventitiously blind and sighted children (McLinden, 1988) isattributed to reliance on movement and a self-referent perception of space (Millar, 1994).However, the information that becomes available to an information gathering system de-pends as much on the spatial layout that is explored as the activities to explore the layout.Therefore, it may be questioned whether a self-referent perception of space and reliance on


movement is inevitable. Just as for the visual system perspective transformations in astructured environment (Gibson, 1979; Pick & Palmer, 1986; Rieser et al., 1986; Rieser,1990) are not tied to the movements used (walking, crawling, car driving etc) to generatethose transformations, the patterns of tactile stimulation that arise on exploring a surface mayalso become uncoupled to the particular movements employed to explore the surface. Whenthe surface is structured, the same pattern may arise on different movements, specifying thespatial layout of the surface.

A surface layout, such as the gradient structure we described above provides a directionalframe. It may guide movements toward different locations. Because density of texture elementsin such a gradient structure changes in a continuous fashion, locations are in principle distin-guishable by their local density and directions to those locations by their change in density. Thus,on rubbing, information could be gathered about whether a change in density was into thedirection of a location or away from it. Moreover, because the same pattern of cutaneousstimulation can arise for different movements, and different patterns of cutaneous stimulation forthe same movement, the body frame can be disentangled from the environmental frameprovided by the surface layout. This makes reliance on solely a body frame superfluous. Italso means that a change in position of the surface relative to the body, either by displacingthe child or the surface, not necessarily needs to perturb way finding on the surface.

Using texture gradients, we studied way finding in blindfolded adults and congenitallyblind 4-year-old children. The studies show that a texture gradient can enhance way findingfor the tactual system. In the following paragraphs we will briefly summarize the results withadults and present data about the study with young blind children.

To study spatial perception in reaching space, two types of tasks are mostly used: detourtasks and rotation tasks. In a detour task, first a detour is taken to reach an end position. Afterthat, the end position has to be reached by its shortest path, without a detour. Investigationsaddress whether and how characteristics of the detour, such as its length and curvature(Brambring, 1976; Lederman et al., 1985; Lederman et al., 1987) affect reproduction of theend position. In rotation tasks, participants search a surface for a location, which can beanywhere on the surface. After that, the experimenter changes the spatial position of theparticipant with respect to the surface, either by rotating the surface, or the participant, andthe participant has to refind the original location (Millar, 1994; Ungar et al., 1995).

Investigations concern the kinds of frame persons use to locate positions in nearby space:movements, a body frame, or external referents. Generally, reliance on a body frame isattributed to blind persons (Millar, 1994) and young children (Acredolo, 1985, 1990), and theuse of external referents to sighted person. However, (Ungar et al., 1995) showed that it isnot the visual status per se that determines performance, but rather the strategies a participantuses to gather the information. We would hypothesize that these strategies depend also on theway the environment is structured and the way the information gathering system is config-ured with respect to the environment.

To investigate manual search of blindfolded adults, Schellingerhout (1998) comparedthree circular surface layouts (see Fig. 4), using a rotation task. One surface was completelysmooth, without any texture elements. The other two surfaces contained lumps as textureelements, homogeneously distributed for one surface and according to a gradient in twoperpendicular directions for the other surface. For the latter surface, density of lumps


increased in two waves, starting at two positions (see Fig. 4). One wave started at 45°counterclockwise to the 0° position and the other wave at 315° counterclockwise the 0°position. From these positions density increased until it reached its maximum at the oppositeend at 135° and 225°. A participant was seated at the 180° position. Considering the twodirections together, density was least around the 0° area and maximum around the 180° area.

The task contained two parts: a search part and a reproduction part. The search partconsisted of locating a small pin on the surface, after free manual exploration of the surfacefor at least 1 min. The reproduction part consisted of reproducing the original position of thepin after the surface had been rotated over different degrees, clockwise as well as counter-clockwise on subsequent trials. In total participants needed to locate and relocate 16 differentpositions that varied in direction and distance. During rotation participants placed their index

Fig. 4. The three round textures.


finger at the center of the surface. During search and reproduction, movements werevideo-recorded and digitized at 50 hertz to calculate the spatial coordinates and trajectory ofthe index finger over the surface.

In analyzing these movement trajectories and participant’s verbal reports, Schellingerhout(1998) discovered different types of strategies whose frequency varied according to thesurface layout that was explored. One of these, which occurred frequently for the gradientstructure, consisted of using the surrounding texture to define the location of the pin. Anotherstrategy was based on estimating the amount of rotation. Analyses of angular error scores andreproduction time showed that errors where least and reproductions times slowest for thetexture gradient, but only when the texture strategy was used.

1.5. Detection of locations in 4-year-old congenitally blind children

To investigate young blind children’s search for locations in a rotation task we presentedthe gradient lump texture and the homogeneous lump texture of Fig. 4 to three congenitallyblind children approximately 4 years of age. By comparing a texture gradient with ahomogeneous texture the contribution of the gradient to young children’s manual search fora location can be investigated. Children had to find a location (specified by a pin) on threeconsecutive trials. The fourth trial differed according to condition: (a) the pin was removed fromthe surface and the child was asked to replace the pin at its’ former location; (b) the child wasasked to find the pin from a changed position with respect to the surface, at either 90° or 180°from the child’s former position or (c) to child was asked find the pin again after a delay of 10 s.

For the texture gradient, rotation with respect to the original position will perturb thepattern of cutaneous stimulation that specified the earlier trajectory toward the location. Suchis not the case for the homogeneous texture. On the other hand, the original location, as wellas the new trajectory for the hand is specified by its surrounding texture in case of the texturegradient, enabling the child to find a new trajectory toward the target position. But findinga new trajectory requires the child to adapt manual search, taking advantage of the waydensity of texture elements systematically changes over the surface. By comparing rotations,which include a delay, with a delay without a change of position, the effect of a change inposition can be disentangled from the time delay connected to the change. Moreover, duringdelay activation for planning the reach toward a particular position in space with respect tothe body may decay, requiring the child to take advantage of the opportunity the gradientstructure provides to recognize the earlier trajectory on the basis of the cutaneous stimulation.In this way, possible effects of the texture gradient may show up in the delay condition.

2. Method

2.1. Participants

Participants were three congenitally blind boys showing no additional disabilities. At thetime of the first session, they were 51 months old (SD � 0.67 months). Causes of blindnesswere Lebers amaurosis and anophthalmos.


3. Materials

Two round textured surfaces with a diameter of 44 cm made of silicon rubber were used(see Fig. 4). The lumps of the gradient texture and homogeneous texture can be described asthe result of the perpendicular crossing of two waves, with a constant amplitude of 2 mm.For the gradient texture, wavelength decreased according to the arithmetic progression: tn �16 - (n-1) * 0.0025 (n e N, 1� n � 56, tn is the wavelength of wave n in mm). For thehomogeneous texture, wavelength was a constant of 10 mm. The first wave started at the135° position, the second wave started at the 225° position (see Fig. 4; the gradient-textureas used in the present experiment was rotated 180° compared to the gradient texture asdepicted in Fig. 4). For the gradient texture this means that for a child at the 0° position, therewas an area of ‘small’ and densely packed lumps at the near end of the surface, and an areaof ‘large’ and less densely distributed lumps at the far end of the surface (the 180° position).

The textures were laid on a round tabletop with upright sides of 5 mm. The tabletop wassupported in the middle by a single round leg, which could be adjusted in height so that theedge of the table was at the height of a child’s belly button. On a texture, a small pin(diameter 7 mm) with a rough head could be pinned. This pin could easily be distinguishedfrom the textures. Four locations were used as search targets for the children. If we take thecenter of the table-top as the origin of an x-y coordinate system (with the y-axis from the 0°position to the 180° position, x-axis perpendicular to the y-axis), the coordinates for locationsA, B, C, and D (in mm) were (163; -44); (0;110); (-100;0), and (0;-110) respectively. Onelocation, ‘B’ was situated in the least dense region of the gradient texture, one location, ‘D’was located in the most dense region, and locations ‘A’ and ‘C’ were in areas of intermediatedensity.

The manual movements of the children on the textured surfaces were recorded by aS-VHS video camera, which was mounted on a large tripod above the surface. The lens ofthe camera was at about 120 cm above the center of the surface. The legs of the tripod wereso far removed from the table that none of the children were aware of the presence of thetripod.

3.1. Procedure

The experiment was conducted at the residential institutions where the children livedduring the week. Sessions took place in a separate room unfamiliar to the children. The childwas led into the room and was positioned so that its belly button was at the 0° position.Before a session started, children were allowed and encouraged to explore the surface for aslong as they wanted. During sessions, children were only allowed to use the dominant hand,which was the right hand for all three children. The time between sessions was about 3–4days (2 sessions every week).

There were four conditions: the 0°-replacement condition, the 90°-rotation condition, the180°-rotation condition and the 0°-time delay condition. For each condition and eachlocation, the child started with an initial search phase consisting of 3 trials. After the initialsearch phase, a fourth trial followed which differed for each of the 4 conditions.

In the initial search phase the task of the child was to find the pin from the 0°-position.


The child was positioned in such a way that his belly button was against the table at the 0°condition. The right hand of the child was also at the 0°-position, lying on the texture againstthe edge of the tabletop. At a signal from the experimenter, who stood behind the child, thechild started to search for the pin by moving his hand across the texture. After the child hadfound the pin, she moved her hand back to the 0°-position. There were three trials in theinitial search phase.

In the 0°-replacement condition, the child removed the pin from the texture after the thirdinitial search trial. Then, from the 0°-position the child tried to place the pin at the originallocation by pinning it to the surface.

In the 90°- and the 180°-rotation conditions the task of the child was to find the pin (whichremained on the surface) from a changed position relative to the texture. After the third initialsearch trial, the experimenter walked to the new position (either the 90° or the 180°condition). The child then joined the experimenter by walking in a counterclockwisedirection along the table with his right hand in constant contact with the surface, until hisbelly button and hand were at the new position. Touching the surface during rotation wasincluded to enhance awareness of the displacement with respect to the surface. From thechanged position, the child was asked to search for the pin. After the child had located thepin, experimenter and child walked in a clockwise direction back to the 0°-position, while theright hand of the child remained in contact with the surface.

In the 0°-time delay condition the task of the child was to find the pin after a time-delay.After completion of the third initial search trial, the child counted aloud to ten (a filledinterval in memory research terms) and subsequently searched again for the pin on thesurface.

3.2. Design

There were 5 sessions, which were completed within 3 weeks. In sessions 2, 3 and 5 thehomogeneous texture was used. In sessions 1, 4 and 5 the gradient texture was used. Sessions1 to 4 started with the 0°– replacement condition, after which a rotation condition followed.In session 1 and 2 the 90°–rotation condition was used. In session 3 and 4 the 180°–rotationcondition was used. In session 5, the children completed the 0°-time delay condition, on boththe homogeneous and the gradient texture. Four locations were used for each session, foreach condition, the order of which was randomized. For sessions 1 to 4, there were 4 trialsfor the 0°– replacement condition and 4 trials for the rotation condition, giving a total of 8trials per session. For session 5, there were 4 trials on the homogeneous texture and 4 trialson the gradient texture, again giving a total of 8 trials.

3.3. Data scoring

The position of the hand of the child relative to the surface was computed every sixth ofa second on the basis of the digitized video-images and software that corrects for distortionsdue to characteristics of the lens of the camera and the video-screen.


4. Results

Although the number of participants is small, analysis of variance will be used to analyzethe results, in line with our earlier analyses of similar experiments (Schellingerhout et al.,1997; Schellingerhout, 1998). However, given the number of participants, the results of thepresent experiment should be regarded as suggestions and guidelines for further research.Results will be discussed under three separate headings: the initial search phase, the0°-replacement condition, and the 90°-rotation, 180°-rotation and the 0°-time delay condi-tion. Where post hoc comparisons are indicated in the following sections, results were furtherexplored by means of the Student-Newman-Keuls test using a univariate setup and an alphaof 0.05.

4.1. Initial search phase

For each of the three trials in the initial search phase, 4 different variables were computed.The search time is the time in seconds between the start and the end of a search for the pin.The initial movement direction is the direction (in degrees) the hand first moves in on thesurface during a search for the pin, relative to the starting position of the hand. This measureis an adaptation of the ‘direction of initial turn’ variable used by (Landau and Spelke, 1985)in their experiment with the blind girl Kelly. The movement direction error is the absolutedifference between the initial movement direction and the true direction of the pin location,again relative to the starting position of the hand. The pathway efficiency is a ratio of (1) thedirect distance between the starting position of the hand on the surface and the location ofthe pin and (2) the length of the actual route that the hand takes from the starting positionto the location of the pin1 (length actual route/direct distance). This measure is adapted fromthe Landau and Spelke (1985) replication experiment by Morrongiello et al. (1995). These4 variables were analyzed by means of a repeated measures Anova with Condition2 (0°-replacement, 90°-rotation, 180°-rotation, 0°-time delay), Texture (gradient, homogeneous),Location (A, B, C, D) and Trial (1, 2, 3) as within subject variables.

Search time showed a main effect for Trial, F(2, 4) � 37.01, p � 0.001. There wereinteractions of Texture and Location, F(3,6) � 5.01, p � 0.05, Texture and Trial, F(2,4) �7.29, p � 0.05, Location and Trial, F(6,12) � 6.28, p � 0.01, and Texture, Location, andTrial, F(6, 12) �3.32, p � 0.05. Fig. 5 gives a graphic representation of this effect. As isclear from this Figure, the search time is largest for the first trial, and drops on trials 2 and3 (all differences between the first trial and trials 2 and 3 were significant, except for LocationB). Fig. 5 also shows that in general, search time for the first trial is shortest on the gradienttexture. For locations B and C, there were significant differences between the gradient andhomogeneous texture for the first trial.

To further investigate why some locations were found sooner on the gradient texture,additional analyses were performed on the first search trials. It was found that the meanmovement speed on the gradient texture was higher than on the homogeneous texture (14.7cm/s relative to 12.1 cm/s), although this difference was not significant. Additionally, it wasscored every sixth of a second for the first search for the pin whether contact with the surfacewas maintained with the whole hand, or with one or several fingers. For the gradient texture,


32% of all the actions of a child were displacements3 with the hand across the surfacewhereas 5.9% consisted of displacements across the surface with the fingers. For thehomogeneous texture, the distribution was 21% displacements with the hand and 7.74%displacements with the finger. This distribution of displacements with the hand versusdisplacements with the fingers was not significantly different for the gradient texture and thehomogeneous texture. If we take all the displacements and the nondisplacements togetherhowever, contact with the surface on the gradient texture was maintained with the hand in84.25% of the cases versus 15.75% with the fingers. For the homogeneous texture contactwith the surface was maintained with the hand in 63.07% of the cases versus 36.93% withthe fingers. This distribution of contacts with the hand versus contact with the fingers wassignificantly different for the gradient texture and the homogeneous texture (X2(1)�11.32,p � 0.05).

The initial movement direction showed a main effect for Trial, F(2, 4) � 35.55, p � 0.01.Fig. 6 gives a graphic representation of this effect. As can be learned from Fig. 6, the initialmovement direction on Trial 1 is to the left, whereas on trials 2 and 3 the initial movementdirection is more towards the right and the middle. The movement direction of Trial 1 differssignificantly from the movement directions of trials 2 and 3, whereas trials 2 and 3 are notsignificantly different.

The movement direction error showed a main effect for Trial, F(2, 4) � 38.45, p � 0.01.Means (SD) for trials 1 to 3 were 52.41 (42.70), 17.80 (15.42), and 22.39 (14.27). Thus, notsurprisingly, the movement direction error is largest for the first trial. The movementdirection error of trial 1 differs significantly from the movement direction errors of trials 2and 3, whereas trials 2 and 3 are not significantly different.

The pathway efficiency showed effects of Trial, F(2, 4) � 34.22, p � 0.01, Location, F(3,6) � 5.03, p � 0.05 and Trial * Location, F(6,12) � 3.55, p � 0.05. Fig. 7 gives a graphicrepresentation of the Trial * Location. As is clear from this Figure, pathways are least

Fig. 5. Mean search time for levels of texture, location and trial in the initial search phase.


efficient for the first trial and drop to values around 2.5 on the second and third trial (alldifferences between first trial and trials 2 and 3 are significant, differences between trials 2and 3 are not significant). Pathway efficiencies on the first trial differ for the differentLocations, which is not strange, given the main effect of movement direction. Locations Aand B differ significantly from locations C and D.

Fig. 6. Initial movement direction for the three trials of the initial search phase.

Fig. 7. Mean pathway efficiency for levels of trial and location in the initial search phase.


4.2. 0°-replacement condition

Two variables were computed to measure the accuracy of the replacement of the pin onthe surface: the angular error and the distance error. The angular error is the angle between(1) the line going through the starting position of the hand and the true location of the pinand (2) the line going through the starting position of the hand and the location where thechild replaced the pin on the surface. If the angle measuring the direction of replacement islarger than the angle measuring for the true location, the directional error is positive, in thereversed situation the directional error is negative. Thus, if the true location of the pin is ina direction of 45°, and the child moves its hand in a direction of 60°, the directional error is15°. The second variable is the distance error, which is the difference in distance between(1) the starting position of the hand and the location where the child replaced the pin and (2)the starting position of the hand and the true location of the pin. A negative distance errorindicates that the child has not moved far enough with respect to the true distance. Theangular error and the distance error are related to a certain extent: if the angular error is large,there will always be a distance between the true location of the pin and the replacement.However, the opposite is not true: the distance error can be large, even when the angular erroris close to zero.

These two variables were analyzed by means of a repeated measures ANOVA, withPresentation4 (First, Second), Texture (gradient, homogeneous), and Location (A, B, C, andD), as within subject variables. Both the angular error and the distance error showed asignificant effect for Location, F(3,6)�9.88, p � 0.01, and F(3,6) � 8.22, p � 0.01respectively. Fig. 8 gives an overview of the positions where the children replaced the pin

Fig. 8. Replacement of the pin in the 0°-replacement condition. The replacement positions indicated are based onthe mean angular and distance errors.


on the surface. As is clear from Fig. 8, the children had a tendency to replace the pin towardsthe middle of the table.

4.3. 90°-rotation, 180°-rotation and 0°-time delay conditions

For the 90°-rotation, the 180°-rotation and the 0°-rotation conditions the same variableswere computed as for the initial search phase. One participant did not complete every trialof the 0°-time delay condition (two trials on the gradient texture, one trial on the homoge-neous texture). Exclusion of this participant would mean that these conditions cannot beanalyzed. Analyses were therefore performed on the averages across levels of Location bymeans of a repeated measures Anova with Texture (gradient, homogeneous), and Condition(90°-rotation, 180°-rotation, and 0°-time delay condition) as within subject variables. Thisprocedure has the advantage that there are no empty cells in the datamatrix, but it has thedisadvantage that effects of Location may not be analyzed. Additional analyses weretherefore performed to investigate the effect of Location. We filled in the three missing trialsby means of the SPSS ‘missing values analysis’ procedure. The resulting data were analyzedby means of a repeated measures Anova with Location (A, B, C, D), Texture (gradient,homogeneous), and Condition (90°-rotation, 180°-rotation, and 0°-time delay condition) aswithin subject variables. Thus, any effect of Location mentioned in the following results isbased on the data with filled in missing values.

There were no effects for search time. Mean search time was 7.56 s.The initial movement direction showed an effect of Condition, F(2, 4) � 99.79, p � 0.01.

Fig. 9 gives a graphic representation of this effect (movement directions from the 90°- and180°-rotation conditions are corrected for the different position of the child with respect tothe table). As can be learned from this Figure, the initial movement direction for the90°-rotation condition is towards the left, the initial movement direction for the 180°-condition is towards the left and the middle, and the initial movement direction for the

Fig. 9. Initial movement direction for the 4th trial in the 90°-rotation, the 180°-rotation, and the 0° time delaycondition.


0°-time delay condition is also towards the left and the middle. All differences betweenconditions are significant. The initial movement direction of the 0°-time delay condition ismost in accordance with the initial movement directions of the second and third trials of theinitial search phase. The analysis with Location yielded no additional results.

There were no effects for the movement direction error. The mean movement directionerror was 45.42 (41.31).

The pathway efficiency showed an interaction for Texture and Condition F(2,4) � 7.13,p � 0.05. Fig. 10 gives a graphic representation of this effect. As is clear from this figure,pathways are more efficient for the gradient texture and the advantage for the gradient textureis smallest in the 90°-rotation condition and largest in the 0°-time delay condition. Differ-ences between the textures are significant for the 180°-rotation condition and the 0°-timedelay condition, but not for the 90°-rotation condition. The analysis with Location yielded anadditional effect. There was an interaction of Condition and Location, F(6, 12) � 4.67, p �0.05, indicating that the pathway efficiency for a certain location differed across conditions.

5. Discussion and conclusions

Exploration involves the cooperative activity of several sensory and motor systems foruncovering what remains invariant in sensory stimulation to the different senses from whatvaries over time. Invariants form information for the system to settle down into a behavioralregime that transforms a state of uncertainty into a state of awareness or preserves a state ofawareness about the relationship with the environment. Surfaces, substances, events, texturesof surfaces and arrangements of surfaces, such as solid objects, form different regions of thedynamic landscape for exploration. This landscape evolves when exploration can be con-figured in new ways with development of the movement apparatus.

Fig. 10. Mean pathway efficiency for levels of Texture and Condition.


Guided by these assumptions, we started to investigate exploration in young children.These children were congenitally blind infants and congenitally blind children of 4 years ofage, for whom the possibility to configure exploration and to guide movements and postureswas severely reduced by the absence of the visual system. To enhance manual exploration,we constructed flat surfaces made of silicon rubber that extended in front of the child,forming a small-scale environment for touching and orientation. We supplied these surfaceswith a texture gradient that affords active awareness of: a) movement of the child’s rubbinghand on the surface, b) its direction of movement and c) relative distance, such that locationsfurther away could be discriminated from a place more nearby.

The texture gradient affords a frame of reference for hand movements that originates fromthe surface as a small-scale environment instead of the body. Uncovering it may allow thechild to disentangle directions and locations in surface space from those in body space. Togather this information, the texture gradient afforded cooperation of two different modalitiesof touch: kinesthesis and the cutaneous sense.

Normally, in sighted children, orientation of postures and movements with respect to theenvironment is made possible by the cooperation of vision and kinesthesis. We reasoned thatproviding young congenitally blind children with texture gradients for manual explorationcould enable the child to configure an exploratory system that to some extent mightcompensate for the loss of vision for spatial orientation of movements and postures5. Ofcourse, the situation is artificial and the scale at which compensation is possible is limited.Surfaces that surround the child normally are not supplied with texture gradients. Neverthe-less, experience with texture gradients during the period that manipulation develops ininfants may enhance tactile exploration and goal directed activities of those blind infants. Inaddition, this small-scale artificial environment can be valuable for investigating basichypotheses about exploration in children, including infants, and how exploration develops.

We discussed results of our research, showing that texture gradients do indeed affordtactual exploration for congenitally blind infants and can be used to enhance exploration. Theresearch also revealed that the dynamic landscape of exploratory activities that 8- to20-month-old blind infants configure to explore texture gradients, changes when manipula-tion develops. By the second half of the second year of life exploratory activities, such asrubbing emerge that are adapted to the texture gradient. Given that such gradients evokeexploration in blind infants, one might speculate about the long-term positive effects of earlyand frequent experiences with such textures for the development of touch in those infants.Communication with parents during the sessions those infants were tested, suggested thatsuch effects might be expected. They reported enhanced exploratory activity in their child’sbehavior that they were happily surprised to see. Enhanced exploratory activity of an infantmight have long lasting positive effects in the sense that it provides opportunities for theinfant to explore the bodily resources for exploration (see also Goldfield, 1995). Moreover,when such enhanced exploration takes place in a period in which important areas of the braindevelop on which the hand projects, its contribution might even be more fundamental thanfacilitating touch at an early age.

One positive effect of experience with a texture gradient could be that it enables the blindinfant to couple the body frame for movements and postures with an environmental frame.Thus, the infant can compensate for the loss of vision for the development of spatial


knowledge and skills. Location and rotation tasks provide means to measure whether such acoupling takes place. By changing the position of the child with respect to an originallocation, they enable investigation of whether search from the new position takes advantageof information about the embedding of a location in the environment.

Investigation of this hypothesis in 4-year-old congenitally blind children, the samechildren that were investigated earlier as infants, showed that locations are more easily foundand recovered on a texture gradient than on a texture consisting of homogeneously distrib-uted elements. Moreover, after three consecutive search trials the same location was moreeasily recovered, after a delay of 10 s as well as after a rotation with respect to the originalposition. Pathway efficiency in recovering the location was more than twice as high fortexture gradient compared to the homogeneous texture in the time delay condition and the180° rotation condition, and slightly higher in the 90° rotation condition. These resultsindicate that the directional information the gradient affords is gathered and that the texturegradient may provide a frame for way finding in near space. We may consider a location asa place on the surface that is specified by a distribution of texture elements and the path fromwhere the child was seated towards this location as a change in this distribution. When weconsider the both rotation conditions from this perspective, relocating the location on thebasis of this distribution and the way it changed seems to be more difficult after a 90° rotationthan after a 180° rotation. Results indeed confirmed such a difference.

The directional angle of the search trajectory did not show an effect for texture. Thisseems reasonable if the search is directed at the distribution of texture elements and guidedby the way the distribution changed. By rubbing, a pattern of changing density could beextracted from the surface specifying a trajectory that would lead toward the target location.On the first three search trials, children needed to discover the trajectory and learn how toconfine its boundaries, making the pathway more efficient. Results showed that efficiencygreatly increased on these three trials, for the gradient texture as well as for the homogeneoustexture. The latter result is not surprising given the fact that kinesthetic information wouldspecify the location, as long as the position from where to search remained unchanged.Results also showed that a delay of 10 s was sufficient to let efficiency drop, not to the levelof the first trial, but nevertheless some retuning of the system was needed. After the delaypathway efficiency turned out to be higher for the gradient texture than for the homogeneoustexture. This shows that retuning could be achieved more easily by taking advantage of thecutaneous sense to rediscover that pathway of changing density.

Upon rotation of the child to a new position the pattern of cutaneous stimulation that ledto the target location must have been disturbed, although not completely. The distribution oftexture elements was changed for the new position from where to search, but not for thetarget location. That location remained embedded in the same environment of a regularlychanging distribution of texture elements. This changing pattern could be rediscovered froma new position. How did the children accomplish such a task? The data on the initialmovement direction for the initial search phase and the delay and rotation phase shed somelight on this question. The data show a bias of the movement apparatus on the first trial forsearching across the midline to the left, which disappears on the second and third trial. Giventhat two locations were at the midline and the other two either to the right or left, one wouldexpect a search into the direction of the midline. However on Trial 2 and 3 there is an


overshoot to the right. In the time delay condition, the search direction changes slightly tothe midline, but stays at the right of it. However in the 180° rotation condition, and evenmore so in the 90° delay condition the initial search direction turns again across the midlineto the left. A return of the initial bias seems highly unlikely given that it did not occur forthe time delay condition and differed for the two rotation conditions. A more likely reasonwould be that in searching for the target location children tried to return to the region fromwhere they searched before they were rotated, especially in the 90°-rotation condition. Itshould be noted that rotation occurred counterclockwise, while the child’s finger padstouched the surface. Thus the direction in which they returned agrees with the direction inwhich rotation took place. For the 180° condition, a return would have made less sense giventhe symmetry between the new and old position from where to search and the larger distanceto move the hand back to the opposite side of the surface. Given that texture was importantduring sessions, search for structure in texture might have become a general strategy for boththe texture gradient and the homogeneous distribution. This might explain why we did notfind a difference between both textures in this respect.

The exploratory activities of the infants and the two 4-year-old children were directed atuncovering structure, at gathering invariants over time in texture. A constant changingdensity of texture over a surface may be quite uncommon for the child, taking the naturalenvironment into account. Texture gradients exist for the visual system as a consequence ofnatural perspective, but at the fine-grained level of texture itself, the environment is ordi-narily not arranged as a gradient.

However, the fact that the gradient was used for way finding in near space when it wasavailable, indicates that the system is not necessarily guided by some a priori knowledgeabout how the environment is structured or should be structured. For the system, theenvironment can be structured in any way as long as it is regular and a behavioralorganization can be configured to uncover the regularity. In case of the texture gradient, agradient of changing density formed the spatial structure of the surface. Space, in ouropinion, is an emerging property of the relationship of an action system with the environ-ment. For the child, space emerged from the changing density discovered by touch as a placeto specify locations and to guide movements toward locations from varying starting positions.

Finding out what remains constant under change requires flexibility to compose explora-tion, taking advantage of the sensory and motor resources that are and become availablewhen the movement apparatus develops. Exploration of the texture gradient required coop-eration of kinesthesis and the cutaneous sense, and activities that exploit their cooperation.Rubbing was the behavioral category that emerged in infants, expressing the cooperationbetween both modalities. It was also the kind of behavior that enabled the 4-year-old childrento find a location.

Finally, results suggest that by structuring the environment in ways that take into accountthe exploratory capacities that infants posses, which include sensory and motor organizationsas well as the flexibility to organize these in different ways, their exploratory capacity canbe enhanced. For handicapped infants and older children this might require particularmodifications. These modifications will enhance the capacity as long as they suit bodilypossibilities that are left for the handicapped. This means that the capacity to exploreinvolves not only the body, but also the environment that suits the body. Normally, acoustics


is exploited and artificial devices for echolocation. We took an alternative route that directlyaffected touch, an important sense blind children rely. Instead of developing a device wechanged the tactile environment to enhance the capacity of perceiving. New studies areunderway, extending the present studies on tactile exploration of texture gradients incongenitally blind infants. It is surprising that this route has not been more thoroughlyexplored during the past, given the importance of touch for the blind.

Notes

1. The length of the route was computed on the basis of the distances between thepositions of the wrist on consecutive video-frames.

2. Every condition started with an initial search phase of three trials.3. Displacements were computed relative to the displacements of the wrist across the

surface. Note that actions such as rubbing with the hand or with the fingers arepossible without displacement of the wrist across the surface.

4. As can be learned from the design, the 0°-replacement condition was completed twicefor each of the textures in the course of the five sessions, whereas the 0°-time delay,the 90°-rotation, and the 180°-rotation conditions were completed once for eachtexture. Presentation refers to the first or second time a child completed the 0°-replacement condition with a given texture.

5. A replication experiment with 8 blindfolded sighted 4 year old children interestinglyenough produced different results than the present experiment with blind children.With the sighted children, we found influence of the gradient texture on the searchpatterns (e.g., an initial movement direction, which followed the gradient). However,there were no effects of texture on efficiency measures, such as the pathway effi-ciency. These different results are in line with the theoretical framework we havepresented here. Because of their different developmental history, sighted childrenhave a different haptic exploratory system than blind children, even when we tem-porarily deprive them of their vision.

Acknowledgments

The authors are grateful to Raoul Bongers, Gerard van Galen and Nienke Smitsman fortheir contribution to parts of this research and comments on an earlier version of thismanuscript. Part of this research was supported by the Dutch Institutes for the Assistance ofthe Visually Disabled Theofaan and Bartimeus and the Dutch Institute for the Blind andVisually Disabled (VNBW).

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Exploratory behavior in blind infants: how to improve touch

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