The gap effect in newborns

Developmental Science 2:2 (1999), pp 174±186

REPORT

The gap effect in newborns

Teresa Farroni,1 Francesca Simion,1 Carlo UmiltÁa2 andBeatrice Dalla Barba3

1. Dipartimento di Psicologia dello Sviluppo e della Socializzazione, UniversitaÁ di Padova, Italy2. Dipartimento di Psicologia Generale, UniversitaÁ di Padova, Italy3. Dipartimento di Pediatria, UniversitaÁ di Padova, Italy

Abstract

In four experiments we investigated the gap effect in infants within the first 3 days of life. Reaction times (RTs) to makea saccade to a peripheral target were measured on gap trials, in which the central fixation stimulus went off 500 msbefore target presentation, and on overlap trials, in which the central fixation stimulus remained on. In every experimentthe fixation stimulus was a flashing light. The target stimulus was a schematic face in Experiment 1, a flashing lightshown at 20� eccentricity in Experiment 2, a flashing light shown at 30� eccentricity in Experiment 3, and an upside-down schematic face in Experiment 4. In Experiments 1±3 a gap effect was found. That is, RT was faster on gap than onoverlap trials. In contrast, the gap effect was absent in Experiment 4. These findings are consistent with the view that thesuperior colliculus plays a major role in producing the gap effect at birth.

Introduction

During our everyday life we make continuous eyemovements to look from one point to another in thevisual field. These high velocity movements are calledsaccades. Disengagement from a central fixation point isa prerequisite for the initiation of a saccade directing thefovea towards a peripheral target. Saccadic reactiontime (RT) is reduced if a temporal gap is introducedbetween the disappearance of the central fixation pointand the appearance of the new lateral target. Inparticular, RT to make a saccade to a peripheral targetis faster on `gap trials', in which a central fixationstimulus goes off before target presentation, than on`overlap trials', in which the central fixation stimulusstays on. In the adult, this phenomenon has becomeknown as the gap effect (e.g. Saslow, 1967; Fischer &Boch, 1983; Fischer & Ramsperger, 1984; Kalesnykas &Hallett, 1987; Braun & Breitmeyer, 1988; see Fischer &Weber, 1993, for a comprehensive review and relatedcommentaries for a thorough discussion).

Two hypotheses have been proposed to explain thegap effect, one attentional and one visuo-motor (alsocalled oculomotor). According to the attentional hy-pothesis (Fischer, 1986; Fischer & Breitmeyer, 1987;Braun & Breitmeyer, 1988, 1990), visual attention blockssaccades to peripheral targets while it is engaged by acentral stimulus. Based on Posner's model (e.g. Posner &Petersen, 1990; Posner & Dehaene, 1994), the hypothesisproposes that performing a saccade from one location toanother in the visual field requires that attention bedisengaged from the starting location before the eyemovement begins. When attention is engaged atfixation, the saccadic system is inhibited and RT isdelayed. When a temporal gap is introduced betweenoffset of the central stimulus and onset of the peripheraltarget, attention is automatically disengaged, inhibitionis removed, and RT is faster.The visuo-motor or oculomotor hypothesis (King-

stone & Klein, 1993; Tam & Stelmach, 1993; Dias &Bruce, 1994; Reuter-Lorenz, Oonk, Barnes & Hughes,1995; Walker, Kentridge & Findlay, 1995; Forbes &Klein, 1996) considers the superior colliculus to be

# Blackwell Publishers Ltd. 1999, 108 Cowley Road, Oxford OX4 1JF, UK and 350 Main Street, Malden, MA 02148, USA.

Address for correspondence: Francesca Simion, Dipartimento di Psicologia dello Sviluppo e della Socializzazione, UniversitaÁ di Padova, via Venezia

8, 35131 Padova, Italy; e-mail: [email protected]

critical for saccade control and maintains that thesuperior colliculus may trigger saccades if the saccade-generating circuit has been partially disinhibited at thetime of target appearance. This disinhibition occursduring the gap, when no stimuli are present in the visualfield. The visuo-motor hypothesis suggests that the gapeffect has two components, in neither of which doesvisual attention play any role (e.g. Kingstone & Klein,1993; Walker et al., 1995). One component, i.e. motorpreparation, is not specific to the oculomotor system.Because of the contingency between offset of the fixationstimulus and onset of the target stimulus, the gap primesthe motor system to make a response. In other words,the gap acts as a warning signal, allowing the subjecttime to prepare a response (also see Ross & Ross, 1980;Reuter-Lorenz, Hughes & Fendrich, 1991; Bekkering,Pratt & Adams, 1996). The other component, instead, isthe fixation offset effect, which is specific to theoculomotor system and causes disinhibition of thesuperior colliculus.1

A phenomenon similar to the gap effect in the adult is`sticky fixation' in young infants (also called `obligatoryfixation'; Stechler & Latz, 1966; Aslin & Salapatek,1975; Mohn & van Hof-Van Duin, 1986; Johnson,Posner & Rothbart, 1991; also see Goldberg, Maurer &Lewis, 1997, for a recent review). Infants between 1 and4 months, and especially at about 2 months, appear tobe compelled to fixate a centrally presented stimulus,and have great difficulty in moving their eyes from it toanother target in the periphery. This phenomenon hasbeen attributed to the development of an inhibitorypathway to the superior colliculus from the primaryvisual cortex, through the substantia nigra and basalganglia (Johnson, 1990, 1995). The improved ability toshift gaze to the periphery after 4 months is insteadassumed to reflect the maturation of parietal and frontalstructures, which exert a disinhibitory influence on thesuperior colliculus (Johnson, 1990; Atkinson, Hood,Wattam-Bell & Braddick, 1992; Hood, 1995; Braddick,Atkinson & Hood, 1996).All these pathways have different stages of maturation

before reaching the adult stage. Subcortical visualstructures are relatively mature at birth compared withcortical structures, and visual development represents atransition from a system predominantly controlled bymature subcortical structures, especially by the superior

colliculus, to a system dominated by cortical visualfunctions (Bronson, 1974; Atkinson, 1984; Johnson,1990, 1995; Hood, 1995; Maurer & Lewis, 1998; Csibra,Tucker & Johnson, in press).Newborns orient to peripheral stimuli if they are

salient enough. At birth, exogenous saccades arepredominantly driven by subcortical structures, notablyby the superior colliculus, and remain so until at least 6months of age (Csibra et al., in press). After 4 months ofage, cortical mechanisms play an increasingly importantrole for endogenously guided saccades. Johnson (1990,1995; also see Maurer & Lewis, 1998) has proposed thefollowing developmental pattern. At 1 month of age,when the baby is fixating a central stimulus, the superiorcolliculus is prevented from initiating a saccade to aperipheral stimulus because of inhibition by the sub-stantia nigra. After 1 month, sticky fixation declinesowing to increasing cortical control over saccades,though it can still be easily observed at 2 months. Byabout 4 months, the premotor areas of the frontal lobes,which contain the frontal eye fields, are mature enoughto disinhibit the colliculus from the inhibition producedby the primary visual cortex. Disinhibition of thesuperior colliculus releases collicular mechanisms forgenerating saccades to peripheral stimuli. Thereforesticky fixation wanes by about 4 months of age.On the basis of the model of maturation of the circuits

governing saccade generation that we have outlined, thegap effect should manifest a pattern of development verysimilar to that of sticky fixation: It should be absent atbirth, when only the superior colliculus is fully mature; itshould appear at about 1 month, as the primary visualcortex matures; it should then decrease from about 3±4months, when saccade generation begins to be influ-enced by the frontal eye fields and other higher corticalareas, like the parietal cortex.A number of studies have reported that 1- to 2-

month-olds are slower to make saccades to the peripheryof the visual field if the central fixation stimulus remainson than if it is turned off before the peripheral targetappears, and that the effect decreases by 3±4 months ofage. Often, babies are also less likely to makedirectionally appropriate eye movements to a peripheraltarget when the central stimulus remains on, again witha diminution of the effect by 3±4 months of age (see thereview in Maurer & Lewis, 1998; also see Matsuzawa &Shimojo, 1997). What remains to be investigated iswhether the gap effect is present at birth. To date, theevidence is scarce, indirect and contradictory.Braddick et al. (1992) tested a baby (PP) who had intact

superior colliculi but had the right cortex removed fortreatment of seizures. The baby was tested in the rightvisual field, which had an intact cortical representation,

1As was pointed out by an anonymous reviewer (also see Munoz &

Wurtz, 1995), the most parsimonious explanation for the fixation

offset component is in terms of competition between orienting and

fixating. On overlap trials, the superior colliculus is activated both by

the stimulus at fixation and by the stimulus at the periphery. Because

fixating and orienting are mutually exclusive, a conflict arises, which is

absent in gap trials.

The gap effect in newborns 175

# Blackwell Publishers Ltd. 1999

and in the left visual field, which had no corticalrepresentation. Note that, as happens in newborns, theleft visual field was served only by the superior colliculus.The testing took place between 7 and 12 months of age,i.e. at ages at which normal infants can easily disengagegaze from a central stimulus to orient to a peripheraltarget. When the central stimulus was removed beforetarget onset, the baby almost always oriented to theperipheral target, even in the affected field. In contrast,when the central stimulus remained on, the baby wasmuch less likely to make a saccade to the peripheraltarget. This finding indicates that the presence of theprimary visual cortex is not necessary for sticky fixationto occur. In accordance with that, Harris and hiscolleagues (Harris & MacFarlane, 1974; MacFarlane,Harris & Barnes, 1976) reported that, even at birth,babies respond to targets less far in the periphery when acentral stimulus remained on than when it was turned off.However, Goldberg et al. (1997) found that the presenceof a central stimulus did not interfere with orienting toperipheral stimuli at birth. The accuracy of newborns'saccades, and thus the estimated extent of the visual field,was not affected by whether the central fixation stimulusremained on or was turned off.It should be noted that none of the studies with

newborns measured saccade latency. Therefore theavailable evidence, besides being contradictory, is scarcelyinformative concerning the presence of the gap effect atbirth. The main purpose of the present series ofexperiments was to explore whether the newborns' latencyto make a saccade toward a peripheral stimulus when acentral fixation stimulus is still present (overlap condition)is longer than when the central stimulus disappears beforethe peripheral stimulus appears (gap condition).

Experiment 1

The first experiment used a typical gap paradigm. In thegap condition the central fixation stimulus was turnedoff before presentation of the peripheral target stimulus(see Figure 1(a)), whereas in the overlap condition thefixation stimulus remained visible while the targetstimulus was presented (see Figure 1(b)). As describedin the method section, the fixation stimulus was aflashing light and the target stimulus was a schematicdrawing of a human face. We recorded the direction andlatency of the first eye movement away from the centretoward the peripheral target.

Method

Subjects

Thirty-seven normal, healthy, full-term newborns wereselected from the maternity ward of the Paediatric Clinicof the University of Padua. Nineteen babies wereexcluded from the final sample for various reasons.Eleven changed state just before the experiment began,or during the experiment, and did not revert to a normalstate within a reasonable amount of time; for one baby atechnical error occurred, and seven babies did not reachthe criterion of at least two valid RTs for each of thefour types of trial (see below).The 18 babies that completed the study met the

screening criteria of normal delivery, a birth weightbetween 2600 and 4000 g and an Apgar score of at least 8at 5 min. All were healthy and free of any knownneurological or ocular abnormality. They were tested afterthe first 24 h of life, the range of ages at time of test beingabout 24±120 h postnatal (mean age about 72 h). Thetesting took place during the hour preceding the scheduled

(b)

(a)

Figure 1 (a) The gap condition and (b) the overlap conditionfor Experiment 1. In the gap condition the central fixationstimulus (the flashing light) was turned off 500 ms beforepresentation of the peripheral target stimulus (the uprightschematic face). In the overlap condition the fixation stimulusremained visible while the target stimulus was presented.

176 Teresa Farroni et al.


feeding time, if the baby was awake and in an alert state.Informed consent was obtained from the parents.

Apparatus and stimuli

The baby sat on a female student's lap in front of ascreen, at a distance of about 30 cm. The student wasunaware of the hypotheses of the study. The screen wasmedium grey in colour, and plain white curtains weredrawn on both sides of the chair where the student satwith the baby in her lap, obscuring irrelevant stimuli.The baby's eyes were aligned with a red LED (4.9 V)

that was located in the centre of the screen. The LEDsubtended about 3� of visual angle and, when turned on,blinked at a rate of 500 ms on and 500 ms off. It had atime-average luminance of 56.5 cd=m2 (alternatingbetween 0.0 and 113.0 cd=m2).This light was used toattract the baby's gaze at the start of each trial. Theperipheral stimuli were provided by two carousel slideprojectors located behind the infant's head and con-trolled by a Quadra 900 computer. The computer alsocontrolled the sequence of the stimuli. A video camera,mounted behind the screen, recorded the baby's eyemovements.The peripheral stimuli were two head-shaped, two-

dimensional white forms, with three blobs (blacksquares) as features of a human face (see Figure 1),taken from previous studies (Johnson & Morton, 1991;Morton & Johnson, 1991; Valenza, Simion, MacchiCassia & UmiltaÁ , 1996; Simion, Valenza, UmiltaÁ &Dalla Barba, 1998b). At the viewing distance of about30 cm, each stimulus subtended about 21� of visualangle horizontally and about 35� vertically. The innerborder of the stimulus was placed at the periphery of thevisual field, about 10� to the left or right of the centrallight. The blobs subtended about 1.5�. The luminancesof the white and black areas of the stimuli were 7.8 cd=m2 and 1.4 cd=m2, respectively. The contrast was 0.70.

Procedure

For half of the subjects the experiment started with thegap condition followed by the overlap condition. Forthe other subjects presentation order was reversed, theoverlap condition preceding the gap condition. There-fore, condition was blocked.2 Considering target side(left versus right), there were four types of trial: leftoverlap, right overlap, left gap, and right gap. For eachsubject and for each condition, the sequence was

pseudo-random, with the constraints that the side ofstimulus presentation was equiprobable and the stimuluswas not presented more than twice in a row on the sameside. Every trial began with the central light flashing. Assoon as the baby fixated the light, one of theexperimenters, the observer, who watched the baby'seyes via a video monitor system, started the sequence ofthe trial by pressing a key on the computer keyboard.This automatically activated the slide projector, whichpresented the face stimulus and turned off the centrallight (gap condition). The interval between disappear-ance of the central light and presentation of theperipheral schematic face was 500 ms.3 In the overlapcondition, the central light remained on while theschematic face was presented. In either condition, theface remained on until the baby initiated an eyemovement or a 10 s interval had elapsed. When thebaby fixated the face, or at the end of the 10 s interval,the observer turned off both the flashing light and theface and, after about 30 s, a new trial was started by theflashing light.

Data collection

Video tapes of the baby's eye movements throughout thetrial were analysed by two independent coders (anexperimenter and a student). The coders could see thecorneal reflection of the fixation stimulus on overlaptrials, but the student was not aware of the purposes ofthe study. They recorded the direction and latency ofeach orienting response. RT was defined as the intervalbetween the onset of the peripheral stimulus and thetime when the eyes started to move. It was measuredframe by frame from the video-recorded tapes. In otherwords, saccadic latencies were determined by calculatingthe number of intervening frames between the appear-ance of the target and the onset of the first lateral eyemovement toward it. Each frame lasted 33 ms; however,owing to poor inter-coder reliability, RT was measuredto the nearest 99 ms (i.e. three frames).We tested every baby at least four times for each type

of trial. Overall, 570 trials were run, but many of them(340) were considered to be invalid because the eyemovement was away from the target stimulus (27 trialsin the gap condition and 40 in the overlap condition),the baby closed the eyes before the peripheral target waspresented (21 in the overlap condition and 37 in the gapcondition), a technical failure occurred (ten in the gapcondition and two in the overlap condition), or the

2 The gap and the overlap conditions were run by different programs.

Therefore different trial types could not be intermixed.

3 This gap duration was chosen on the basis of the Matsuzawa and

Shimojo (1997) study with older infants, and considering that a 200 ms

gap is the most effective one with adults.



experimenter started the trial when the baby was moving(129 in the gap condition and 74 in the overlapcondition). In addition, trials in which the saccadeoccurred before target onset or with a latency of 99 msor shorter were excluded as anticipatory. There was atime limit of 10 s for performing the saccade. In eithercondition, however, no saccade was lost for exceedingthis limit. Of the 230 valid trials for which RT wasmeasured, 110 were for the gap condition and 120 forthe overlap condition. The average number of trials foreach subject was 6.7 in the overlap condition and 6.1 inthe gap condition, with a minimum of two trials.

Results and discussion

A preliminary analysis of variance showed that order ofcondition (gap condition first or overlap condition first)and target side (left or right visual field) wereimmaterial. Therefore these factors were dropped fromthe subsequent analysis. Average saccadic RT wascalculated for each subject as a function of condition.It was found that mean RT was 1308 ms (SD� 443) onoverlap trials and 874 ms (SD� 259) on gap trials.Because every baby produced at least two scorable

RTs for each of the four types of trial, a t-test analysisbetween condition (gap or overlap) could be conducted.It revealed a significant effect of condition, t(17)� 3.95,p< 0.001, demonstrating that the gap condition pro-duced shorter RTs than the overlap condition (i.e. a gapeffect of 434 ms).Clearly, saccadic latency was longer in the overlap

than in the gap condition. To our knowledge, noprevious study has measured the RT gap effect ininfants younger than 1±1.5 months (Hood & Atkinson,1993; Goldberg et al., 1997; Matsuzawa & Shimojo,1997). Experiment 1 demonstrated that this effect ispresent in newborns, when saccade generation ispredominantly, if not exclusively, controlled by thesuperior colliculus. The implications of this finding willbe considered in the general discussion. As of now, it issufficient to note that the longer saccadic latency in theoverlap than in the gap condition cannot be attributedto the fact that, in the overlap condition, the baby keptfixating the more interesting central stimulus. In fact,the schematic drawing of a human face we used as thetarget stimulus is able to attract the newborns' gazepromptly when presented in isolation or simultaneouslywith another lateralized competing stimulus (Johnson &Morton, 1991; Morton & Johnson, 1991; Valenza et al.,1996; Simion et al., 1998b).

Experiment 2

The purpose of Experiment 2 was to replicate andextend the findings of Experiment 1 by using differentstimuli. Considering that faces are `special' for new-borns, as well as for adults (e.g. Simion, Valenza &UmiltaÁ , 1998a), it was important to demonstrate that thegap effect is present at birth even when the newborn isrequired to perform a saccade to a less special stimulus.Therefore in this experiment the peripheral targetstimulus was a flashing light, very similar to the centralfixation stimulus.

Method

Subjects

Twenty-seven normal, healthy, full-term newborns wereselected from the maternity ward of the Paediatric Clinicof the University of Padua. The final sample, however,comprised only 16 babies because 11 had to be excludedfrom the study. Five changed state before the experi-ment began, three were discharged from the hospitalbefore being tested in one of the two experimentalconditions, one had a position bias to the right, one hadpoor ocular coordination and one was never engaged bythe central stimulus.The 16 babies that completed the study met the

screening criteria of normal delivery, a birth weightbetween 2600 and 4000 g and an Apgar score of at least8 at 5 min. They were tested after the first 24 h of life,the range of ages at the time of test being about 24±120 h postnatal (mean age about 72 h). The testing tookplace during the hour preceding the scheduled feedingtime, if the baby was awake and in an alert state.Informed consent was obtained from the parents.

Apparatus, stimuli and procedure

The apparatus and procedure were identical to thosedescribed for Experiment 1, whereas the stimuli weredifferent.The central fixation stimulus and the peripheral target

stimulus were red lamps inserted into a plastic tube5.0 cm long and with a diameter of 3.2 cm (about 6� at adistance of about 30 cm). The lamps flashed at a rate of500 ms on and 500 ms off. They had a time-averagedluminance of 34.0 cd=m2, alternating between 0.0 and68.0 cd=m2. The centre of the peripheral lamp waslocated about 20� from the centre of the fixation lamp.



Data collection

Saccadic latency was measured as described for Experi-ment 1. We tested every baby 10±11 times for each typeof trial. The total number of trials should have been 664.However, many trials (387) were lost because the eyesmoved away from the target (38 trials in the gapcondition and 49 in the overlap condition), no saccadeoccurred within 10 s from target onset (nine, 2.7%, inthe gap condition and three, 0.9%, in the overlapcondition), the baby changed state before completingthe experiment (16 in the gap condition and 13 in theoverlap condition), a technical failure occurred (11 inthe gap condition and six in the overlap condition), orthe experimenter started the trial when the baby wasmoving (131 in the gap condition and 111 in the overlapcondition). Trials in which the saccade occurred beforetarget onset or with a latency of 99 ms or less wereexcluded as anticipatory responses. All trials in which asaccade was performed within 10 s from target onsetwere retained. Out of 277 valid trials for which RT wasmeasured, 123 were for the gap condition and 154 forthe overlap condition. The average number of trials foreach subject was 9.6 in the overlap condition and 7.7 inthe gap condition, with a minimum of two trials.


A preliminary analysis of variance showed that order ofcondition (gap condition first or overlap condition first)and target side (left visual field or right visual field) did notaffect the results. Therefore these factors were droppedfrom the subsequent analysis. Average saccadic RT wascalculated for each subject as a function of condition. Itwas found that mean RT was 2024 ms (SD� 496) onoverlap trials and 1241 ms (SD� 323) on gap trials.Every baby produced at least three RTs for each of

the four types of trial. Therefore a t- test was performed.It revealed a significant effect of condition, t(15)� 5.58,p< 0.0001, demonstrating that the gap condition pro-duced shorter RTs than the overlap condition (i.e. a gapeffect of 783 ms).Experiment 2 confirmed the results of Experiment 1,

showing that RT was slower in the overlap than in thegap condition. The gap effect was 783 ms. It is thereforeapparent that special target stimuli, like faces, are notneeded for the gap effect to manifest itself in newborns.Overall RT was slower in Experiment 2 than in

Experiment 1 (1632 vs 1091 ms). It is likely that theperipheral stimuli of Experiment 1 (schematic faces)were more salient than those of Experiment 2 (flashinglights). The greater salience of the stimuli of Experiment1 can be attributed to several factors: faces are special

for newborns, they were much larger than the lamps(21� � 35� vs 6� � 6�), and closer to fixation (about 10�

vs about 20�).

Experiment 3

This experiment tried to further extend the findings ofthe previous two experiments by using still differentstimulation conditions. In it, the peripheral targetstimulus was smaller and more eccentric than inExperiment 2, but its luminance was greater.

Method

Subjects

Nineteen normal, healthy, full-term newborns wereselected from the maternity ward of the PaediatricClinic of the University of Padua. Ten babies did notcomplete testing because seven changed state before theexperiment began, for two a technical error occurred,and one was discharged from the hospital before beingsubmitted to the gap condition.The nine babies that completed the study met the

screening criteria of normal delivery, a birth weightbetween 2600 and 4000 g, and an Apgar score of at least8 at 5 min. They were tested after the first 24 h of life,the range of ages at the time of test being about 24±120 h postnatal (mean age about 72 h). The testing tookplace during the hour preceding the scheduled feedingtime, if the baby was awake and in an alert state.Informed consent was obtained from the parents.


The apparatus and procedure were identical to thosedescribed for the two previous experiments, whereas thestimuli were similar, but not identical, to those that wereused in Experiment 2.The central fixation stimulus and the peripheral target

stimulus were red LEDs (4.9 V). They flashed at a rateof 500 ms on and 500 ms off, at the viewing distance ofabout 30 cm subtended about 3�, and had a time-averaged luminance of 56.5 cd=m2 (alternating between0.0 and 113.0 cd=m2). The central and the peripheralstimulus were separated by 30�.

Data collection

Saccadic latency was measured as described for theprevious experiment. We tested every subject nine to tentimes for each type of trial. That produced 340 trials



overall, of which 258 were lost because the eye move-ment was in the direction opposite to the target (32 trialsin the gap condition and 40 in the overlap condition),the baby changed state (two in the gap condition andtwo in the overlap condition), the baby did not shift gazewithin 10 s from target onset (seven, 4.4%, in the gapcondition and three, 1.6%, in the overlap condition), atechnical failure occurred (one in the gap condition and13 in the overlap condition), the baby closed the eyesbefore presentation of the peripheral target (five for theoverlap condition and five for the gap condition), or theexperimenter started the trial when the baby was moving(71 in the gap condition and 77 in the overlapcondition). Trials in which the saccade occurred beforetarget onset or with a latency of 99 ms or less wereexcluded as anticipatory responses. All trials in which asaccade was performed within 10 s from target onsetwere retained. Out of 82 valid trials for which RT wasmeasured, 41 were for the gap condition and 41 for theoverlap condition. The average number of trials for eachsubject was 4.6 in either condition, with a minimum oftwo trials.


A preliminary analysis of variance showed that order ofcondition (gap condition first or overlap condition first)and side of target presentation (left or right visual field)did not matter. Therefore these factors were droppedfrom the subsequent analysis. Average saccadic RT wascalculated for each subject as a function of condition. Itwas found that the mean RT was 2594 ms (SD� 1158)on overlap trials and 1696 ms (SD� 648) on gap trials.Because every baby produced at least two RTs for

each type of trial, a t- test was conducted. It revealed asignificant effect of condition, t(8)� 2.3, p< 0.05,demonstrating that the gap condition produced shorterRTs than the overlap condition (i.e. a gap effect of898 ms).The results of Experiment 3 fully confirmed those of

the previous two experiments. After three experimentsthat produced the same outcome, there can be littledoubt that the gap effect can be observed in newborns.The further increase in overall RT that was observed inExperiment 3 was probably due to the fact that thetarget stimuli were more eccentric than in Experiment 2(about 30� vs about 20�).

Experiment 4

The peripheral flashing lights we used in Experiments 2and 3 were well suited to producing an activation of the

superior colliculus. Whether the superior colliculus issensitive to stationary patterned stimuli is more con-troversial. Whereas most authors (e.g. Schiller, 1985;Stein & Meredith, 1994) believe it is not, there are atleast two studies (Rizzolatti, Buchtel, Camarda &Scandolara, 1980; Arendes, 1994) that showed thatneurones in the superior colliculus selectively respond tospecific patterns, like faces or other objects.Morton and Johnson (1991; also see Johnson &

Morton, 1991) suggested that the superior colliculuscontains a mechanism (Conspec, in their terminology)that allows the newborn to direct the eyes to any facelikepattern that is present in the visual field.4 We (Simion etal., 1998b) showed that newborns preferentially orient tofacelike patterns in the temporal hemifield but not in thenasal hemifield. If the temporal±nasal asymmetryindexes processing by the superior colliculus (Rafal,Henik & Smith, 1991, Maurer & Lewis, 1998; Simion etal., 1998a, 1998b), the presence of the asymmetry isevidence that the superior colliculus plays an importantrole in processing facelike patterns. Therefore, also theperipheral stationary schematic faces we used inExperiment 1 might have been well suited for activatingthe superior colliculus.In Experiment 4 we asked whether the gap effect was

present when the peripheral targets were stationarypatterns that presumably did not cause a specificactivation of the superior colliculus. The stimuli wechose were upside-down schematic faces (see Figure 2).They possess psychophysical characteristics that arevery similar to those of the upright schematic faces ofExperiment 1, but do not produce the temporal±nasalasymmetry that is thought to index mediation by thesuperior colliculus (Simion et al., 1998b).Newborns are known to orient to upside-down

schematic faces like those we used in the presentexperiment (e.g. Johnson & Morton, 1991; Morton &Johnson, 1991; Valenza et al., 1996; Simion et al., 1998b).In the absence of a specific collicular mechanism forprocessing them, orienting to upside-down faces mustoccur through cortical mediation. This indicates that anynecessary cortical structures, although still immature, areto some extent already functional at birth. These corticalstructures are the primary visual cortex, the parietalcortex and, but much less likely, the frontal eye fields.Note that there is already evidence that the immatureprimary visual cortex can influence visual behaviour at

4 It must be stressed that Conspec, as conceived by Morton and

Johnson (1991), does not mediate face discrimination. It only

discriminates between faces and non-faces. Thus, in the absence of

cortical mediation, recognizing faces is impossible.



birth (Atkinson, Hood, Wattam-Bell, Anker & Trickle-bank, 1988; Slater, Morison & Somers, 1988).If the gap effect we found in Experiments 1±3 is

strictly collicular, i.e. it is due to the opposite influencesof reflexive orienting and fixation maintaining mechan-isms within the superior colliculus (see, for example,Munoz & Wurtz, 1995), it may be predicted that the gapeffect should disappear if the processing of the targetstimuli does not involve the superior colliculus. This isbecause, as was suggested by Csibra et al. (in press),cortical mechanisms which are called into play bystimuli that are not processed by the superior colliculusmight compensate for the collicular gap effect. Note,however, that Csibra et al. argued that the gap effect isreduced, not eliminated, by cortical activity.

Method

Subjects

Twenty-six normal, healthy, full-term newborns wereselected from the maternity ward of the Paediatric Clinicof the University of Padua. Ten babies were excluded

from the final sample because five changed state justbefore or during the experiment and did not revert to anormal state within a reasonable amount of time, andfive did not reach the criterion of at least two valid RTsfor each of the four types of trials.The 16 babies that completed the study met the

screening criteria of normal delivery, a birth weightbetween 2600 and 4000 g and an Apgar score of at least8 at 5 min. All were healthy and free of any knownneurological or ocular abnormality. They were testedafter the first 24 h of life, the range of ages at the time oftest being about 24±120 h postnatal (mean age about72 h). The testing took place during the hour precedingthe scheduled feeding time, if the baby was awake and inan alert state. Informed consent was obtained from theparents.


The apparatus, stimuli and procedure were identical tothose already described for Experiment 1, apart from thefact that the target schematic faces were presentedupside-down, i.e. they were rotated by 180� (seeFigure 2; also see Johnson & Morton, 1991; Morton &Johnson, 1991; Valenza et al., 1996; Simion et al.,1998b).

Data collection

The procedure for measuring RT was as described forthe previous experiments.The total number of trials should have been 354, but

many trials (174) were not valid because the eyemovement was away from the target stimulus (eighttrials in the gap condition and nine in the overlapcondition), the baby changed state after trial initiation(two in the gap condition), the baby closed the eyesbefore presentation of the peripheral target (20 for theoverlap condition and 12 for the gap condition), atechnical failure occurred (two for the overlap condi-tion), the experimenter started the trial when the babywas moving (72 in the gap condition and 43 in theoverlap condition), no saccade occurred within 10 sfrom target onset (two, 1.0%, in the gap condition), orthere was no agreement between the two coders (three inthe gap condition and one in the overlap condition).Trials in which the saccade occurred before target onsetor with a latency of 99 ms or shorter were excluded asanticipatory responses. All trials in which a saccade wasperformed within 10 s from target onset were retained.Of the 182 valid trials for which RT was measured, 92were for the gap condition and 90 for the overlapcondition. The average number of trials for each subject

(b)

(a)

Figure 2 (a) The gap condition and (b) the overlap conditionfor Experiment 4. In the gap condition the central fixationstimulus (the flashing light) was turned off 500 ms beforepresentation of the peripheral target stimulus (the upside-downschematic face). In the overlap condition the fixation stimulusremained visible while the target stimulus was presented.



was 5.6 in the overlap condition and 5.8 in the gapcondition, with a minimum of two trials.


Once again, a preliminary analysis of variance showedno indication that order of condition (gap condition firstor overlap condition first) and side of stimuluspresentation (left or right visual field) had any effectson RT. Therefore these factors were not included in themain analysis. Average saccadic RT was calculated foreach subject as a function of condition. It was foundthat mean RT was 1038 ms (SD� 284) on overlap trialsand 1011 ms (SD� 388) on gap trials.Because every baby produced at least two scorable

RTs for each of the four types of trial, a t- test betweencondition (gap or overlap) was conducted. It revealedthat condition had no effect on RT, t(15)� 0.30. The(non-significant) advantage of the gap condition overthe overlap condition was just 27 ms.As was confirmed by a comparison between Experi-

ment 1 and Experiment 4 (see below), the gap effect thathad been found with upright schematic faces becamemuch smaller, or disappeared altogether, when the sameschematic faces were presented upside-down. If oneconsiders that, very probably, the superior colliculuscontains a mechanism that processes facelike patternsbut not similar non-facelike patterns (Johnson &Morton, 1991; Morton & Johnson, 1991; Simion et al.,1998b), the clear implication is that the gap effect innewborns strictly depends on whether the targetstimulus is processed by the superior colliculus.

Analyses between experiments

The gap effect was present in the first three experiments,but its magnitude was different. It increased from434 ms in Experiment 1, to 783 ms in Experiment 2, to898 ms in Experiment 3. To explore whether thesedifferences in magnitude were reliable, an omnibusanalysis of variance was performed. In it, the between-subjects factor was experiment (Experiment 1, 2 or 3),whereas the within-subjects factor was condition (gap oroverlap).As expected, the condition main effect was significant,

F(2, 40)� 41.22, p< 0.001. Also the experiment maineffect was significant, F(2, 40)� 20.05, p< 0.001,whereas the interaction was not. The same analysiswas conducted on log transformed RTs and yieldedidentical results.These results indicated that overall RT was faster in

Experiment 1 than in Experiments 2 and 3, whereas the

gap effect was not reliably greater in Experiments 2 and3 than in Experiment 1.The gap effect was present in Experiment 1 and absent

in Experiment 4. To make sure that the difference wasnot due to Type I or Type II errors, an analysis ofvariance was performed to compare the two experi-ments. In it, the between-subjects factor was experiment(Experiment 1 or 4), whereas the within-subjects factorwas condition (overlap or gap condition). The conditionmain effect, F(1, 31)� 10.11, p< 0.001, and the interac-tion, F(1, 31)� 7.82, p< 0.01, were both significant.Thus, it is clear that the gap effect was greater inExperiment 1 than in Experiment 4. Probably, it waspresent only in Experiment 1.

General discussion

In the following discussion we will not distinguishbetween the attentional hypothesis (e.g. Fischer, 1986;Braun & Breitmeyer, 1990) and the visuo-motor, oroculomotor, hypothesis (e.g. Kingstone & Klein, 1993;Walker et al., 1995). Both hypotheses share the criticalfeature of assuming the existence of a neural mechanismthat exhibits a change in its activity level during the gapperiod, reflecting the disengaging of visual attention orthe disinhibition of a neural structure that governssaccadic generation. Thus, the experiments reported inthe present study did not allow us to investigate whetherthe gap effect reflects disengaging of attention oroculomotor competition or both. It must be said,however, that an explanation that maintains that thedelay in the overlap condition is due to the time neededto disengage attention from fixation before initiating thesaccade to the peripheral target is not convincing in thecase of newborns (or very young infants either). Ofcourse, newborns were not instructed to execute aspeeded saccade. Apparently, they shifted gaze to thetarget in an entirely automatic fashion, without anyobvious need to disengage attention. In addition, thefinding that the gap effect is absent when the target is anupside-down face is inconsistent with the view thatattributes the effect to attentional disengagement. Infact, it would seem that, if an attentional disengagementis needed, it should occur regardless of the nature of thetarget stimulus. Finally, it is worth noting that theabsence of the gap effect with upside-down faces clearlyrules out the possibility of a general warning orpreparation effect, which in any case would seem to beextremely unlikely in the case of newborns.The present study was the first demonstration to date

that the gap effect is present at birth. In our experiments,the magnitude of the effect ranged between 434 and



898 ms. If compared with the results of previous studies(Hood & Atkinson, 1990, 1993; Johnson et al., 1991;Atkinson et al., 1992; Hood, 1995; Matsuzawa &Shimojo, 1997; Csibra et al., in press; but see Goldberget al., 1997, for a failure to find a difference between theoverlap and the gap conditions), it appears that innewborns the gap effect is smaller than in 1- or 2-month-olds. With regard to 6-month-olds, the results arecontradictory. Hood and Atkinson (1990, 1993) founda gap effect greater than the one we found in newborns.In contrast, other studies found a gap effect smaller(Matsuzawa & Shimojo, 1997) or even much smaller(Csibra et al., in press).The model we have outlined in the introduction (e.g.

Johnson, 1990, 1995; Maurer & Lewis, 1998) attributesthe onset of the gap effect (and of sticky or obligatoryfixation) at about 1 month of age to the development ofan inhibitory influence from the maturing primaryvisual cortex on the superior colliculus, which is theneural structure that plays the major role in saccadeproduction from birth through adulthood (e.g. Munoz& Wurtz, 1995). The fact that the gap effect decreases atabout 4 months is assumed to reflect the disinhibitoryinfluence on the superior colliculus from the parietaland, especially, frontal cortices (the frontal eye fields, inparticular), which reach at that age a more advancedmaturation stage. In other words, the collicular saccadeproduction mechanisms are thought to be regulated bycortical input, which first is inhibitory (i.e. primaryvisual cortex) and then disinhibitory (i.e. the frontal eyefields).On this model, the gap effect should be smaller at

birth than it is at 1±2 months, as the superior colliculusis relatively mature compared with the primary visualcortex. The results of the present experiments supportthis prediction by showing that the gap effect we foundin newborns is smaller than that found in previousstudies of 1- or 2-month-olds. The model assumes that,before the frontal eye fields can influence the superiorcolliculus, the magnitude of the gap effect is inverselyrelated to the maturation of the primary visual cortex.The primary visual cortex is less mature at birth than at1 month. Therefore the gap effect is smaller at birth thanat 1 month.Alternatively, one might think that, whereas at 1

month the gap effect reflects inhibition of the superiorcolliculus by the primary visual cortex, at birth the gap

effect depends on purely collicular mechanisms, as hasbeen proposed by some to account for the effect in theadult too (Kingstone & Klein, 1993; Walker et al., 1995).This explanation is in accord with the fact that, when thesuperior colliculus is not involved in processing thetarget stimulus (see our Experiment 4), the gap effectdisappears or becomes much smaller.5

There are apparent parallels between the gap effectand a phenomenon known as inhibition of return (IOR;however, see Reuter-Lorenz, Jha & Rosenquist, 1996,for a contrary view). In particular, Abrams and Dobkin(1994) reported that IOR and fixation point offset haveinteractive effects on saccadic RT, suggesting a commonlocus for these two effects.IOR was first identified by Posner and Cohen (1984)

and refers to the fact that, in spatial precue paradigms, aperformance decrement (i.e. inhibition) can emerge atthe precued location after an initial facilitation (e.g.Tassinari, Aglioti, Chelazzi, Marzi & Berlucchi, 1987;Rafal, Calabresi, Brennan & Sciolto, 1989). It isgenerally assumed that IOR is an attentional phenom-enon that prevents spatial attention from continuing toexplore already explored locations (e.g. Tassinari &Berlucchi, 1993; Reuter-Lorenz et al., 1996). However,we (Valenza, Simion & UmiltaÁ , 1994; Simion, Valenza,UmiltaÁ & Dalla Barba, 1995) have demonstrated thatIOR occurs in newborns and depends on oculomotormechanisms. Note that the act of programming an eyemovement is probably sufficient to evoke IOR even inthe adult (Rafal et al., 1989; Klein & Taylor, 1994).More interesting for the present discussion, at birth

IOR is present when the precue falls in the temporalhemifield and absent when it falls in the nasal hemifield(Simion et al., 1995). If it is accepted that the temporal±nasal asymmetry is a marker of collicular mediation (e.g.Rafal, Henik & Smith, 1991; Simion et al., 1998b; inparticular see Maurer & Lewis, 1998, for a thoroughdiscussion of the issue), then it can be concluded thatIOR depends on the activity of the superior colliculus.Here we have presented evidence that is not inconsistentwith the notion that the gap effect may also depend onthe superior colliculus. This is because the onlyexperiment in which the gap effect was not found (e.g.Experiment 4) was also the only experiment in which thetarget stimulus was not such as to produce an activationof the superior colliculus. Of course the fact that the gapeffect is absent with stimuli that probably do notactivate the superior colliculus is far from beingunequivocal evidence for the exclusive, or even pre-dominant, role of this neural structure in the gap effect.In view of this serious weakness, the conclusion that thegap effect depends on the superior colliculus must betaken as very tentative.

5One might wonder why an inverted face did not activate the superior

colliculus, as a flickering spot of light did. We suggest that the superior

colliculus is not very sensitive to a static spot of light, unless it is an

upright face, for which there is a specific collicular mechanism (i.e.

Conspec).



It would be very important, therefore, to investigatewhether the gap effect is temporal±nasal asymmetric atbirth. Very recently, Goldberg et al. (1997) did exactlythat by presenting, in a gap paradigm, the targetstimulus in the temporal or nasal hemifield. They testednewborns, 1-month-olds and 4-month-olds. However,none of their age groups demonstrated the gap effect.Moreover, they did not measure saccade latency innewborns. Therefore the issue of whether the gap effectis temporal±nasal asymmetric in newborns needsfurther investigation. The prediction would be that, ifthe gap effect is due to the inhibitory influence of theprimary visual cortex, then it should not be temporal±nasal asymmetric. Conversely, if the gap effect is purelycollicular, then it should be temporal±nasal asymmetric.

Acknowledgements

The authors thank the nursing staff at the PaediatricClinic of the University of Padua for their collaboration,Sandro Bettella for writing the software, and AnnaOrsini and Maria Grazia Pasinato for help in the testingof babies. The authors thank also Mark Johnson and ananonymous reviewer for extremely helpful comments onan earlier version of the paper. This research wassupported by grants from the Consiglio Nazionale delleRicerche, the Ministero dell'UniversitaÁ e della RicercaScientifica e Tecnologica, and CEE-Biomed, N.BMH4-97-2032.

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Received: 11 June 1998

Accepted: 15 December 1998



The gap effect in newborns

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