Acta Psychologica 50 (1982) 73-82 North-Holland Publishing Company

73

FOREPERIOD AND VISUAL STIMULUS INTENSITY: A REAPPRAISAL *

Pekka NIEMI and Erkki LEHTONEN University of Turku, Finland

Accepted April 1981

The joint effects of stimulus modality, stimulus intensity, and foreperiod on simple RT were investigated. In experiment 1 an interaction was found between stimulus intensity, both visual and auditory, and a variable FP such that the intensity-effect on RT was largest at the shortest FP. Experiment 2 provided a successful replication with smaller and weaker visual stimuli. No interaction was observed with a constant FP, although the visual stimuli were identical and the auditory ones psychophysically equivalent to the visual stimuli of experiment 1.

It is proposed that an additive or interactive relationship between stimulus intensity and FP can be inferred only when the mental processes called for by the various uses of FP are simulta- neously considered. Another precondition is an adequate sampling of the intensity-continuum with special reference to the retinal size of visual stimuli.

Introduction

The purpose of this paper is to evaluate critically recent findings con- cerning the effects of stimulus intensity and foreperiod (FP) on the hu- man simple reaction process. A common observation in a constant-FP situation is that the longer the FP, the longer the RT (Klemmer 1956; Teichner 1954; Woodrow 1914). However, Sanders and Wertheim ( 1973) observed that a response to an auditory stimulus is less suscep- tible than one to a visual stimulus for the effects of FP prolongation. In line with this finding, Sanders (1975, 1977) and Niemi (1979) reported an interaction between auditory stimulus intensity and FP. In these studies, soft auditory stimuli yielded similar FP-effects to the

* This study was supported in part by the Alfred Kordelin Foundation, Finland. The authors are indebted to A.F. Sanders for useful comments.

Requests for reprints should be addressed to P. Niemi, Department of Psychology, Univer- sity of Turku, Kasarmink. 6, rak. 11, 20500 Turku 50, Finland.

OOOl-6918/82/0000-0000/$02.75 0 1982 North-Holland

14 P. Niemi, E. Lehtonen / Forepetiod and visual stimulus intensity

visual ones, whereas loud stimuli showed a smaller effect: Both studies revealed additivity between visual stimulus intensity and FP. These findings are important for the stage-analysis of ,the reaction process as they suggest a reconciliation among contradictory empirical results. Changes of luminance appear to influence only the d’, whereas auditory intensity has more extensive (e.g. criterion-based) effects (see Niemi 1979; Sanders 1977).

Several authors have offered an explanation of modality-effects of this type. Strong auditory reaction stimuli (RS) are said to exert an “immediate arousing effect” (Bertelson and Tisseyre 1969; Sanders 1975, 1977; Sanders and Wertheim 1973). An equivalent formulation is that they are “automatically alerting” (Posner et al. 1976). Niemi and Naatanen ( 198 1) suggest that loud stimulation produces a startle reflex which makes it possible for the subject to launch a rapid response even in an unprepared state. This reflex may actually be a significant part, in proportion to the stimulus intensity, of a simple-RT response.

Although the dissimilarity of FP-effects on RT appears well-estab- lished between audition and vision, for two reasons the issue cannot be regarded as decided. These deal with the adequate control of visual

stimulus intensity from the arousal point of view and the optimal use of the FP-variable in the experimental design.

Interaction between FP and auditory intensity has been convincingly demonstrated (for a review, see Niemi 1979; Sanders 1977). However, demonstrating the additivity between FP and visual intensity is prob- lematic. Thus the psychophysical intensity-continuum may not be ade- quately covered by the luminance values chosen in the various studies. As a matter of fact, luminances of less than 0.5 cd/m2 are rarely used. Yet, according to Stevens’ (1955) scale, this corresponds to about 62 dB re lo-” lambert, which is psychophysically equivalent to 62 dB re 0.0002 dyne/m2 of auditory intensity. This loudness value is consider- ably above the weak tones commonly used (35-55 dB). On the other hand, large luminances (280-650 cd/m2) have been employed by San- ders (1975, 1977) that are equivalent to about 89993 dB of loudness. This author suggested that an interaction between luminance and FP might be found with very intense stimuli exceeding, say, 650 cd/m2. However, the cross-modal comparison of strong intensities meets a further problem. Unlike audition, the visual channel is not totally occupied by RS. The size of a visual RS affects at least the subjective side of the reaction process. Niemi and Valitalo (1980) observed that

P. Niemi, E. Lehtonen / Foreperiod and visual stimulus intensity 75

the subjective response speed was increased by a large and bright RS relative to a dim one of the same size. No such effect was found with small RSs of the same luminances. Moreover, Kohfeld ( 197 l), applying Stevens’ (1955) scale, used a large 38 X 38 cm light source. He found equal RTs for the same dB-levels of visual and auditory intensity, pro- vided that the luminance level exceeded the photopic threshold. This finding differs from those reporting a faster RT to auditory stimuli than to visual ones (Elliott 1968; Todd 1912). Consequently, a possibility which deserves testing is that intense and large enough visual stimuli may have an immediate arousal property similar to that possessed by loud auditory ones.

In studies dealing with stimulus intensity and FP, a constant-FP design is commonly used, i.e., the duration of FP is not changed within a block of trials. In this type of situation, the subject’s time uncertainty is due to his imperfect timekeeping ability (Klemmer 1956). It is worth noting that the cross-modal asymmetry described above was demon- strated with constant FPs (Niemi 1979; Sanders 1975; Sanders and Wertheim 1973). An alternative design is that of variable FPs in which the frequency of RS-occurrence is predetermined at each possible FP- moment on a trial. Uncertainty as to whether an RS will be presented after a given FP is usually greatest at the shortest FPs. Uncertainty decreases gradually as the time from the WS elapses and the probability of RS-occurrence reaches unity at the longest FP, provided that no catch trials are used. Hence the uncertainty experienced by the subject comes from two sources, i.e., his imperfect time-estimation ability and the changing conditional probability of RS-occurrence. The more un- certainty there is concerning the delivery of RS, the less prepared the subject is for a response. It is not surprising that variable FPs tend to yield longer average RTs than constant FPs of equal duration (Bevan et

al. 1965; Trumbo and Gaillard 1975). Thus the effects of variable-FP should be particularly sensitive to arousal which promptly increases the subject’s responsiveness.

Using auditory stimuli and a variable FP, Kellas et al. (1969) ob- served a strong Intensity X FP interaction such that the intensity-effect on RT was largest at the shortest FP. There are two similar visual studies. Niemi (1979) failed to find an interaction. This was to be expected in view of the above considerations as he used very small stimuli (about 1” of arc). A similar outcome reported by Raab et al.

(1961) is more interesting as they employed very bright stimuli of

76 P. Niemi, E. Lehtonen / Foreperiod and visual stimulus intensity

3000, 30, and 0.30 foot-lambert, equalling approximately 105, 85, 65 dB of auditory intensity (Stevens 1955). However, their stimuli were also small (1” 10’ of arc).

In summary, it appears that the relationship between FP and visual stimulus intensity has not yet been comprehensively investigated and results suggesting additivity are not decisive. An optimal situation for detecting an interaction between these factors obviously entails large and strong lights to which the subject responds after a variable FP. Moreover, strong and weak RSs should occur randomly within a same session (Grice and Hunter 1964).

Experiment 1

Condition 1 attempts to replicate the Auditory Intensity X FP interaction of Kellas’ et al. ( 1969) study, while condition 2 is its visual equivalent.

Method

Subjects

Eight Ss, 5 males and 3 females, participated in condition 1, and 10 Ss, 6 males and 4 females, in condition 2. All were undergraduate students from the University of Turku, had normal hearing and normal or corrected vision. Most Ss had no previous experience in RT-experiments.

Apparatus and stimuli

The S was sitting in a dimly lit room wearing headphones (Best SH-1300) and pres- sing the response key with the index finger of the preferred hand. In the auditory condition 1, the WS was a red light emitting diode with the luminance of 4.5 cd/m’. The RSs were 47 and 82 dB(A), 1000 Hz tones. In the visual condition 2, the WS was a 70 dB(A), 1000 Hz tone. The size of the visual RSs was 21 X 23 cm sub- tending a visual angle of 29’ horizontally and 32’ vertically. They were produced by replacing the rear panel of one channel of a Gerbrands tachistocope with a sheet of glossy paper. The light sources were two Sylvania F4TS/CWX lamps with a rise time of 20 psec. The base level luminance of the lamps was not visible through the paper. The intensities of RSs, as measured from the S’s eye position, were 0.5 and 130 cd/m’. The durations of WS (0.5 set), variable FP (2, 4, or 6 set), RS (0.1 set), and IT1 (4 set) were equal in both experiments. The viewing distance was 40 cm. The control apparatus was located in a near-by room comprising a Gerbrands 300 Millisecond Timer, a Clock/Timer, a Lamp Drive Circuit, and a Logic Interface.

Procedure

The S was instructed to press the response key as soon as possible after detecting the RS. The FPs were presented 100 times each in an irregular order. There were

P. Niemi, E. Lehtonen / Foreperiod and visual stimulus intensity II

10 additional trials for warm-up. The total duration of one session was 45 min. A 5 min break was given in the middle.

Results

The mean RT of S in each subcondition was used as a cell in the ANOVA that was computed over Ss and subconditions with RS-modality as a between-Ss variable. The visual and auditory groups did not differ significantly from each other. The effects of intensity (F= 126.24; df= 1, 16; p < 0.001) and FP (F= 71.46; df= 2, 32; p < O.Oolj were clear as expected. The interaction Intensity X FP was also significant (F= 14.81; df = 2, 32; p < 0.001). The results of both conditions are illustrated in fig. 1 the inspection of which appears to suggest other interactions as well. However, none of these reached statistical significance.

Experiment 2

The purpose of experiment 2 was to establish more firmly the observed interaction between visual stimulus intensity and FP. In order to make the replication conser- vative, smaller and weaker stimuli were used than those of the visual condition of experiment 1.

cm% 270- 47d0 AUDITORY VISUAL

260 - 25,; 0.5 cd/m2

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230 -

220 -

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260

250

230

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L 1 I I 1 I I I 2 L 6 2 lo 6

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Fig. 1. Mean RTs as a function of stimulus intensity, modality, and variable FP in experiment 1.

78 P. Niemi, E. Lehtonen / Foreperiod and visual stimulus intensity

Method

Subjects

Ten additional students, one female and nine males, served as Ss. They had normal or corrected vision, None had previous experience in RT-experiments.

Apparatus and procedure

The strong RS (34 cd/m2) was a group of six red light emitting diodes projected through a diffusing lens of a 5 cm diameter with the visual angle of 7’ of arc. The weak RS (0.5 cd/m2) was one diode receiving a reduced voltage. The other details were the same as in the visual condition of experiment 1.

Results

Table 1 summarizes the results which replicated those obtained in the visual condi- tion of experiment 1. The effects of intensity (F= 87.74;df= 1, 9;p < O.OOl), FP (F= 17.79; df= 2, 18;~ < 0.001) were clear. The interaction Intensity X FP, albeit small in absolute terms, was also significant (F = 7.57, df= 2, 18;~ < 0.01).

Discussion of experiments 1. and 2

The results show that an interaction between visual stimulus intensity and FP can be obtained. In this respect they differ from the findings of Niemi (1979) and San- ders (1975, 1977). Within the visual modality, this type of outcome is new, except that it was mentioned in passing by Sanders (1977: 17). He observed more pro- nounced effects of visual intensity at 1-set variable FP than at longer ones. How- ever, he did not analyze the finding further. In contradiction to the present results, Niemi (1979: exp. 1) and Raab et al. (1961) reported additivity between visual intensity and variable FP. Niemi’s finding evidently was due to his rather small and weak stimuli, yielding average RTs above 240 msec for practiced Ss. Raab et al.

(1961) had small (1” 10’) but strong stimuli. Although the size is sufficient for the spatial summation, some minimum area of retinal stimulation may have to be exceeded before arousal effects occur. Another possibility is that the high degree of preparation left no room for facilitation by arousal in their experiment. This

Table 1 Mean RTs across subjects in experiment 2 as a function of stimulus intensity and FP.

Intensity (cd/m2)

FP (set)

2 4 6

34 232 219 215 0.5 272 247 248

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80 P. Niemi, E. Lehtonen / Foreperibd and visual stimulus intensity

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between intensity and FP (F= 3.39, df= 1, 9; p < 0.10). The reason for the weak- ness of the trend is not that the stimulation was too weak to produce immediate arousal. The 86 dB(A) intensity corresponds well to the ones used in the previous studies. This is confirmed by the small 15 msec FP-effect for the strong RS. The absence of interaction is best accounted for by the small FP-effect for the weaker RS (21 msec). Since the purpose of the present experiment was to provide a psy- chophysically controlled comparison to the interactive visual results of experiments 1 and 2, a less intense lower loudness level, say, 47 dB(A) was not used.

General discussion

It is misleading to infer the existence of a fundamentally additive or interactive relationship between stimulus intensity and FP unless atten- tion is paid to the psychological processes brought about by the various uses of these variables. A variable FP produces more time uncertainty than a constant one and creates a situation more sensitive to arousal effects. This happens through continuous changes in the conditional probability of RS occurrence to which the subject adjusts.

Concerning FP-effects on RT, there is no sharp functional asymm-

P. Niemi, E. Lehtonen / Foreperiod and visual stimulus intensity 81

etry between audition and vision, provided that the stimuli are matched psychophysically. In vision, there appears to be a trade-off between the area and the intensity of the stimulus such that strong but small stimuli do not evoke an alerting effect to counteract the detrimental effects of decreased responsiveness (e.g. Raab et al. 196 1) whereas such an alerting is obtained with somewhat dimmer but larger stimuli. This evidently is the source of the discrepant results between the two modalities in simple-RT experiments.

A problem in a psychological auditory-to-visual matching is that, with constant hue and pitch, the visual stimulus still has two relevant physical dimensions, area and intensity, in contrast to intensity alone in audition. A possible measure to meet this difficulty is the troland which is a product of the luminance and retinal projection of the stimulus. However, its straightforward application does not deal adequately with the trade-off described above. For instance, the strongest stimulus of Raab et al.‘s (1961) study was 6-7 times stronger in terms.of trolands than the one of the present experiment 2, and yet it failed to produce an interaction between FP and intensity. Hence the present results sug- gest that the arousing effects of visual stimulation in simple-RT experi- ments are due to two physical dimensions, area and intensity, which both appear to have an arousal threshold.

If fundamental functional dissimilarities in terms of stimulus-inten- sity effects are sought for between auditory and visual reaction pro- cesses, the possible differences are subtler than has been proposed previ- ously. This suggests the need for a psychophysical re-evaluation of the cross-modal asymmetries described in the introduction. However, the established asymmetry, although theoretically in question, may be ecqlogically valid. Unexpected noises are not too infrequent in the living and working environments, whereas visual stimuli rarely impress a person with their sudden occurrence, large size, and high luminance at the same time.

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