Temporal brightness enhancement: Studies of individual differences

We have previously identified categorical individual differences in the occurrence of temporal brightness enhancement (TBE) by using a simultaneous brightness discrimination paradigm (Bowen & Markell, 1980).TBE is a nonmonotonic relation between brightness and pulse duration, pulses of intermediate duration (75–125 msec) can appear brighter than longer or shorter pulses of the same luminance. Three classes of observers can be defined based on whether they perceive TBE under one of two conditions of temporal asynchrony between a short test pulse and a longer (500 msec) comparison pulse:simultaneous onset of the pulses orsimultaneous offset. Type A observers show TBE for both asynchrony conditions; Type B observers show the effect for simultaneous offset but not simultaneous onset; Type C observers do not show TBE for either asynchrony. In the present study, we show that Type A and Type C observers maintain a constant brightness-duration relation as the asynchrony between test and comparison pulses is varied from simultaneous onset to simultaneous offset. Type B observers show a gradual shift in the brightness-duration relation as asynchrony changes. In a separate experiment, we find that practice has little effect on Type A and Type B observers but that Type C observers may change in classification to Types A and B over as few as five experimental sessions. The hypothesis that individual differences are due to differential “weighting” of chromatic (sustained) and achromatic (transient) visual channels is discussed.     

Temporal brightness enhancement studied with a large sample of observers: Evidence for individual differences in brightness perception

Brightness vs. duration relations were measured for 80 naive observers using a method in which a short pulse (10–500 msec) was compared to a pulse 500 msec longer in duration at the same luminance. Pulses were presented under two conditions of pulse asynchrony: simultaneous onset and simultaneous offset. Three classes of observers were found: Type A observers (57% of all observers) showed temporal brightness enhancement (the Broca-Sulzer effect) for both simultaneous onset and offset conditions. Type B observers (33% of the sample) showed brightness enhancement for simultaneous offset but not onset. Type C observers (10% of the sample) did not exhibit brightness enchancement under either asynchrony condition. The distribution of these classes was not affected by the specificity of instructions concerning the brightness judgment or by the sequencing (ordered vs. random) of stimuli. We analyze how these classes of observers might be explained either on the basis of differing neurophysiological mechanisms or responses or on the basis of differing psychophysical criteria. Further, we present a model of the data which illustrates how behaviorally distinct types of observers could be generated by continuous distributions within the general population of either perceptual criteria or neurophysiological responses.     

Individual differences in pulse brightness perception

When brightness-pulse duration relations are studied with a simultaneous brightness discrimination procedure, three classes of observers emerge (Bowen & Markell, 1980). These classes are defined by whether or not observers perceive temporal brightness enhancement (the Broca-Sulzer effect) under two asynchrony conditions for pulses to be compared: simultaneous onset and simultaneous offset. Type A observers perceive brightness enhancement for both asynchrony conditions; Type B observers perceive brightness enhancement for simultaneous offset of pulses but not for simultaneous onset; Type C observers do not generate the Broca-Sulzer effect under either asynchrony condition. Here we present supplementary measures on observers of all three types: (1) magnitude estimation of the brightness of single pulses of light of varying duration, (2) modulation sensitivity for sin~wave flicker, and (3) contrast sensitivity for moving sine-wave gratings. The magnitude estimation data differentiated the three types of observers, but flicker and motion sensitivity did not. The three classes of observers probably differ in the perceptual criteria they employ in judging the brightness of isolated pulses of light; they probably do not differ in their underlying neurophysiological responses.     

A comparison of two psychophysical methods using animals

A discrimination box containing two levers with a light above each was used to train eight rats to press beneath the brighter light for a milk reinforcer. The brighter light was held constant and the comparison light was varied to produce 12 brightness differences. The animals were run under two experimental methods: the block method in which each brightness level comparison was presented for a block of 11 contiguous trials, and the staircase method in which the sequence of brightness comparisons was determined by the correctness of the response on the preceding comparison. The block method produced a smaller differential brightness threshold and a larger change in discrimination performance for stimulus magnitude changes than did the staircase method.     

Frequency recognition in temporal interference tasks: A comparison among four psychophysical procedures

The frequency discrimination of a 20-msec test tone was measured for the same subjects in four psychophysical paradigms: a single-interval procedure, a two-alternative forced-choice procedure, a same-different procedure, and a method-of-adjustment procedure. In each paradigm, the 20-msec test tone was preceded or followed by a 500-msec, 800-Hz interfering tone. The interfering tone occurred 50 or 5 msec before the onset of the test tone (forward interference) or 100 or 5 msec after the offset of the test tone (backward interference). In each of the four paradigms and for each of the interference conditions the value of Δf for the test tone was varied symetrically around 800 Hz to obtain an estimate of the frequency discrimination thresholds. In all psychophysical paradigms except for the single-interval procedure, there were small or no significant differences in observers’ frequency discrimination behavior among the interference conditions. The thresholds for Δf from these conditions were approximately the same as those obtained without an interfering tone. In the single-interval task, when the test tone preceded the interfering tone (backward interference) by 5 msec, an increase in the value of Δf required for discrimination over that required in the other conditions was measured. These results suggest that the effect of backward interference on a target tone does depend on psychophysical procedure.     

Visual temporal integration and simple reaction time

Intensity-time reciprocity for simple RT to foveal pulses of light was demonstrated up to 11 msec by using two experimental paradigms. The first paradigm was designed to separate two possibly confounded factors displayed by previous studies investigating the effects of increased stimulus duration on RT: (1) an asymptotic RT as a function of the increasing energy of a pulse as its duration is increased, and (2) the breakdown of integration as the pulse duration is increased. The second paradigm was designed to avoid the first factor so as to maximize the possibility of finding partial integration at long durations. In this paradigm, partial integration was demonstrated for additional light input presented as long as 64 msec after stimulus onset. The failure of other studies to demonstrate temporal integration for RT is discussed in terms of these paradigms.     

Anatomy of a flash. 1. Two-peak masking and a temporal filling-in

A modified paradigm of Crawford masking was used to link masking to brightness fluctuation, as distinct from flash brightness. Thresholds were measured for a 10 ms incremental pulse (the 'probe') presented before, during, or after a 500 ms pulse (the 'flash'). Both pulses were spatially coextensive with the background field, thus the criterion for probe detection was purely temporal. The flash occurred either in the tested eye, the opposite eye, or in both eyes. In all conditions, masking was strongly bimodal: thresholds peaked near flash onset and flash offset. The flash was perceived as a unitary event. Bimodal masking is attributed to cortical on-and off-effects, as (i) dichoptic masking was strong and (ii) the same incremental probe was masked by either incremental or decremental flashes. Strikingly, monocular probe thresholds were about equally elevated by binocular as by monocular flashes, although the binocular flashes were brighter. Therefore, some monocular features can be preserved in the larger net binocular response. A general conclusion is that masking depends on the same transient neural responses that bring about a brightness fluctuation, whereas the appearance of the flash as a single event, a unitary change of brightness, depends on a different mechanism, perhaps a sustained response that performs a temporal filling-in.     

Discrimination of visually perceived intervals of time

The discrimination of short intervals of time, demarcated by a foveally presented spatially distinct double pulse of light, was studied under several conditions of pulse intensity, angular diameter, and duration. We defined temporal acuity as a measure of discrimination capacity in terms of d′ values. It is shown that the acuity mechanism uses largely integrated information—in the spatial and temporal domain, up to at least 56′ and 32 msec, respectively. Acuity increases slightly with increasing integrated pulse energy, but seems quite independent of the presence of an adapting field of appreciable brightness. Studies on the effect of the foveal site aimed at by the pulses of light have shown that temporal projections lead to significantly poorer acuity values than nasal projections. Monoptic and dichoptic stimulation, however, are fully equivalent.     

The effects of temporal waveform upon temporal darkness enhancement

Temporal darkness enhancement refers to the finding that decremental flashes of 50–140 msec appear darker than longer flash decrements. The present experiment determined the effects of temporal waveform upon darkness enhancement by obtaining darkness judgments for flashes that had abrupt onset/abrupt offset, abrupt onset/gradual offset, gradual onset/abrupt offset, and gradual onset/gradual offset. Temporal darkness enhancement was found only for flashes that had abrupt onsets regardless of offset waveform. These results are discussed in terms of the role of transients in the coding of perceived darkness.     

Brightness of paired photic pulses with pulse duration and separation varied

The brightness of pairs of photic pulses, differing in pulse duration and pulse separation. was measured. The data were in general agreement with the neurophysiological model of Bishop and Clare. Pulse duration appeared to have little systematic influence on the shape of the curves. Time between onset of the first and second pulse appeared to be a prime factor determining the receptivity of the cortex to the second pulse. The two pulses were seen as single below about .03 sec separation. The second pulse was always judged brighter than the first.     

Left hemisphere selectivity for processing duration in normal subjects

The role of the left cerebral hemisphere for the discrimination of duration was examined in a group of normal subjects. Two tasks were presented: the first required a reaction-time response to the offset of monaural pulse sequences varying in interpulse duration, and the second required the discrimination of small differences in durations, within a delayed-comparison paradigm. In each task a right-ear advantage was obtained when the durations were 50 msec or less. No ear advantage was obtained for the larger durations of 67 to 120 msec. Since the perceptual distinctiveness of phonemes may be provided by durations approximating 50 msec, the nature of the relationship between the left hemisphere's role in temporal processing and speech processing may be elaborated.     

Attentional mechanisms in visual signal detection: the effects of simultaneous and delayed noise and pattern masks

The attentional cuing effects in detection and some discrimination tasks depend on the use of backward masks and on the presence of external noise in the display. These effects have been attributed to an interruption masking mechanism, which terminates stimulus processing prematurely, and an external noise exclusion mechanism, which minimizes the perceptual effects of noise. To test whether the dependencies on masking and external noise are expressions of a single mechanism, observers detected grating patch stimuli, masked with noise masks or pattern masks, presented either simultaneously or after a delay of 60-90 msec. Contrary to an external noise exclusion account, but consistent with an interruption masking account, cuing effects were largest when the masks were delayed. However, weaker cuing effects were obtained with simultaneous masks, contrary to an interruption masking account. These results suggest that attentional effects in simple visual judgments are affected by mechanisms of both kinds.     

Some temporal factors in the successive comparison of auditory amplitudes.

Under certain conditions the discrimination of successively presented auditory amplitudes appears to be primarily a function of the time intervals between stimulus onsets rather than of either the duration of the stimulus or the silent interval between the tones (interstimulus interval). The results of this study are interpreted in terms of a model containing an acquisition process which continues for a set interval (about 600 msec) subsequent to the onset of the stimulus, even if stimulus offset occurs prior to this time. The onset of another stimulus during this period terminates acquisition. The model also includes a memory process which does not start until acquisition is complete.     

Continuous measurement of visible persistence

In the synchrony judgment paradigm, observers judge whether a click precedes or follows the onset of a light flash and, on other trials, whether or not a click precedes light termination. The interclick interval defines the duration of visible persistence. An elaboration of this method consists of two phases: In Phase 1, the luminance of a reference stimulus is psychophysically matched to the peak brightness of the test flash. Five luminance values between .1 and 1.0 of the reference stimulus are used subsequently. In Phase 2, a random one of the five reference stimuli, a test flash, and a click are presented; the observer judges whether the click occurred before or after the brightness of test flash reached the reference value (on onset trials) or decayed below it (on termination trials). This method was validated on 3 subjects with test stimuli whose luminance rises and decays slowly in time, and then was used to trace out the precise subjective rise and decay (temporal brightness response function) of brief flashes.     

Visual onset and offset latencies

An experiment was designed to test the hypothesis that the latency of response to the offset of light can sometimes be shorter than the latency of response to the onset. The subjects' task involved the temporal order discrimination of the offset of one light and the onset of another. The results indicated that offsets were perceived about 40 ms earlier than onsets. Reasons were suggested for the shorter onset latencies found by other investigators, and a model was proposed to account for changes in the relative latencies of onsets and offsets as a function of stimulus duration and intensity.     

Errorless learning of a conditional temporal discrimination

In the present study we extended errorless learning to a conditional temporal discrimination. Pigeons' responses to a left-red key after a 2-s sample and to a right-green key after a 10-s sample were reinforced. There were two groups: One learned the discrimination through trial and error and the other through an errorless learning procedure. Then, both groups were presented with three types of tests. First, they were exposed to intermediate durations between 2 s and 10 s, and given a choice between both keys (stimulus generalization test). Second, a delay from 1 s to 16 s was included between the offset of the sample and the onset of the choice keys (delay test). Finally, pigeons learned a new discrimination in which the stimuli were switched (reversal test). Results showed that pigeons from the Errorless group made significantly fewer errors than those in the Trial-and-Error group. Both groups performed similarly during the stimulus generalization test and the reversal test, but results of the delay test suggested that, on long stimulus trials, responding in the errorless training group was less disrupted by delays.     

Discrimination of stimulus aperiodicity as a function of temporal patterning: a discrepancy between psychophysical and sensory-neural assessments.

Tactile discrimination of stimulus aperiodicity was investigated in a two-alternative forced-choice task by having subjects report which of two successively presented trains of mechanical pulses, one with complete pulse periodicity and one in which a single pulse was temporally displaced from its regular placement, was temporally irregular or aperiodic. Trains contained three, four, or five pulses and were presented at interpulse intervals of 60, 75, 85, 100, 125, and 150 ms. Aperiodic trains had one pulse, other than the first or last pulse, temporally advanced or receded by 40% of the interstimulus interval. Results showed that: (i) although the percentage of correct reports of aperiodicity decreased as presentation rate increased (smaller interpulse intervals), the amount of decrease was a function of the number of pulses in a train; (ii) the discrimination of aperiodicity was dependent upon the specific location of the irregularity within the stimulus temporal pattern; and (iii) all patterns with complete temporal reversals of their successive interpulse intervals yielded nearly equivalently accurate reports of aperiodicity. These data suggest that central, attentional factors are involved in the discrimination of stimulus aperiodicity.     

Visual long-term memory for spatial frequency?

It has been suggested that a visual long-term memory based on a sensory representation of the stimulus accounts for discrimination performance when the reference and the test stimuli are separated in time. Decision processes involved in setting response criteria, however, may also contribute to discrimination performance. In the present study, it is shown that under proper control, spatial frequency discrimination thresholds from a group of observers, each performing on a single trial, are significantly higher for a 2-h than for a 5-sec retention interval, whereas thresholds from individual observers performing in repeated trials with a 2-h retention interval are considerably lower. The results suggest that discrimination performance may depend on the retention of task-relevant information, such as a response criterion, rather than on visual memory of the stimulus. It is concluded that it is risky to postulate ahigh-fidelity long-term visual memory for spatial frequency on the basis of psychophysical group discrimination thresholds.     

The induced asynchrony effect: Its role in visual judgments of temporal order and its relation to other dynamic perceptual phenomena

A context-induced “illusion” in visual judgments of temporal order, termed the induced asynchrony effect (IAE), is reported. It consists of an apparent ordering in time of two simultaneous light onsets, produced by the preceding, asynchronous offsets of two other lights. The joint effect of a real stimulus onset asynchrony and a preceding stimulus offset asynchrony bn judgments of onset order appears to be additive, given a Gaussian transformation of response probability. This result is shown to be consistent with a simple statistical decision model, which provides a conceptual framework for drawing inferences from temporal order judgment data. However, it is emphasized that certain interpretations of such models are not empirically testable on the basis of temporal order data alone. An attempt is made to relate the IAE to three other dynamic perceptual phenomena; all four effects may reflect a tendency of observers to perceive the velocity of apparent motion as being constant. Questions raised by the demonstration of the IAE are discussed, and directions for further research are suggested.     

Visual discrimination with a single manipulandum following temporal ablations in the monkey

The aim of this investigation was to determine what effect variation in the conditions of cue presentation and response availability has on the visual discrimination performance of monkeys with bilateral inferior temporal ablations. Six animals were trained on one of two versions of a test of successive brightness discrimination. Common to both versions of this test was the availability of a single lever, manipulation of which in the presence of the positive cue constituted the correct response. Performance on this test was compared with performance on a pattern discrimination, for which the customary simultaneous two-choice procedure was used. It was found that the temporal removals were followed by definite impairment on the pattern discrimination (as expected), but no consistent change in efficiency at brightness discrimination could be attributed to the lesions. Alternative interpretations, making reference to the amount of pre-operative training on the brightness discrimination or the lack of differentiation between the correct and alternative responses in this test, are discussed.