Decoupling stimulus duration from brightness in metacontrast masking: data and models

A brief target that is visible when displayed alone can be rendered invisible by a trailing stimulus (metacontrast masking). It has been difficult to determine the temporal dynamics of masking to date because increments in stimulus duration have been invariably confounded with apparent brightness (Bloch's law). In the research reported here, stimulus luminance was adjusted to maintain constant brightness across all durations. Increasing target duration yielded classical U-shaped masking functions, whereas increasing mask duration yielded monotonic decreasing functions. These results are compared with predictions from 6 theoretical models, with the lateral inhibition model providing the best overall fit. It is tentatively suggested that different underlying mechanisms may mediate the U-shaped and monotonic functions obtained with increasing durations of target and mask, respectively.     

Perceived brightness as a function of flash duration in the peripheral visual field

Using a method of direct magnitude estimation, perceived brightness was measured in the dark-adapted eye with brief flashes of varying duration (1–1,000 msec), size (16’–116’), and retinal loci (0°–60°) for the lower photopic luminance levels covering the range between 8.60 and 86 cd/m² in steps of .5 log units. Perceived brightness increased as a function of flash duration as well as luminance up to approximately 100 msec, then remained constant above 100 msec. The enhancement of brightness at about a 50-msec flash duration has been observed not in the fovea but in the periphery. Target size also has been found to be effective on brightness.     

Does rate of processing determine ease of target detection?

In a complex target identification task, Krumhansl (1982) reported that ease of target detection was inversely related to the exposure duration of contextual elements in the display. This relationship held at brief exposures of the contextual elements. However, in Krumhansl's design, brightness and duration covaried. Hence, the results cannot be ascribed unambiguously to the effect of duration alone. In the present two experiments, brightness and duration were varied independently. The outcome showed that brightness differences were both a necessary and a sufficient condition for producing the relationships observed by Krumhansl. In fact, when brightness was equalized across all durations the relationship was reversed. Although unable to account for the effects of duration, Krumhansl's formal model remains viable as a predictor of the effects of brightness.     

Duration discrimination of brief light flashes

The data from four experiments indicate that when Os discriminate between light flashes of different durations, for durations for which Bloch’s law has been shown to hold, their discriminations are frequently made on the temporal information available in the flashes rather than on their apparent brightness. A model for duration discrimination which specifies that discriminability depends only on the difference in duration between the two brief flashes, and is independent of their durations, is presented and applied to the data.     

The effect of small brightness differences on the after-effect of seen movement

Two experiments are reported. In the first experiment the after-effect of seen movement has been shown to occur with a measurable duration when there was a small difference in brightness between adjacent parts of the stimulus pattern. In the second experiment the duration of the after-effect was observed under two conditions of brightness difference; one small and the other relatively large. The difference between the durations of the aftereffect for the two conditions of brightness difference was significant. Results from these experiments have been discussed together with data from an earlier study in which the effects of brightness differences of greater magnitude were investigated. The probable function of the duration of the after-effect with respect to brightness differences over the full range of differences has been considered.     

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.     

Brightness and loudness as functions of stimulus duration

The brightness of white light and the loudness of white noise were measured by magnitude estimation for sets of stimuli that varied in intensity and duration. Brightness and loudness both grow as power functions of duration up to a critical duration, beyond which apparent magnitude is essentially independent of duration. For brightness, the critical duration decreases with increasing intensity, but for loudness the critical duration is nearly constant at about 150 msec. Loudness and brightness also grow as power functions of intensity. The loudness exponent is the same for all durations, but the brightness exponent is about half again as large for short durations as for long. The psychophysical power functions were used to generate equal-loudness and equal-brightness functions, which specify the combinations of intensity E and duration T that produce the same apparent magnitude. Below the critical duration ET equals k for equal brightness, and ET^a equa Is k for equal loudness. The value a is about 0.7 for threshold and about 1.25 for supraliminal loudness.     

Brightness and loudness as functions of stimulus duration

The brightness of white light and the loudness of white noise were measured by magnitude estimation for sets of stimuli that varied in intensity and duration. Brightness and loudness both grow as power functions of duration up to a critical duration, beyond which apparent magnitude is essentially independent of duration. For brightness, the critical duration decreases with increasing intensity, but for loudness the critical duration is nearly constant at about 150 msec. Loudness and brightness also grow as power functions of intensity. The loudness exponent is the same for all durations, but the brightness exponent is about half again as large for short durations as for long. The psychophysical power functions were used to generate equal-loudness and equal-brightness functions, which specify the combinations of intensity E and duration T that produce the same apparent magnitude. Below the critical duration ET equals k for equal brightness, and ET^a equals k for equal loudness. The value a is about 0.7 for threshold and about 1.25 for supraliminal loudness.     

Does backward masking by visual noise stop stimulus processing?

The identification of one, two, and four random letters was studied under three procedures: (1) backward masking by a visual noise; (2) concurrent masking by a visual noise; and (3) no masking. With backward masking the number of letters correctly identified was independent of the number presented. Direct judgments of the duration, brightness, contrast, sharpness, and texture of the letters were also made. Under backward masking the letters appeared to be on for a very brief duration, but with high apparent contrast. The results indicate that backward masking impairs identification by interrupting the stimulus processing, not by degrading the stimulus input.     

Time discrimination in Columba livia and Homo sapiens.

Pigeons' ability to discriminate stimulus duration, focusing on stimuli less than 1 s in duration, was evaluated in 4 experiments. In Experiment 1, the performances of pigeons and humans were compared with a staircase technique, and in Experiment 2, the method of constant stimuli was used. Both experiments produced similar results: The pigeon and human data were well described by the generalized form of Weber's law (Getty, 1975). Experiment 3 demonstrated that the birds did not use perceived brightness to mediate the discrimination of brief visual durations. Experiment 4 used a modified staircase procedure that yielded a continuous measure of discrimination from absolute threshold (0 s) to about 1 s. The difference thresholds were constant over a considerable range, similar to findings reported by Kristofferson (1980) for human timing.     

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.     

On the brightness of short and long flashes

The equation proposed by S. S. Stevens to describe the relations among brightness, intensity, and duration involves assumptions that are here made explicit. The equation is shown to apply to earlier studies designed to measure critical duration.     

Brightness function: Role of the pupil

Pairs of brightness functions were generated by magnitude estimation, one pair for each of three flash durations (0.5,1.0, and 3.0 sec). Each pair comprised a function obtained with an artificial pupil and a function obtained with the uncontrolled natural pupil. The pupil turned out to have little or no effect on the form and slope of the power functions for brightness, even when flash duration was as long as 3.0 sec.     

Duration,luminance, and the brightness exponent

The relation of brightness to duration and luminance has been studied by matching one brightness to another and also by matching numbers to brightnesses (magnitude estimation). The two methods concur in confirming certain well-known visual functions: Bloch’s law, the Broca-Sulzer effect, and the shift of the Broca-Sulzer enhancement to shorter durations when luminance increases. It is shown that the shift with luminance requires the exponent of the power function for short-flash brightness to be larger than the exponent for stimuli of longer duration. An attempt is made to analyze some of the reasons why the procedure advocated by Graham may not give comparable results.     

Luminous calibration of oscilloscopic displays

Procedures are described for adjusting the intensity of computer-driven oscilloscopic displays. The adjustment maintains a fixed intensity level on different occasions and equates the brightness of brief stimuli displayed for different durations (i.e., compensates for the effects of time-intensity reciprocity).     

A diffusion model account of response time and accuracy in a brightness discrimination task: fitting real data and failing to fit fake but plausible data

A brightness discrimination experiment was performed to examine how subjects decide whether a patch of pixels is “bright” or “dark,” and stimulus duration, brightness, and speed versus accuracy instructions were manipulated. The diffusion model (Ratcliff, 1978) was fit to the data, and it accounted for all the dependent variables: mean correct and error response times, the shapes of response time distributions for correct and error responses, and accuracy values. Speed-accuracy manipulations affected only boundary separation (response criteria settings) in the model. Drift rate (the rate of accumulation of evidence) in the diffusion model, which represents stimulus quality, increased as a function of stimulus duration and stimulus brightness but asymptoted as stimulus duration increased from 100 to 150 msec. To address the argument that the diffusion model can fit any pattern of data, simulated patterns of plausible data are presented that the model cannot fit.     

The effect of simultaneous and sequential presentation of stimulus dimensions on absolute judgment accuracy

The effects of spatial stimulus repetition, sequential stimulus repetition, spatially separated dimensional redundancy, and sequentially presented dimensional redundancy on absolute judgment accuracy of hue and brightness were compared. Two exposure durations, 0.1 and 2.0 sec, were used. While spatial repetition did not improve accuracy for either dimension, the sequential repetition of brightness produced a small increase in accuracy. The spatial presentation of correlated values of both dimensions increased accuracy only at the 2.0-sec duration. The sequential presentation of both dimensions increased accuracy, but only at the 2.0-sec duration was this gain substantial and greater than that provided by the sequential repetition of brightness alone.     

Discrimination learning with and without “errors”

Responses to S− (“errors”) are not a necessary condition for the formation of an operant discrimination of color. Errors do not occur if discrimination training begins early in conditioning and if S+ and S− initially differ with respect to brightness, duration and wavelength. After training starts, S−'s duration and brightness is progressively increased until S+ and S− differ only with respect to wavelength. Errors do occur if training starts after much conditioning in the presence of S+ has occurred or if S+ and S− differ only with respect to wavelength throughout training. Performance following discrimination learning without errors lacks three characteristics that are found following learning with errors. Only those birds that learned the discrimination with errors showed (1) “emotional” responses in the presence of S−, (2) an increase in the rate (or a decrease in the latency) of its response to S+, and (3) occasional bursts of responses to S−.     

Exposure duration and effective figure-ground contrast

The pre-and post-exposure fields in the tachistoscopic presentation are assumed to reduce the apparent contrast of the figure by brightness summation. A matching procedure was used to measure this effect. Apparent contrast rises linearly with duration, but only in the upper range. Further observations confirm the suggestion that the pre-and post-exposure fields retard the formation of bounding contours with a further reduction of apparent contrast at short durations as a result. It is indicated that the contrast-matching method provides a short-cut technique for the measurement of the temporal range of brightness summation.     

Effects of hue,brightness, contrast with background,and observation time on figural dominance in ambiguous patterns

Ambiguous patterns composed of two alternate crosses of different hues and brightness on two different brightness backgrounds were viewed for 120 sec by 10 female college students. Each subject observed 92 pattern presentations (23 patterns, each pattern presented in 2 orientations and on each of 2 backgrounds). Effects of hue and brightness contrast with background were clearly demonstrated: blue was the most dominant, red the least, and green and yellow located in between. Brightness contrast of patterns with background accentuated figural dominance of the darker figures. The number of alternations increased over the observation time for hues of equal brightness; however, the relation of this measure to total duration of seeing a figure in studies of figural dominance is unclear. Theories of neural satiation, fatigue, and interaction were used in interpreting the results.