The latency of response in relation to Bloch’s law at threshold’

Bloch’s law failed to hold for latencies of response in a threshold experiment where frequencies ofresponse did, nevertheless, obey the law. Dark-adapted Ss were instructed to respond as soon as they detected flashes of various luminances and durations. The frequency of response increased with increases in stimulus energy (luminance times duration); it was constant when energy was constant. The latency of response, measured from stimulus onset. varied inversely with energy; it also varied inversely with the luminance of flashes that were constant in energy. The results were consistent with data from earlier threshold and simple reaction time experiments.     

“Backward masking” of simple detection latencies

Simple detection latencies were determined with stimuli that comprised two successive flashes. The second flash either equaled or exceeded the first in luminance, duration, or both. When a low-luminance flash preceded either a low-or a high-luminance flash, the flashes summated, yielding latencies that were shorter than the latency for the first flash alone. In the limit, when the second flash was intense and followed the first by a short time, the latency for the paired flashes behaved as if it was determined by the second flash alone. This limit was analogous to the retroactive interference in studies of the neurophysiological concomitants of backward masking. However, the S in masking studies detects the first flash despite interference by the second, whereas the present S detected the onset of the stimulus light. The need for an analysis of the S’s response under masking is thus indicated.     

Frequencies and latencies in detecting two-flash stimuli

Dark-adapted Ss responded as soon as they detected near-threshold stimuli, which usually comprised two flashes that were equal in luminance and duration. The percentage of trials on which the S responded and the median latency of responses were determined as functions of the interval between flash onsets. At short intervals: the percentage was independent of the interval and attributable to the stimulus energy alone (Bloch’s law); the latency increased gradually to the value for the first flash alone and was attributable to not only the energy, but also to its temporal distribution. At long intervals: the percentage decreased to a steady level that was either equal to or higher than the value for the first flash; the latency increased to a peak and then decreased to the value for the first flash; both response measures suggested inhibition and probability summation between the flashes.     

Successive luminance difference thresholds and brightness as a function of the interstimulus interval and durations of successive flashes

Monocular,successive luminance difference thresholds (ΔI) and brightness matches (PSE) were obtained by the method of constant stimuli for two flashes successively presented to the same retinal area. Variations in interstimulus interval first-flash duration,and second-flash duration were the independent variables investigated. ΔI decreased as a function of ISI,while PSE remained relatively uninfluenced. An intensity-duration reciprocity was observed with increases in either first- or second-flash duration. Equal increases in duration of both flashes led to a constant value of ΔI. A Broca-Sulzer effect was also noted. In another study, a 10-msec, variable-luminance Standard was followed after 500 msec by either a 10-msec or a 320-msec test flash that was compared to the Standard. The results indicated that the rate of change of brightness with changes in luminance of the Standard was faster for the 10-msec flash than for the 320-msec flash. The rate-of-change hypothesis would predict that the 10-msec flash should have the smaller ΔI. The results for two Ss indicated the opposite: AI was smaller for 320-msec than for 10-msec flashes. A modification of the hypothesis was suggested such that it may be the energy increment (Δlt) required for detection that is related to the rate at which brightness changes with energy.     

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.     

Binocular summation of equal-energy flashes of unequal duration

To determine if binocular summation occurs when increment flashes are of equal energy (Bloch’s law) but unequal in luminance-duration parameters, three Ss made temporal forced-choice judgments: (1) monocularly, (2) binocularly when the flashes to each eye were identical, (3) binocularly when the flashes to each eye were of equal energy but different in terms of their luminance and duration parameters, and (4) binocularly when flashes to each eye were separated by 100 msec. Binocular detection rates were consistently superior to monocular detection rates. Similarity in performance between Conditions 2 and 3 indicates that the binocular visual system responds only to the total energy of each monocular flash. The data from two Ss reveal that binocular performance was greater than that predicted on the basis of probability summation.     

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.     

Nonverbal representation of time and number in adults

A wealth of human and animal research supports common neural processing of numerical and temporal information. Here we test whether adult humans spontaneously encode number and time in a paradigm similar to those previously used to test the mode-control model in animals. Subjects were trained to classify visual stimuli that varied in both number and duration as few/short or many/long. Subsequently subjects were tested with novel stimuli that varied time and held number constant (eight flashes in 0.8-3.2s) or varied number and held time constant (4-16 flashes in 1.6s). Adult humans classified novel stimuli as many/long as monotonic functions of both number and duration, consistent with simultaneous, nonverbal, analog encoding. Numerical sensitivity, however, was finer than temporal sensitivity, suggesting differential salience of time and number. These results support the notion that adults simultaneously represent the number and duration of stimuli but suggest a possible asymmetry in their representations.     

Visible persistence following a brief increment in stimulus luminance

In previous studies, it has been demonstrated that visible persistence--the period for which the perceived duration of a stimulus exceeds its physical duration--can be extended by briefly incrementing the luminance of the stimulus immediately prior to offset. Using a two-component pattern integration task, we show that this effect is not an artifact of change in the total luminous flux within the stimulus. Visible persistence was unaffected by overall luminance of the stimulus. It was also time-locked to the luminance increment. Visible persistence is seen to result from a process that is initiated by stimulus onset and that can be either wholly or partially reinitiated by the onset of the luminance increment. The duration of this process (which determines the duration of stimulus visibility) can be modified in a graded fashion by stimulus events that occur after its initiation. We outline a single-process inhibitory feedback model of the persistence mechanism that accounts for the present findings.     

Visual sensitivity and mindfulness meditation

Practitioners of the mindfulness form of Buddhist meditation were tested for visual sensitivity before and immediately after a 3-mo. retreat during which they practiced mindfulness meditation for 16 hr. each day. A control group composed of the staff at the retreat center was similarly tested. Visual sensitivity was defined in two ways: by a detection threshold based on the duration of simple light flashes and a discrimination threshold based on the interval between successive simple light flashes. All light flashes were presented tachistoscopically and were of fixed luminance. After the retreat, practitioners could detect shorter single-light flashes and required a shorter interval to differentiate between successive flashes correctly. The control group did not change on either measure. Phenomenological reports indicate that mindfulness practice enables practitioners to become aware of some of the usually preattentive processes involved in visual detection. The results support the statements found in Buddhist texts on meditation concerning the changes in perception encountered during the practice of mindfulness.     

The use of chromatic information for motion segmentation: differences between psychophysical and eye-movement measures

Previous psychophysical studies have shown that chromatic (red/green) information can be used as a segmentation cue for motion integration. We investigated the mechanisms mediating this phenomenon by comparing chromatic effects (and, for comparison, luminance effects) on motion integration between two measures: (i) directional eye movements with the notion that these responses are mediated mainly by low-level motion mechanisms, and (ii) psychophysical reports, with the notion that subjects' reports should employ higher-level (attention-based) mechanisms if available. To quantify chromatic (and luminance) effects on motion integration, coherent motion thresholds were obtained for two conditions, one in which the signal and noise dots were the same 'red' or 'green' chromaticity (or the same 'bright' or 'dark' luminance), referred to as homogeneous, and the other in which the signal and noise dots were of different chromaticities (or luminances), referred to as heterogeneous. 'Benefit ratios' (theta(HOM)/theta(HET)) were then computed, with values significantly greater than 1.0 indicating that chromatic (or luminance) information serves as a segmentation cue for motion integration. The results revealed a high and significant chromatic benefit ratio when the measure was based on psychophysical report, but not when it was based on an eye-movement measure. By contrast, luminance benefit ratios were roughly the same (and significant) for both measures. For comparison to adults, eye-movement data were also obtained from 3-month-old infants. Infants showed marginally significant benefit ratios in the luminance, but not in the chromatic, condition. In total, these results suggest that the use of chromatic information as a segmentation cue for motion integration relies on higher-level mechanisms, whereas luminance information works mainly through low-level motion mechanisms.     

The role of transparency in perceptual grouping and pattern recognition.

The shortest stimulus exposure time for which transparency can be seen was examined. In the first experiment, overlapping digits were presented for 120 ms and the luminance in the overlapping regions was varied. Subjects reported, in separate blocks of trials, either the apparent transparency of the digits or the identity of the digits. When the luminance was set so that one set of digits appeared to be seen through the other, recognition of the digits was high. When the luminance in the overlapping regions did not produce impressions of transparency, digit recognition was low. In the second experiment, digit identification at several stimulus durations was compared between stimuli that had luminance that was valid for transparency and stimuli that had invalid luminance. Performance was found to be higher in the valid luminance condition than in the invalid condition after as little as 60 ms exposure duration. This result suggests that the impression of transparency requires only relatively short exposure durations.     

The frequency accrual speed test (FAST): A new measure of ‘mental speed’?

This paper presents a rationale for a new measure of ‘mental speed’, based on a task in which subjects discriminate the relative frequency of flashes on one or the other of two lamps. In this ‘frequency accrual speed test’ (FAST), accuracy is interpreted as a measure of the rate at which flashes are registered. By assuming that sub-optimal performance is due to a random failure to register flashes, predictions are derived for a binomial and a hypergeometric version of the test. A procedure for estimating inspection time (IT) is given, and optimal values for the task parameters suggested. Results from eight studies are presented. These show that accuracy in the FAST procedure provides a stable, reliable, and robust measure, not obviously susceptible to different strategies or to practice. In four out of six studies, accuracy in this type of task showed a significant positive correlation with tests of intellectual performance, and in five out of five studies accuracy was negatively correlated with IT (significantly in two). These results show that accuracy in the FAST procedure may provide a useful measure of the speed with which sensory information is sampled. A programme for further testing is outlined.     

The pre-reflective experience of “I” as a continuously existing being: The role of temporal functional binding

The present moment is of infinitesimally brief duration. In the brain, however, there are perceptual processes that bind together events occurring at different times, on a time scale of milliseconds, into a coherent and integrated temporal representation. These processes include temporal integration, as in perception of biological motion, synchronisation, and change detection. These processes are also responsible for temporal integration and coherence in inner mental life, such as in mental imagery. I argue that this gives rise to the pre-reflective experience of the self as a continuously existing being. Temporal integration is also a feature of the experience of action–outcome relations, and I argue that this produces a pre-reflective experience of the self, not just as continuously existing, but also as the doer of both physical and mental actions. This is the foundation on which the idea of the self as continuously existing on longer time scales – the narrative self – is built.     

Cortical dynamics of visual persistence and temporal integration

In a temporal integration experiment, subjects must integrate two visual stimuli, presented at separate times, to perform an identification task. Many researchers have assumed that the persistence of the leading stimulus determines the ability to integrate the leading and trailing stimuli. However, recent studies of temporal integration have challenged that hypothesis by demonstrating that several theories of persistence are incompatible with data on temporal integration. This paper shows that an account of visual persistence given by a neural network model of preattentive vision, called the boundary contour system, explains data on temporal integration. Computer simulations of the model explain why temporal integration becomes more difficult when the display elements are separated by longer interstimulus intervals or are of longer duration or of higher luminance, or are spatially closer together. The model suggests that different mechanisms underlie the inverse duration effects for leading and for trailing elements. The model further predicts interactions of spatial separation, duration, and luminance of the trailing display.     

The reaction-time/luminance relationship for pigeons to lights of different spectral compositions

Pigeons learned to peck a key when it was illuminated during a 2-sec trial. A white-noise ready signal preceded the onset of the light; a response terminated the trial and occasionally produced reinforcement. For every trial, reaction time was recorded as the temporal interval between light onset and keypeck response. The initial experiment used “white” light; subsequent experiments used monochromatic lights of 525 and 625 nm. Within each session, the luminance of the light stimulus varied randomly over a three-log-unit range. For white light, overlapping ranges were used to extend the total luminance variation to six log units. Resulting reaction-time/luminance functions for white light were decreasing over most of the range. However, a rise in reaction time with increasing luminance was seen in the midluminance region and again at very high values. At 625 nm, the function decreased rapidly at low luminances and then leveled off or rose; at 525 nm, it was relatively flat at low luminances, where reaction times were lower than they were to photopically matched 625-nm values. Sensory and nonsensory factors might contribute to the shapes of these functions, which may be too complex to be used for psychophysical scaling.     

An N-channel stroboscopic tachistoscope for typewritten stimuli

The n-channel tachistoscope described employs the principle of “stopping” a continuously moving list of words by means of stroboscopic flashes of light. It can present as many as 50 stimuli per trial at any rate between 5 and 40 items per second. Stimulus materials can be prepared on an electric typewriter. It provides only partial control over the various time parameters.     

Binocular summation in detection of contrast flashes

We studied monocular and binocular detection of foveal flashes of different contrast. When background contours were binocularly fused, detectability (d’) of binocular test flashes was, on the average, twice the detectability of monocularly presented flashes. The precise amount of binocular advantage varied with test contrast: binocular improvement exceeded full summation for low test contrast, but fell below full summation at higher test contrasts. In the absence of contours in one eye, background luminances are not expected to sum, yet binocular detection is an average of 41.5% better than monocular detection. This indicates a difference in the functional organization of the fused binocular channel and a monocular channel.     

Temporal summation in human vision: Simple reaction time measurements

Simple reaction time IRT) was measured as a function of stimulus intensity for a brief light pulse (1 msec) and a long one (300 msec). Target size, retinal position, and adapting luminance of the stimulus were varied parametrically, and the luminance value required to produce a RT of 50 msec greater than the asymptotic RT was calculated to obtain the critical duration or limit of time-intensity reciprocity. It was found that: the critical duration, even at the fovea, tends to increase with decreasing target size; the critical duration is shortest at the fovea and increases sharply with distance from the fovea; and as the adapting luminance increases, the critical duration decreases. These findings indicate that the RT technique is a sensitive measure for the stimulus conditions explored.     

Invisible motion contributes to simultaneous motion contrast

The purpose of the present study was two-fold. First we examined whether visible motion appearance was altered by the spatial interaction between invisible and visible motion. We addressed this issue by means of simultaneous motion contrast, in which a horizontal test grating with a counterphase luminance modulation was seen to have the opposite motion direction to a peripheral inducer grating with unidirectional upward or downward motion. Using a mirror stereoscope, observers viewed the inducer and test gratings with one eye, and continuous flashes of colorful squares forming an annulus shape with the other eye. The continuous flashes rendered the inducer subjectively invisible. The observers’ task was to report whether the test grating moved upward or downward. Consequently, simultaneous motion contrast was observed even when the inducer was invisible (Experiment 1). Second, we examined whether the observers could correctly respond to the direction of invisible motion: It was impossible (Experiment 2).