Temporal factors in the discrimination of coherent motion

When observers view the relative movements of a pair of bars defined by the difference of spatial Gaussian functions (DOGs), they can accurately discriminate coherent movements over a range of temporal frequencies and temporal asynchronies. Of particular interest is the fact that performance accuracy is maintained even when the two bars differ in spatial-frequency content and contrast. On each trial, observers viewed two brief presentation intervals in which a pair of vertically oriented DOGs moved randomly back and forth within a restricted range. During one observation interval, both elements moved in the same direction and by the same magnitude (correlated), and in the other interval, the movements were independent (uncorrelated). Temporal asynchronies were introduced by delaying the displacement of the right bar relative to that of the left bar in each interval. Observers were able to discriminate correlated versus uncorrelated movements up to a 45–60-msec temporal delay between the two elements’ relative displacements. If motion processing is accomplished by mechanisms operating over multiple spatial and temporal scales, the visual system’s tolerance of temporal delays among correlated signals may facilitate their space-time integration, thereby capitalizing on the perceptual utility of coherent-motion information for image segmentation and interpolating surface structure from the movements of spatially separated features.     

Detection and discrimination of coherent motion

When viewing a pair of bars defined by the difference of spatial Gaussian functions (DOGs), human observers can discriminate accurately the relative movements of the bars, even when they differ in spatial frequency. On each trial, observers viewed two brief presentation intervals in which a pair of vertically oriented DOGs moved randomly back and forth within a restricted range. During one interval, both bars moved in the same horizontal direction and by the same magnitude (correlated movements); in the other interval, their movements were uncorrelated. When discrimination accuracy is related to the simultaneous detection of two independent movements, it was found that, if observers can detect the movements of spatially separated bars, they can tell whether their relative movements are correlated. Performance remained remarkably accurate even when the two bars differed in spatial frequency by more than two octaves or were presented separately to the two eyes. Apparently, the accurate discrimination of coherent motion involves an efficient spatial integration of optical motion information over multiple spatial locations and multiple spatial scales.     

Temporal discrimination in the split brain

Divided visual field studies of neurologically normal adults indicate that the left hemisphere is superior to the right in making temporal judgments. Some neuroimaging and neuropsychological studies, however, have suggested a role for the right hemisphere in temporal processing. We tested the divided hemispheres of a split-brain patient in two tasks requiring temporal judgments about visually presented stimuli. In one task, the patient judged whether two circles presented to one visual field appeared for the same or different durations. In the second task, the patient judged whether the temporal gaps in two circles occurred simultaneously or sequentially. In both tasks, the performance of the right hemisphere was superior to that of the left. This suggests that the right hemisphere plays an important role in making temporal judgments about visually presented stimuli.     

Temporal discrimination and a free-operant psychophysical procedure.

Pigeons were presented a series of keylight time periods (separated by blackouts) during which two response keys were lit, one by blue light and the other either by orange or green. Blue-key responses changed the color on the other key. Orange-key responses sometimes produced food during the first half of a time period; green-key responses sometimes produced food during the second half. In three experiments, the probability of a green-key response increased as a function of elapsed time. Experiment 1 compared performance when the duration of the keylight periods was varied across a wide range. Discrimination of performance was similar across the range of durations. Experiment 2 varied both relative reinforcement rate and the local reinforcement rate for orange-key and green-key responses. These manipulations produced changes in response bias but not discrimination sensitivity. Experiment 3 varied the local temporal placement of reinforcers within time periods and demonstrated that choice behavior was affected by differential reinforcement at different points during the time periods. The results were consistent with previous research on duration discrimination that used psychophysical trials procedures.     

Sufficient conditions for the discrimination of motion

When one half of a randomly contoured pattern is displaced in one of four directions between successive exposures, Ss are capable of discriminating the direction of the displacement. Accuracy of discriminating decreases as a function of the relative displacement within the pattern, independent of the visual angle subtended by the pattern. Supplementary data suggest that Ss are unrealistically confident in the accuracy of their discrimination. The results demonstrate that identification of specific contours is not necessary for the discrimination of motion and suggest that some type of correlational process is employed by the visual system in dealing with spatially and temporally displaced patterns.     

Manual discrimination of force using active finger motion.

In these experiments, two plates were grasped between the thumb and forefinger and squeezed together along a linear track. An electromechanical system presented a constant resistance force during the squeeze up to a predetermined location on the track, whereupon the force effectively went to infinity (simulating a wall) or to zero (simulating a cliff). The task of the subject was to discriminate between two alternative levels of the constant resistance force (a reference level and a reference-plus-increment level). Results of these experiments indicate a just noticeable difference of roughly 7% of the reference force using a one-interval paradigm with trial-by-trial feedback over the ranges 2.5 less than or equal to F0 less than or equal to 10.0 newtons, 5 less than or equal to D less than or equal to 30 mm, 45 less than or equal to S less than or equal to 125 mm, and 25 less than or equal to V less than or equal to 160 mm/sec, where F0 is the reference force, D is the distance squeezed, S is the initial fingerspan, and V is the mean velocity of the squeeze. These results, based on tests with 5 subjects, are consistent with a wide range of previous results, some of which are associated with other body surfaces and muscle systems and many of which were obtained with different psychophysical methods.     

Culture-general and culture-specific factors in the discrimination of melodies.

We examined effects of a culture-general factor, pattern redundancy (number of repeated tones), on the discrimination of 5-tone melodies that differed in their adherence to Western tonal conventions. Experiment 1 evaluated the ability of 9-month-old infants to differentiate "standard" melodies from subtly altered "comparison" melodies. Greater redundancy of the standard melodies was associated with enhanced infant performance, but musical conventionality had no effect. Experiment 2 evaluated comparable abilities in 5-year-old children and musically untrained adults. Children's performance was enhanced by the redundancy of standard melodies, but the effect was greater in conventional than in unconventional contexts. The redundancy of standard melodies facilitated adults' performance in conventional but not in unconventional contexts. Thus, increasing musical exposure seems to attenuate the effects of culture-general factors such as pattern redundancy while amplifying the influence of culture-specific factors.     

Temporal discrimination increases in precision over development and parallels the development of numerosity discrimination

Time perception is important for many aspects of human behavior, and a large literature documents that adults represent intervals and that their ability to discriminate temporal intervals is ratio dependent. Here we replicate a recent study by vanMarle and Wynn (2006) that used the visual habituation paradigm and demonstrated that temporal discrimination in 6-month-old infants is also ratio dependent. We further demonstrate that between 6 and 10 months of age temporal discrimination increases in precision such that by 10 months of age infants succeed at discriminating a 2:3 ratio, a ratio that 6-month-old infants were unable to discriminate. We discuss the potential implications of the fact that temporal discrimination follows the same developmental progression that has been previously observed for number discrimination in infancy (Lipton & Spelke, 2003).     

Segregation by color/luminance does not necessarily facilitate motion discrimination in the presence of motion distractors.

Under what circumstances is the common motion of a group of elements more easily perceived when the elements differ in color and/or luminance polarity from their surround? Croner and Albright (1997), using a conventional global motion paradigm, first showed that motion coherence thresholds fell when target and distractor elements were made different in color. However, in their paradigm, there was a cue in the static view of the stimulus as to which elements belonged to the target. Arguably, in order to determine whether the visual system automatically groups, or prefilters, the image into different color maps for motion processing, such static form cues should be eliminated. Using various arrangements of the global motion stimulus in which we eliminated all static form cues, we found that global motion thresholds were no better when target and distractors differed in color than when they were identical, except under certain circumstances in which subjects had prior knowledge of the specific target color. We conclude that, in the absence of either static form cues or the possibility of selective attention to the target color, features with similar colors/luminance-polarities are not automatically grouped for global motion analysis.     

Temporal integration in duration and number discrimination

Temporal integration in duration and number discrimination by rats was investigated with the use of a psychophysical choice procedure. A response on one lever ("short" response) following a 1-s white-noise signal was followed by food reinforcement, and a response on the other lever ("long" response) following a 2-s white-noise signal was also followed by food reinforcement. Either response following a signal of one of five intermediate durations was unreinforced. This led to a psychophysical function in which the probability of a long response was related to signal duration in an ogival manner. On 2 test days, a white-noise signal with 5, 6, 7, 8, or 10 segments of either 0.5-s on and 0.5-s off or 1-s on and 1-s off was presented, and a choice response following these signals was unreinforced. The probability of a long response was the same function of a segmented signal and a continuous signal if each segment was considered equivalent to 200 ms. A quantitative fit of a scalar estimation theory suggested that the latencies to initiate temporal integration and to terminate the process are both about 200 ms, and that the same internal accumulation process can be used for counting and timing.     

Temporal interference effects in auditory amplitude discrimination

The efficiency, y,η of performance in amplitude discrimination is examined as a function of the temporal separation, Γ, of the two signals to be discriminated. Performance in a monaural amplitude discrimination task is compared with that in a dichotic amplitude discrimination task, in which the first of the two signals was always presented to one ear and the second signal to the other ear. The difference in the shape of the resulting η versus Γ functions for the monaural and dichotic cases is interpreted in terms of peripheral and central interference effects.     

Temporal sequence effects in auditory oddity discrimination

The oddity method was used in assessing pitch, loudness, simultaneous tone, successive tone, and speech sound discrimination in 35 normal children and 15 children with learning problems. With this method three auditory stimuli are presented, two of which are identical, and the S is required to indicate the temporal position of the odd stimulus. For both groups, discrimination was most accurate when the odd stimulus was in the third position. These results could be explained by assuming that the oddity response was based upon successive Same-Different judgments of the first and second stimuli and the second and third stimuli, since a correct response to third-position oddity would require only a Same judgment of the first and second stimuli. Other findings were not as easily explained by this simple model, and alternative hypotheses are discussed.     

Temporal and spatial effects in luminance discrimination

Luminance difference thresholds (ΔI) were obtained by a successive and a simultaneous method of presenting the stimuli Spatial and temporal separation between the fields was the independent variable while other variables as stimulus size, luminance, retinal area of stimulation and duration were kept constant ΔI was less for simultaneously presented stimuli than for successively presented stimuli This was related to spatial and temporal interaction effects such that the greater the spatial interaction between simultaneously presented fields, the greater the discriminability while the greater the temporal interaction between successively presented fields, the less the sensitivity to luminance differences. It was suggested that the basis of the luminance discrimination may be different under conditions of temporal interaction than under conditions of spatial interaction between the fields.     

Perception of coherent motion in random dot displays by pigeons and humans.

Pigeons and humans were required to discriminate coherent from random motion in dynamic random dot displays. Coherence and velocity thresholds were determined for both species, and both thresholds were found to be substantially higher for pigeons than for humans. The results are discussed with reference to differences in motion processing in mammals and birds. It is suggested that the inferior motion sensitivity of pigeons can be attributed to poorer spatiotemporal motion integration.     

Temporal properties in masking biological motion

The perception of biological motion using point light animation techniques was investigated in several experiments. Animations simulating walking were presented with additional masking dots. The temporal properties of the walking motion or the temporal relationship between the walking and masking motions were systematically manipulated. Results showed that (1) perception of biological motion was sensitive to even small temporal perturbation within the walker, (2) the effectiveness of a mask depended upon the temporal phase difference between the mask and point light walker, (3) relatively small temporal differences between the mask and point light walker decreased the effectiveness of the mask, and (4) these effects were not due simply to observers detecting the phase offsets in the display. Temporal properties of the motion are important in perceiving the human form in action, just as in other types of figure-ground segregation. This information may be processed by both motion and form pathways for processing biological motion.     

Temporal integration in structure from motion

A temporal integration model is proposed that predicts the results reported in 4 psychophysical experiments. The main findings were (a) the initial part of a structure-from-motion (SFM) sequence influences the orientation evoked by the final part of that sequence (an effect lasting for more than 1 s), and (b) for oscillating SFM sequences, perceived slant is affected by the oscillation frequency and by the sign of the final gradient. For contracting optic flows (i.e., rotations away from the image plane), the sequence with the lowest oscillation frequency appeared more slanted; for expanding optic flows (i.e., rotations toward the image plane), the sequence with the highest oscillation frequency appeared more slanted.     

Binocular contrast interactions in two-frame motion discrimination

Previous studies have shown that two-frame motion detection thresholds are elevated if one frame's contrast is raised, despite the increase in average contrast--the "contrast paradox". In this study, we investigated if such contrast interactions occurred at a monocular or binocular site of visual processing. Two-frame motion direction discrimination thresholds were measured for motion frames that were presented binocularly, dichoptically or interocularly. Thresholds for each presentation condition were measured for motion frames that comprised either matched or unmatched contrasts. The results showed that contrast mechanisms producing the contrast paradox combine contrast signals from both eyes prior to motion computation. Furthermore, the results are consistent with the existence of monocular and binocular contrast gain control mechanisms that coexist either as combined or independent systems.