The effect of exposure duration on the apparent contrast of sinusoidal gratings

Apparent contrast determined for sinusoidal grating targets which were flashed for varying durations appears to follow a temporal course similar to a Broca-Sulzer curve, and this temporal course is also influenced by the spatial frequency of the grating target. These results were discussed in terms of differential latency and temporal integrating characteristics of spatial frequency channels.     

Contrast constancy revisited: The perceived contrast of sinusoidal gratings above threshold

The contrast sensitivity function of the human visual system, measured with sinusoidal luminance gratings, has an inverted U shape with a peak around 2–4?c/deg. Above threshold, it is thought that luminance gratings of equal physical contrasts but of distinguishably different spatial frequencies are all perceived as having similar contrasts, a phenomenon that has been termed contrast constancy. However, when suprathreshold contrast matches were measured for pairs of luminance gratings whose spatial frequencies were indistinguishable, the matching curves were not flat and followed a similar inverted U shape form as the contrast sensitivity function at threshold. It was therefore suggested that contrast constancy may only be revealed when matching the contrasts of clearly distinguishable spatial frequencies. Here, observers matched the perceived contrasts of suprathreshold luminance gratings of similar but visibly different spatial frequencies between 0.25 and 16?c/deg. The results show that, much like the contrast sensitivity function at threshold, observers are more sensitive to intermediate spatial frequencies (1–6?c/deg) than they are to either higher or to lower spatial frequencies. This tuning is evident when matching reference contrasts of 30–80%, implying a significant role in everyday vision. To demonstrate that these results were not due to local adaptation, the experiment was repeated with shorter stimulus duration, producing the same results. The extent of departure from contrast constancy found in the present study is compared to previously reported suprathreshold measurements. The results are also discussed with consideration to limitations with display apparatus such as monitor blur.     

A simple method of projecting mach bands,color mixtures,and variable contrast sinusoidal gratings

This paper describes a simple method of projecting Mach bands, color mixtures, and variable-contrast sinusoidal gratings.     

Amplitude and spatial-period discrimination in sinusoidal gratings by dynamic touch

We measured discrimination thresholds for sinusoidal gratings using active dynamic touch. In the first experiment we measured the thresholds for amplitude discrimination as a function of amplitude and spatial period. Thresholds ranged between 10.8%, and 15.8% of the standard amplitude. We showed that amplitude differences as small as 2 microm can be detected. We found that Weber fractions for amplitude discrimination are constant over a range of amplitudes, but are influenced by the spatial period of the grating; discrimination improved with increasing spatial period. In the second experiment we determined the thresholds for spatial-period discrimination. We used the same design as in the first experiment. Weber fractions ranged from 6.4% to 11.8%. Amplitude was found to have no effect on the Weber fractions for spatial-period discrimination. However, the spatial period had an effect on the Weber fractions: larger spatial periods yielded lower Weber fractions.     

Adaptation effects on the apparent "squareness" of square-wave gratings

Adaptation to a 9 cycles deg-1 sine wave makes the apparent brightness profile of a 3 cycles deg-1 square wave look less "square" even when it does not look like that of a sine wave. Adaptation to a 3 cycles deg-1 sine wave has no noticeable effect. Hypotheses based on "excitation patterns" across spatial-frequency-selective channels can account for these results; it is not clear whether variations in local light adaptation can also account for them.     

The effect of stimulus duration on the persistence of gratings

The persistence of gratings varying in spatial frequency and exposure duration was measured using a stimulus-blank alternation method. Persistence was found to lengthen with increasing spatial frequency and to shorten with increasing exposure duration. For each spatial frequency, persistence decreased linearly with a slope of approximately —.75 as duration increased for short stimulus durations. For longer stimulus durations, the rate of decline in persistence with increasing duration was reduced, the slope being approximately —.13. The stimulus duration at which the change in slope of the persistence-duration relationship occurred was shown to increase with increasing spatial frequency and was approximately equivalent to the critical duration for each spatial frequency. The data were consistent with an interpretation of persistence in terms of a temporal integration component and a second, possibly cortically located, component.     

Apparent contrast of suprathreshold gratings varies with stimulus orientation

Human contrast sensitivity is known to vary with both stimulus spatial frequency and orientation. Previous research has indicated, however, that the apparent contrast of a sinusoidal grating is independent of spatial frequency at suprathreshold levels. We have found that the apparent contrast of suprathreshold gratings is not independent of its orientation. Observers adjusted the apparent contrasts of vertical and oblique gratings to match a 40% standard. Contrast match deviations were found at a number of spatial frequencies, indicating a lack of contrast constancy with stimulus orientation. The apparent contrast of gratings in different orientations could not be predicted in any simple fashion from either contrast threshold differences or ratios.     

T-junctions,apparent depth,and perceived lightness contrast

Observers performed lightness matches for physically equivalent gray targets of a simultaneous lightness contrast display and displays in which both targets were on the same background. Targets either shared a common line-texture pattern with their respective backgrounds or did not. Results indicate that when targets share a line-texture pattern with their respective backgrounds, a contrast effect is obtained. However, when the target's pattern is different than the background's pattern, perceived contrast is significantly reduced and the target appears as a separate 3-D entity. This result applies to both vertically and horizontally oriented displays, to targets that are increments or decrements, and to line-texture patterns that are black or white. Line patterns that are shared by targets and backgrounds result in T-junctions that provide occlusion information. We conclude that targets and backgrounds perceived to be on separate planes because of T-junctions are less likely to be perceptually grouped together and that their luminance values are less likely to be compared with one another.     

The effect of negative stimulus presentations on observing-response rates

Theories of observing differ in predicting whether or not a signal for absence of reinforcement (S−) is capable of reinforcing observing responses. Experiments in which S− was first removed from and then restored to the procedure have yielded mixed results. The present experiments suggest that failure to control for the direct effect of presenting S− may have been responsible. Pigeons and operant procedures were used. Experiment 1 showed that presentations of S−, even when not contingent on observing, can raise the rate of an observing response that was reinforced only by presentations of a signal (S+) that accompanied a schedule of food delivery. Experiment 2 showed that this effect resulted from bursts of responding that followed offsets of S−. Experiment 3 showed that, when the presence of S− was held constant, lower rates occurred when S− was dependent on, rather than independent of, observing. These results support theories that characterize S− as incapable of reinforcing observing responses.     

The effect of suspense and its resolution on the appreciation of dramatic presentations

In a factorial design, degree of suspense (low, intermediate, high) of an adventure presented audiovisually was varied with resolution of the suspense (resolved, lingering). The variations were accomplished by producing different versions of the same program. Appreciation of the presentation was measured in the viewers' facial displays, in structured interviews, and in ratings. Consistently across measures, it was found that under conditions of resolved suspense, appreciation substantially increased with degree of suspense. Under conditions of lingering suspense, appreciation also increased with degree of suspense but to a significantly smaller extent. Excitatory responses were measured in skin temperature and heart rate. A significant differentiation of heart rate as a function of degree of suspense was observed. In addition, both skin temperature and heart rate disclosed sudden declines in excitation with the effective resolution of suspense.     

The effect of sound on visual apparent movement

Twelve subjects found the longest possible interstimulus interval (ISI) at which they perceived continuous apparent motion of one light instead of partial motion or succession between two lights. In the visual condition, two lights only were presented. In the bimodal conditions, binaurally presented tones were presented synchronously with the lights, and the lights and tones were either spatially congruent (in phase) or incongruent (180 degrees out of phase). Bimodal presentations lowered the upper ISI threshold for the perception of visual apparent motion, and the reduction was greater when the tones and lights were spatially congruent. The threshold reduction may be caused by a perceptual inference about localization of the lights in space or by a change in visual persistence.     

Effects of luminance and contrast on direction of ambiguous apparent motion

A study is reported of the role of luminance and contrast in resolving ambiguous apparent motion (AM). Different results were obtained for the short-range (SR) and the long-range (LR) motion-detecting processes. For short-range jumps (7.5 min arc), the direction of ambiguous AM depended on brightness polarity, with AM only from white to white and from black to black. But for larger jumps, or when an interstimulus interval (ISI) was introduced, AM was less dependent on polarity, with white often jumping to black and black jumping to white. Two potential AMs were pitted against each other, one carried by a light stimulus and the other by a dark stimulus. The stimulus whose luminance differed most from the uniform surround captured the AM. Visual response to luminance was linear, not logarithmic. When the stimulus was modified to give continuous AM in one direction it was followed by a negative aftereffect of motion only when the spatial displacement was 1 min arc. A larger displacement (10 min arc) gave good AM but no motion aftereffect. Thus only short-range motion adapts motion-sensitive channels.     

The effect of apparent movement on mental rotation

Eleven subjects were timed as they judged whether a small bar perpendicular to one side of a clockhand would point left or right if the hand was pointing upward (i.e., at the "12 o'clock" position). The clockhand was shown in two successive orientations 30 degrees apart, so that it was perceived to jump from one to the other in either a clockwise or a counterclockwise direction. Reaction times were consistent with the interpretation that the subjects "mentally rotated" the clockhand from its perceived orientation back to the upright before making their decisions. The direction of the jump influenced perceived orientation but did not influence either the direction or rate of mental rotation itself.