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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. [ABSTRACT FROM AUTHOR]

Campbell and Howell (1972) reported an effect called 'monocular pattern alternation.' They found that a pattern composed of two orthogonal sinusoidal gratings, one horizontal and the other vertical, underwent rivalry when viewed monocularly for a period of time. In the present study, it has been shown that monocular pattern alternation depends upon the orientation of the pattern and the spatial frequency of its components. Fewer reversals were found for an obliquely oriented pattern than for a pattern with components in the horizontal and vertical meridians. Alternation rate was higher when the gratings were similar in frequency but differed in orientation than when the components of the pattern differed in both dimensions. It was concluded that pattern alternation reflects an antagonistic interaction between interdependent channels in the human visual system that respond to orientation and spatial frequency. [ABSTRACT FROM AUTHOR]

The relationship between local/global and high/low spatial-frequency processing in hemispheric asymmetries was explored. Subjects were required to judge the orientation of a high- or low-spatial-frequency component of a compound grating presented in the left visual field (LVF) or right visual field (RVF). In Experiment 1, attention was focused on one or the other component. A signal detection analysis indicated that sensitivity ( d′) to the high-spatial-frequency target was reduced more by the presence of the low-spatial-frequency component when both were presented in the LVF rather than in the RVF. In Experiment 2, subjects determined whether a target orientation was present, independent of spatial frequency at only a single level (i.e., at the high- or low-spatial-frequency level), as opposed to both or neither level. An RVF/LH (left hemisphere) advantage was found when the decision was based on the orientation of the high-frequency component. The asymmetrical influence of visual field of presentation and spatial frequency upon sensitivity is discussed in terms of hemispheric differences in the magnitude of inhibition between spatial-frequency channels and in the role of transient channel activity to capture and direct higher order attentional processes. [ABSTRACT FROM AUTHOR]

The time needed to decide whether the second of two successively presented sinusoidal gratings was of a higher or lower spatial frequency than the first was measured for spatial frequencies of 1, 2, 4, 8, and 12 cycles per degree (cpd) presented in either the left visual field (LVF) or right visual field (RVF). A LVF advantage was found for discriminating within the low-spatial-frequency range (i.e., 1 and 2 cpd), whereas a RVF advantage was found for discriminating within the high-spatial-frequency range (i.e., 4-12 cpd). These findings support the conclusion that hemispheric asymmetries in the processing of gratings arise when comparisons-are -made between the output of spatial-frequency channels. [ABSTRACT FROM AUTHOR]

The processing of sine-wave gratings presented to the left and right visual fields was examined in four experiments. Subjects were required either to detect the presence of a grating (Experiments 1 and 2) or to identify the spatial frequency of a grating (Experiments 3 and 4). Orthogonally to this, the stimuli were presented either at threshold levels of contrast (Experiments 1 and 3) or at suprathreshold levels (Experiments 2 and 4). Visual field and spatial frequency interacted when the task required identification of spatial frequency, but not when it required only stimulus detection. Regardless of contrast level (threshold, suprathreshold), high-frequency gratings were identified more readily in the right visual field (left hemisphere), whereas low-frequency gratings showed no visual field difference (Experiment 3) or were identified more readily in the left visual field (right hemisphere) (Experiment 4). Thus, hemispheric asymmetries in the processing of spatial frequencies depend on the task. These results support Sergent's (1982) spatial frequency hypothesis, but only when the computational demands of the task exceed those required for the simple detection of the stimuli. [ABSTRACT FROM AUTHOR]

This experiment was designed to determine whether there were hemispheric and/or hemiretinal (nasal/temporal) differences in contrast sensitivity and the oblique effect. Contrast sensitivity functions were measured in the left and right eyes for vertically (90°) and obliquely (45°) oriented sinusoidal gratings presented in the right and left visual fields. There were no hemispheric differences in contrast sensitivity for vertically or obliquely oriented gratings. However, sensitivity was lower for obliquely oriented gratings. Thus, the cerebral hemispheres do not appear to differ in sensitivity to contrast or in the magnitude of the oblique effect. The implications of these results are discussed in terms of the role of spatial frequency channels in information processing asymmetries between the left and right cerebral hemispheres. [ABSTRACT FROM AUTHOR]

Monocular pattern alternation refers to the changes over time in the apparent contrast of the components of a pattern formed by superimposing two sinusoidal gratings. In Experiment 1, alternation rate was measured as a function of the orientation, spatial frequency, and mean luminance of the pattern. Experiment 2 determined pattern alternation rate as a function of orientation, spatial frequency, and contrast. For both experiments, alternation was greater for patterns whose components were horizontal and vertical than for patterns whose components were obliquely oriented. These differences were more pronounced at intermediate than at high and low spatial frequencies. With decreases in mean luminance, there was a decline in both the rate of alternation and the differences in rate due to orientation. There was no significant effect of contrast upon alternation rate. Experiment 3 determined the effects of viewing time upon pattern alternation. For both horizontal-vertical and oblique patterns, the frequency of alternation increased with time up to 100-200 sec and then leveled off. However, the rate of increase with time was greater for horizontal-vertical patterns. These results were discussed in terms of the antagonistic interaction between channels tuned to spatial frequency and orientation. [ABSTRACT FROM AUTHOR]

The effect of duration upon the apparent contrast of a 0.7- and 7.0-c/deg sinusoidal grating was determined for two modes of temporal presentation: phase reversals and on-off flashes. For both spatial frequencies at durations of less than 60 msec, the apparent contrast of phase-reversed presentations was less than that of on-off flashes. However, for the 0.7-c/deg target at durations of 100-300 msec, the apparent contrast of the phase-reversed presentations was significantly greater than that of on-off presentations. There was no difference in the effect of the two modes of temporal presentation upon apparent contrast at longer durations with the 7.0-c/deg grating. These results are discussed in terms of the role of response transients in the perceived contrast of suprathreshold gratings. [ABSTRACT FROM AUTHOR]

Observers detected 20-msec sinusoidal gratings presented at various asynchronies relative to a decrement in the mean luminance of a uniform field. Brief luminance decrements have no effect upon contrast thresholds determined with high-spatial-frequency gratings. With low-spatial-frequency gratings, transient threshold elevations were found at decrement onset and offset. For longer decrements, threshold changes were found for both high- and low-spatial-frequency gratings. However, the time courses of these changes differed. With high-spatial-frequency gratings, thresholds changed more gradually, whereas with low-spatial-frequency gratings, the time courses contained marked transients at the onset and offset of the decrement. These results are discussed in terms of contribution of multiple spatial-frequency channels to temporal resolution. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Temporal contrast enhancement refers to the finding that intermediate-duration, low-spatial-frequency gratings are perceived to have a greater contrast than long-duration gratings of similar spatial frequency. Kitterle and Corwin (1979) suggested that this effect reflects primarily activity in transient channels. To test this, 30 subjects were run in one of three groups consisting of either adaptation to a steady or flickering low-spatial-frequency grating or no adaptation. Temporal contrast enhancement was found for both steady and no-adaptation conditions. Flicker adaptation abolished temporal contrast enhancement. It was suggested that flicker adaptation may decrease the contribution of transient channels to perceived contrast and may cause processing of the test gratings to be switched to sustained channels. The implications of these results for understanding the coding of brightness are discussed. [ABSTRACT FROM AUTHOR]

Evidence that the magnitude of simultaneous contrast depends upon retinal orientation of a stimulus configuration is presented in two experiments. More specifically, the magnitude of contrast was found to be greater for stimuli presented horizontally and vertically than for stimuli presented obliquely. These results were interpreted in terms of a cortical analysis of simultaneous contrast. [ABSTRACT FROM AUTHOR]

Temporal contrast enhancement (TCE) refers to the finding that the apparent contrast of flashed low-spatial-frequency gratings is higher for flashes of intermediate duration [i.e., 50120 msec) than for flashes of longer duration. The present experiment investigated the effect of temporal waveform upon TCE. We measured the apparent contrast of a .6-cycles/deg sinusoidal grating that varied in duration. Four temporal waveforms were used: abrupt onset/ abrupt offset, abrupt onset/gradual offset, gradual onset/abrupt offset, and gradual onset/ gradual offset. TCE was found only for targets with abrupt onsets, regardless of offset waveform. These results have implications about the role of onset transients in the coding of brightness. [ABSTRACT FROM AUTHOR]