Frederick L. Kitterle

Hemispheric differences are found in the identification, but not the detection, of low versus high spatial frequencies

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.

Hemispheric asymmetry in the processing of absolute versus relative spatial frequency

Observers indicated whether a stimulus presented to one visual field or the other consisted of two sine-wave gratings (the baseline stimulus) or those same two gratings with the addition of a 2 cycle per degree (cpd) component. When the absolute spatial frequencies of the baseline stimulus were low (0.5 and 1.0 cpd), there was a left visual field-right hemisphere (LVF-RH) advantage in reaction time (RT) to respond to the baseline stimulus which disappeared when the 2 cpd component was added (i.e., the stimulus consisted of 0.5, 1.0, and 2.0 cpd components). When the absolute spatial frequencies of the baseline stimulus were moderate to high (4.0 and 8.0 cpd), a right visual field-left hemisphere advantage in RT to respond to the baseline stimulus approached significance and shifted to a significant LVF-RH advantage when the 2 cpd component was added (i.e., the stimulus consisted of 2.0, 4.0, and 8.0 cpd components. That is, adding the same 2 cpd component caused opposite shifts in visual laterality depending on whether 2 cpd was a relatively high or relatively low frequency compared to the baseline.

Visual hemispheric asymmetries depend on which spatial frequencies are task relevant

Observers classified sine-wave and square-wave gratings on the basis of fundamental frequency (Are the bars wide or narrow?) or on the basis of higher harmonic frequencies (Are the bars sharp or fuzzy?). Stimuli were presented in either the left (LVF) or right (RVF) visual field. When the classification was made on the basis of the fundamental frequencies (1 or 3 c/deg), there was a LVF/right hemisphere advantage. However, when the classification was on the basis of a sharp/fuzzy distinction which involved searching for the higher harmonic frequencies, then a RVF/left hemisphere advantage was found.

Visual field effects in the discrimination of sine-wave gratings

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.

Hemispheric symmetry in contrast and orientation sensitivity

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.

Psychophysics of lateral tachistoscopic presentation

Human visual performance depends upon the retinal position to which a target is delivered. A general finding is that performance measured in a variety of psychophysical tasks deteriorates as a target is presented to more eccentric retinal regions. One purpose of this paper is to describe differences between foveal and peripheral vision in a number of psychophysical tasks. A second purpose is to review studies which have attempted to account for the fall off in visual performance between central and peripheral target presentations. A third purpose is to consider the contribution of the periphery to perception since targets which are sufficiently large project not only on receptors in the fovea but also on those in the periphery. In addition, stimuli presented to the peripheral retina can influence the processing of a target presented to the central retinal region. A fourth purpose is to review studies which have attempted to compensate for foveal and peripheral differences by scaling the target in size or some other attribute in proportion to the cortical magnification factor. A final purpose of this paper is to consider whether the fovea and the periphery are specialized for different functions.

The effects of spatial frequency, orientation, and color upon binocular rivalry and monocular pattern alternation

The purpose of this investigation was to determine whether or not monocular and binocular rivalry are mediated by the same mechanism. The question was approached by examining, in a series of two experiments, the spatial frequency and orientation tuning characteristics for both forms of rivalry, as well as the effects of color upon monocular and binocular rivalry. The results of Experiment 1 indicate that binocular rivalry is insensitive to spatial frequency and color. In Experiment 2, the effects of spatial frequency and orientation were examined. The results of this experiment indicate that binocular rivalry is less sensitive to changes in orientation than is monocular alternation. There is no effect of color upon binocular rivalry. It was concluded that monocular alternation and binocular rivalry are mediated by different mechanisms.

Pattern alternation: Effects of spatial frequency and orientation

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.

The effects of temporal waveform upon apparent contrast

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.

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.