Singly and doubly contingent after-effects involving color, orientation and spatial frequency

Two singly contingent after-effects were observed: an orientation effect contingent on spatial frequency and a spatial frequency effect contingent on orientation. These effects may be interpreted in terms of channels specific for orientation and spatial frequency. Two doubly contingent after-effects were also observed: a color effect contingent jointly on orientation and frequency, and a color effect contingent on apparent frequency contingent on orientation. These effects may be interpreted in terms of channels specific for color, orientation and spatial frequency.     

The role of spatial frequency in color induction

Color induction was measured for test and inducing chromaticities presented in spatial square-wave alternation, with spatial frequencies of 0.7, 4.0, 6.0 and 9.0 cpd. Observers matched the test chromaticities to a rectangular matching field using haploscopic presentation. Data were collected and analyzed within the framework of a cone chromaticity space, allowing analysis of spatial frequency effects on post-receptoral spectral opponent pathways. Assimilation, a shift of chromaticity toward the inducing chromaticity, was found at the highest spatial frequency (9.0 cpd). Contrast, a shift of chromaticity away from the inducing chromaticity, occurred at the lowest spatial frequency (0.7 cpd). The spatial frequency at the transition point from assimilation to contrast was near 4 cpd, independent of the cone axis. Assimilation was unaffected by the presence of a neutral surround and could be described by a spread light model. Contrast was reduced in the presence of a neutral surround. The data suggested that retinal contrast signals are important determinants in the perception of chromatic contrast.     

Angle-contingent color aftereffects

Visual aftereffects of color, contingent on the stimulus pattern, provide evidence that anglecontingent aftereffects may be mediated by broadly tuned mechanisms different from those which mediate line orientation. Angle-contingent aftereffects generalize to test patterns made up of curves, and curvaturecontingent aftereffects generalize to angles. Some characteristics of angle-contingent aftereffects can be explained by a neural substrate with characteristics like those that have been reported for higher-order hypercomplex cells in the visual cortex.     

The role of afterimages in visual spatial aftereffects

Displacement aftereffects were obtained with alternate presentations of an inducing figure and its complement which is a finding contrary to an explanation based on retinal afterimages. Both the strength and decay rate of the aftereffects obtained for this condition were comparable to those found when a single inducing figure was presented for the entire adaptation period. For all conditions a stronger and longer aftereffect resulted for binocular stimulus presentation than was the case for dichoptic presentation. The results suggest that central processes, sensitive to relative figure-ground contrast regardless of polarity, are responsible for both the temporal protraction of the inducing figure and the displacement of the test figure.     

Phase perception in the high spatial frequency range

A compound waveform was composed of a fundamental and a higher harmonic with 46/15 times the frequency. No phase slip can be observed when the frequency of the fundamental is 6 c/deg. Phase perceptibility is therefore frequency dependent, and may be limited, in the high frequency range, by a 2′ tolerance for absolute position.     

Color contingent motion aftereffects: Single or multiple levels of processing?

The monocular spiral aftereffect can be observed in the stimulated and unstimulated eye. When the aftereffect is made contingent upon a specific color, transfer does not occur. The experiments reported show this to be the result of separate coding for color and movement, whereby the color coding occurs prior to binocular interaction.     

Movement aftereffects contingent on the colour or pattern of a stationary surround

Other studies have shown that movement aftereffects (MAEs) contingent on several visual dimensions, such as colour, texture, luminance or depth, can be obtained from suitable stationary test fields if the appropriate characteristics of the moving stimulus are varied during adaptation. We have obtained CMAEs from an adaptation paradigm in which only the direction of movement of a moving stimulus is changed while the colour or pattern of a surrounding stationary area is varied. Stationary test fields then elicit MAEs contingent on the colour or pattern of their surround.     

Independent coding of object motion and position revealed by distinct contingent aftereffects

Despite several findings of perceptual asynchronies between object features, it remains unclear whether independent neuronal populations necessarily code these perceptually unbound properties. To examine this, we investigated the binding between an object's spatial frequency and its rotational motion using contingent motion aftereffects (MAE). Subjects adapted to an oscillating grating whose direction of rotation was paired with a high or low spatial frequency pattern. In separate adaptation conditions, we varied the moment when the spatial frequency change occurred relative to the direction reversal. After adapting to one stimulus, subjects made judgments of either the perceived MAE (rotational movement) or the position shift (instantaneous phase rotation) that accompanied the MAE. To null the spatial frequency-contingent MAE, motion reversals had to physically lag changes in spatial frequency during adaptation. To null the position shift that accompanied the MAE, however, no temporal lag between the attributes was required. This demonstrates that perceived motion and position can be perceptually misbound. Indeed, in certain conditions, subjects perceived the test pattern to drift in one direction while its position appeared shifted in the opposite direction. The dissociation between perceived motion and position of the same test pattern, following identical adaptation, demonstrates that distinguishable neural populations code for these object properties.     

Interocular generalization of orientation specific color aftereffects.

McCollough effects were generated by monocular exposure to alternating red vertical and green horizontal gratings, and observed with achromatic orthogonal gratings that formed concentric square bands. Chromatic aftereffects on appropriately oriented gratings viewed by the contralateral eye were obtained when binocular scanning of the test pattern preceded testing with the contralateral eye. The red-green configuration was reversed between the two eyes. The chromatic aftereffects that appear with the contralateral eye are proposed to be “second order McCollough Effects”.     

Disparity range for local stereopsis as a function of luminance spatial frequency

The disparity range for stereo sensitivity was investigated with spatially filtered bars, tuned narrowly over a broad range of spatial frequencies. When measured with narrow (high spatial frequency) bars the disparity range for stereopsis exceeded two orders of magnitude. The range was reduced with broad (low spatial frequency) bars by an elevation of stereothreshold that increased according to a constant 6deg phase disparity. The upper disparity limit also increased at broad spatial periods but at a lower rate. These size disparity correlations illustrate quantitative stereopsis along a continuum from fine to coarse disparities (± 2deg). A disparity matching task revealed that greater amounts of uncrossed than crossed disparity were required to match suprathreshold disparities. The ratio of standard/matched disparity was lower in both directions for small suprathreshold disparities subtended by broad than by narrow spatial periods. This selective reduction of stereo-efficiency ratio illustrates tuning to coarse disparities subtended by broad (low spatial frequency) stimuli and accounts for the marked reduction of stereoacuity caused by spectacle blur.     

Orientation and spatial frequency selectivity of adaptation to color and luminance gratings

Prolonged viewing of sinusoidal luminance gratings produces elevated contrast detection thresholds for test gratings that are similar in spatial frequency and orientation to the adaptation stimulus. We have used this technique to investigate orientation and spatial frequency selectivity in the processing of color contrast information. Adaptation to isoluminant red-green gratings produces elevated color contrast thresholds that are selective for grating orientation and spatial frequency. Only small elevations in color contrast thresholds occur after adaptation to luminance gratings, and vice versa. Although the color adaptation effects appear slightly less selective than those for luminance, our results suggest similar spatial processing of color and luminance contrast patterns by early stages of the human visual system.     

Gaze direction modulates visual aftereffects in depth and color

Prior physiological studies indicate that gaze direction modulates the gain of neural responses to visual stimuli. Here, we test gaze modulation in the perceptual domain using color and depth aftereffects. After confirming retinotopy of the effects, we employed a balanced alternating adaptation paradigm (adaptation alternates between opponent stimuli) to demonstrate that opposite color and depth aftereffects can co-develop at the same retinal location for different gaze directions. The results provide strong evidence for (a) gaze modulation of aftereffects, (b) generality of gaze modulation across two visual attributes, and (c) perceptual correlates of the modulation of neural activity by gaze direction.     

Adaptation duration affects the spatial selectivity of facial aftereffects

Adaptation processes in human early visual cortical areas are sensitive to the exposure time of the adaptor stimulus. Here we investigated the effect of adaptation duration at the higher, shape-specific stages of visual processing using facial adaptation. It was found that long-term (5s) adaptation evokes facial aftereffects consisting of a position invariant as well as a position-specific component. As a result of adaptation to a female face, test faces were judged more masculine when they were displayed in the same location as the female adaptor face, as compared to that when they were presented in the opposite visual hemifield. However, aftereffects evoked by short-term (500 ms) adaptation were found to be entirely position invariant. In accordance with these behavioral results, we found that the adaptation effects, measured on the amplitude of the N170 ERP component consisted of a position-specific component only after long-term, but not after short-term adaptation conditions. These results suggest that both short and long exposure to a face stimulus leads to adaptation of position invariant face-selective processes, whereas adaptation of position-specific neural mechanisms of face processing requires long-term adaptation. Our findings imply that manipulating adaptation duration provides an opportunity to specifically adapt different neural processes of shape-specific coding and to investigate their stimulus selectivity.     

Shifts in perceived periodicity induced by temporal modulation and their influence on the spatial frequency tuning of two aftereffects

Temporal modulation of sinewave grating stimuli is known to alter their apparent periodicity. It is shown that this may occur without significant loss in spatial frequency discrimination performance. Two aftereffects are shown to have different spatial selectivity when a temporally modulated grating is used as an adapting stimulus: one, contrast threshold elevation, is specific to the true spatial frequency of the adapting stimulus; the other, the spatial frequency shift, is specific to the apparent spatial frequency. The implications of these findings for the visual processing of contour and pattern are discussed.     

EEG oscillations in the gamma and alpha range respond differently to spatial frequency

Physical properties of visual stimuli affect electrophysiological markers of perception. One important stimulus property is spatial frequency (SF). Therefore, we studied the influence of SF on human alpha (8-13 Hz) and gamma (>30 Hz) electroencephalographic (EEG) responses in a choice reaction task. Since real world images contain multiple SFs, an SF mixture was also examined. Event related potentials were modulated by SF around 80 and 300 ms. Evoked gamma responses were strongest for the low SF and the mixture stimulus; alpha responses were strongest for high SFs. The results link evoked and induced alpha and evoked gamma responses in human EEG to different modes of stimulus processing.     

The contrast sensitivity gradient across the human visual field: with emphasis on the low spatial frequency range

The regional variation of contrast sensitivity along the greater extent of each of the four principal hemi-meridia of the normal human eye was determined under photopic conditions using horizontally-orientated sinusoidal grating stimuli. The stimuli were well localized in space and frequency, and special attention was paid to the low spatial frequency range. The results confirm that contrast sensitivity is maximal for central vision for all test spatial stimuli. Extra-foveal fall-off in sensitivity can be represented as a linear function of eccentricity if the latter is expressed in relative units (i.e. periods of the stimulus). The regional variation parameter depends upon whether the horizontal or vertical field is tested and upon the spatial frequency of stimulation. The visible spatial frequency range (0.05-24 c/deg) can be approximately described by just three different rules. The fact that more than one rule is found bears upon current models of the functional organization of the visual system.