Orientation-specific aftereffects and illusions in the perception of brightness

Orientation-specific brightness aftereffects were found when vertical and horizontal gratings of the same space-average luminance were viewed following alternate exposure to vertical and horizontal gratings that differed in space-average luminance. The vertical test grating appeared bright following exposure to a dim vertical grating, and dim after a bright vertical grating had been viewed. This aftereffect did not occur when the adaptation gratings had been seen by one eye and the test gratings by the other eye. An orientation-specific illusion in the perception of brightness was also found, with the white sectors of a vertical grating appearing brighter against a background of horizontal lines than they did against a background of vertical lines. Both distortions imply that there are detectors in the human visual system that are conjointly tuned to luminance and contour orientation.     

Effect of luminance contrast on the motion aftereffect

The effects of luminance contrast and spatial frequency on the motion aftereffect were investigated. The point of subjective equality for velocity was measured as an index of the motion aftereffect. The largest effect was observed when a low contrast grating (5%) was presented as a test stimulus after adaptation to a high contrast grating (100%) in the low spatial frequency condition (0.8 cycle deg.-1). On the whole, the effect increased with increasing adapting contrast and with decreasing test contrast or spatial frequency. Small effects were observed at high test contrasts. These results were inconsistent with those of Keck, Palella, and Pantle in 1976. Analysis showed that there was no saturation on velocity of the motion aftereffect above 5% of the contrast although Keck, et al. (1976) found that the incremental increases of the effect above 3% adapting contrast were small.     

Psychophysical evidence for an extrastriate contribution to a pattern-selective motion aftereffect

A stationary vertical test grating appears to drift to the left after adaptation to an inducing grating drifting to the right, this being known as the motion aftereffect (MAE). Pattern-specific motion aftereffects (PSMAEs) induced by superimposed pairs of gratings in which the component gratings drift up and down but the observer sees a single coherent plaid drifting to the right have been investigated. Two experiments are reported in which it is demonstrated that the PSMAE is tuned more to the motion of the pattern than to the orientation and direction of motion of the component gratings. However, when subjects adapt to the component gratings in alternation, aftereffect magnitude is dependent upon the individual grating orientations and motion directions. These results can be interpreted in terms of extrastriate contributions to the PSMAE, possibly arising from the middle temporal area, where some cells, unlike those in striate cortex (V1), are tuned to pattern motion rather than to component motion.     

Luminance as a factor in the ability of achromatic gratings to interfere with MeCollough effects

McCollough effects were reduced in assessed strength by having observers view achromatic gratings that varied in space-average luminance. Whether the gratings were presented before or after induction of McCollough effects, high luminance gratings interfered with the effects more than did low luminance gratings. If considered within the context of a conditioning model, this finding may be interpreted as inconsistent with other characteristics of McCollough effects.     

Contour specificity of the McCollough effect

Subjective estimates of McCollough aftereffect strength are significantly reduced when certain spatial features of the line grating patterns are manipulated. Results are dependent upon whether the spatial parameters of the test or inspection patterns are altered. Changing the angular slant, contour sharpness, or contour completeness of the inspection gratings does not affect aftereffect strength, but changing the spatial frequency, contour sharpness, or contour completeness of the test gratings does. The implications of these results are discussed with regard to theories offered to explain the McCollough effect.     

Motion over the retina and the motion aftereffect.

The motion aftereffect (MAE) was measured with retinally moving vertical gratings positioned above and below (flanking) a retinally stationary central grating (experiments 1 and 2). Motion over the retina was produced by leftward motion of the flanking gratings relative to the stationary eyes, and by rightward eye or head movements tracking the moving (but retinally stationary) central grating relative to the stationary (but retinally moving) surround gratings. In experiment 1 the motion occurred within a fixed boundary on the screen, and oppositely directed MAEs were produced in the central and flanking gratings with static fixation; but with eye or head tracking MAEs were reported only in the central grating. In experiment 2 motion over the retina was equated for the static and tracking conditions by moving blocks of grating without any dynamic occlusion and disclosure at the boundaries. Both conditions yielded equivalent leftward MAEs of the central grating in the same direction as the prior flanking motion, ie an MAE was consistently produced in the region that had remained retinally stationary. No MAE was recorded in the flanking gratings, even though they moved over the retina during adaptation. When just two gratings were presented, MAEs were produced in both, but in opposite directions (experiments 3 and 4). It is concluded that the MAE is a consequence of adapting signals for the relative motion between elements of a display.     

Perceptual, oculomotor, and neural responses to moving color plaids

Moving plaids constructed from two achromatic gratings of identical luminance contrast are known to yield a percept of coherent pattern motion, as are plaids constructed from two identical chromatic (e.g. isoluminant red/green) gratings. To examine the interactive influences of chromatic and luminance contrast on the integration of visual motion signals, we constructed plaids with gratings that possessed both forms of contrast. We used plaids of two basic types, which differed with respect to the phase relationship between chromatic and luminance modulations (after Kooi et al, 1992 Perception 21 583-598). One plaid type ('symmetric') was made from component gratings that had identical chromatic/luminance phase relationships (e.g. both components were red-bright/green-dark modulation). The second plaid type ('asymmetric') was made from components that had complimentary phase relationships (i.e. one red-bright/green-dark grating and one green-bright/red-dark grating). Human subjects reported that the motion of symmetric plaids was perceptually coherent, while the components of asymmetric plaids failed to cohere. We also recorded eye movements elicited by both types of plaids to determine if they are similarly affected by these image cues for motion coherence. Results demonstrate that, under many conditions, eye movements elicited by perceptually coherent vs noncoherent plaids are distinguishable from one another. To reveal the neural bases of these perceptual and oculomotor phenomena, we also recorded the responses of neurons in the middle temporal visual area (area MT) of macaque visual cortex. Here we found that individual neurons exhibited differential tuning to symmetric vs asymmetric plaids. These neurophysiological results demonstrate that the neural mechanism for motion coherence is sensitive to the phase relationship between chromatic and luminance contrast, a finding which has implications for interactions between 'color' and 'motion' processing streams in the primate visual system.     

Recovery from adaptation to moving gratings

Short-term adaptation to moving sinusoidal gratings results in a motion aftereffect which decays in time. The time decay of the motion aftereffect has been measured psychophysically, and it is found to depend on (i) the spontaneous recovery from the adapted state, and (ii) the contrast of the test grating. We have measured the decays for various test conditions. An extrapolation of the measurements allows us to obtain a decay which represents the time course of the spontaneous recovery of the direction-sensitive mechanisms.     

Purely chromatic perception of motion in depth: two eyes as sensitive as one

Motion hyperacuity (phase) thresholds were measured for both lateral and stereoscopic oscillatory motion in both luminance and equiluminant red/green gratings of 2 cycles per degree. Thresholds for lateral chromatic motion did not exhibit the inhibitory fall-off at low temporal frequencies that was found for luminance motion. Phase thresholds for purely chromatic motion were substantially higher than those for luminance gratings, in proportion to the ratio of cone signal modulation, but they could be predicted from the corresponding contrast sensitivities for both types of stimulus. Stereomovement thresholds in luminance gratings showed the stereomovement suppression effect relative to monocular motion sensitivity previously reported for line stimuli, but purely chromatic gratings did not. Together with the lack of an inhibitory fall-off, these results imply that chromatic and luminance motion are processed by different neural pathways, and that the chrominance pathway is capable of supporting a strong percept of stereoscopic motion from purely chromatic gratings.     

Color—luminance relationships and the McCollough effect

The McCollough effect is an orientation-specific color aftereffect induced by adapting to colored gratings. We examined how the McCollough effect depends on the relationships between color and luminance within the inducing and test gratings and compared the aftereffects to the color changes predicted from selective adaptation to different color—luminance combinations. Our results suggest that the important contingency underlying the McCollough effect is between orientation and color—luminance direction and are consistent with sensitivity changes within mechanisms tuned to specific color—luminance directions. Aftereffects are similar in magnitude for adapting color pairs that differ only in S cone excitation or L and M cone excitation, and they have a similar dependence on spatial frequency. In particular, orientation-specific aftereffects are induced for S cone colors even when the grating frequencies are above the S cone resolution limit. Thus, the McCollough effect persists even when different cone classes encode the orientation and color of the gratings.     

Coherence and transparency of moving plaids composed of Fourier and non-Fourier gratings

We examined the perceptual coherence of two-component moving plaids. The gratings that constituted the plaids were either standard Fourier gratings (F), in which luminance was determined by a drifting sinusoid, or non-Fourier gratings (NF), in which the contrast of a random background was modulated by a drifting sinusoid. These NF gratings are examples of stimuli that generate a compelling percept of motion, even though they fail to elicit a motion signal from motion analyzers based on standard cross-correlation (Chubb & Sperling, 1988). Naive observers viewed three types of stimuli consisting of superpositions of these two components: (1) two standard drifting gratings (F/F), (2) two non-Fourier drifting gratings (NF/NF), and (3) one standard and one non-Fourier drifting grating (F/NF). As expected, the F/F stimulus yielded a compelling percept of coherent motion. The dominant percept of all the observers for the NF/NF stimulus was one of coherent motion, provided that both gratings were visible and of approximately equal contrast. None of the observers reported a dominant percept of coherent motion for the F/NF condition, over a wide range of contrasts for the two grating components and across two varieties of NF gratings. In view of the results of Albright (1992) and Albright and Chaudhuri (1989), that show that single cells in macaque V1 and MT respond to both F and NF motion, one cannot interpret our findings as evidence that F and NF motion are processed independently. Alternative, “higher level” interpretations based on the intrinsically ambiguous nature of the stimuli and physical laws governing the appearance of transparent objects are discussed.     

Coherence and transparency of moving plaids composed of Fourier and non-Fourier gratings.

We examined the perceptual coherence of two-component moving plaids. The gratings that constituted the plaids were either standard Fourier gratings (F), in which luminance was determined by a drifting sinusoid, or non-Fourier gratings (NF), in which the contrast of a random background was modulated by a drifting sinusoid. These NF gratings are examples of stimuli that generate a compelling percept of motion, even though they fail to elicit a motion signal from motion analyzers based on standard cross-correlation (Chubb & Sperling, 1988). Naive observers viewed three types of stimuli consisting of superpositions of these two components: (1) two standard drifting gratings (F/F), (2) two non-Fourier drifting gratings (NF/NF), and (3) one standard and one non-Fourier drifting grating (F/NF). As expected, the F/F stimulus yielded a compelling percept of coherent motion. The dominant percept of all the observers for the NF/NF stimulus was one of coherent motion, provided that both gratings were visible and of approximately equal contrast. None of the observers reported a dominant percept of coherent motion for the F/NF condition, over a wide range of contrasts for the two grating components and across two varieties of NF gratings. In view of the results of Albright (1992) and Albright and Chaudhuri (1989), that show that single cells in macaque V1 and MT respond to both F and NF motion, one cannot interpret our findings as evidence that F and NF motion are processed independently. Alternative, "higher level" interpretations based on the intrinsically ambiguous nature of the stimuli and physical laws governing the appearance of transparent objects are discussed.     

The effects of adaptation to square-wave gratings as a function of grating orientation

An adaptation technique was used to measure the selectivity or tuning for grating orientation in the visual system for different orientations of the inspection stimulus. Duration thresholds for grating patterns of constant luminance were determined for 13 test gratings oriented from ±5 to 90 deg away from each of five adaptation gratings: 0, 22, 45, 67, and 90 deg. Threshold data obtained for test gratings without prior adaptation indicated higher sensitivity for gratings oriented along the horizontal and vertical axis than along the oblique axis. After adaptation, thresholds increased (sensitivity was reduced) for gratings having similar orientations as the test gratings. However, the functions relating sensitivity reduction to degree of angular disparity between test and adaptation grating did not vary across the five inpsection orientations, i.e., selectivity or tuning for grating orientation appeared to be independent of the orientation of the adapting stimulus.     

Lightness, equivalent backgrounds, and anchoring

Observers compared two center/surround configurations haploscopically. One configuration consisted of a standard surface surrounded by two, three, or four surfaces, each with a different luminance. The other configuration consisted of a comparison surface surrounded by a single annulus that varied in luminance. Center surfaces always had the same luminance but only appeared to have the same lightness with certain annuli (equivalent backgrounds). For most displays, the luminance needed to obtain an equivalent background was close to the highest luminance in the standard surround configuration. Models based on the space-average luminance or the space-average contrast of the standard surround configuration yielded poorer fits. Implications for computational models of lightness and for candidate solutions to the anchoring problem are discussed.     

Induction of the McCollough effect II: Two different mechanisms

An orientation-specific chromatic aftereffect was observed when a single colored grating was used as an induction stimulus. The magnitude of the aftereffect was compared to that obtained when alternating orthogonal gratings in complementary hues were used as induction stimuli. The two-stimulus condition produced a stronger aftereffect than a single-stimulus condition. This facilitation was also obtained when a colored plain square with no grating was substituted for the second colored grating in the two-stimulus condition. The results suggest that the McCollough effect involves adaptation of two different mechanisms, one which is orientation-specific and one which is not.     

Storage of spatially specific threshold elevation

The decay of several visual aftereffects may be prolonged by interposing a period of light-free or pattern-free viewing between adaptation and testing. We demonstrate that this storage phenomenon can be observed using the threshold elevation aftereffect that follows inspection of a high-contrast grating pattern. Control experiments comparing thresholds for vertical and horizontal gratings after adaptation to a vertical grating reveal that the stored aftereffect, like its unstored counterpart, is pattern-selective. Storage is equally pronounced with stimuli that are detected by pattern-analyzing or movement-analyzing visual channels. Unlike other aftereffects, the threshold-elevation aftereffect requires that the storage period be light-free; no storage is seen if a blank field is inspected between adaptation and testing. The results are discussed with respect to the nature of visual aftereffects, and possible cognitive or physiological models of storage.     

Contrast and spatial frequency components in visual preferences of newborns

A preferential-looking procedure was used to investigate newborns' responses to square-wave gratings varying in spatial frequency and contrast. A preliminary study confirmed that the gratings used in the experiment were suprathreshold. In the experiment newborns' preference for a grating of 0.1 cycle deg-1 within the peak contrast sensitivity range was examined. Reduction in the contrast of this grating led to a transfer of the preference to a high-contrast grating of the same space-averaged luminance with a spatial frequency outside this range (0.42 cycle deg-1). The findings are discussed with reference to the role of the contrast sensitivity function in pattern preferences of newborns: it is suggested that contrast and spatial frequency interact in determining pattern preferences.     

Contrast and frequency competition for orientation-contingent color aftereffects

A “competition” paradigm was developed to examine separately the effects of pattern contrast and spatial frequency characteristics on the strength of orientation-contingent color aftereffects (McCollough effects). After adapting to alternately presented red/black and green/black square-wave gratings (one horizontal, one vertical), 11 subjects viewed seven different kinds of test patterns. Unlike Standard McCollough effect test stimuli, the present patterns had variable luminance profiles running both horizontally and vertically within each test pattern area. Forced choice responses were used to determine which aftereffect color (red or green) appeared, as characteristics of vertical and horizontal luminance profiles were varied separately among test stimulus types. We conclude that pattern contrast and human contrast sensitivity account for aftereffect colors in such stimuli. When contrast is taken into consideration, aftereffects are not predicted by similarity between adaptation and test pattern Fourier characteristics, nor are they predicted by the width, per se, of pattern elements.     

Brightness enhancement and the Talbot level in stationary gratings

At low spatial frequencies, the perceived brightness of the light phase of a stationary square-wave grating is greater than the brightness of a solid field of equal physical luminance. That increase in the perceived brightness of a grating at low spatial frequencies is analogous to the brightness enhancement observed in a flickering light at low temporal frequencies. At or above the critical spatial frequency—the visual resolution threshold—the brightness of a grating is determined by its space-average luminance, just as the brightness of a flickering light at or above the critical flicker frequency is determined by its time-average luminance in accordance with Talbot’s law. Thus, Talbot’s law applies in the spatial as well as the temporal domain. The present study adds to the evidence that temporal and spatial frequency play analogous roles in some aspects of brightness vision.     

Apparent velocity of motion aftereffects in central and peripheral vision

Adapting to a drifting grating (temporal frequency 4 Hz, contrast 0.4) in the periphery gave rise to a motion aftereffect (MAE) when the grating was stopped. A standard unadapted foveal grating was matched to the apparent velocity of the MAE, and the matching velocity was approximately constant regardless of the visual field position and spatial frequency of the adapting grating. On the other hand, when the MAE was measured by nulling with real motion of the test grating, nulling velocity was found to increase with eccentricity. The nulling velocity was constant when scaled to compensate for changes in the spatial 'grain' of the visual field. Thus apparent velocity of MAE is constant across the visual field, but requires a greater velocity of real motion to cancel it in the periphery. This confirms that the mechanism underlying MAE is spatially-scaled with eccentricity, but temporally homogeneous. A further indication of temporal homogeneity is that when MAE is tracked, by matching or by nulling, the time course of temporal decay of the aftereffect is similar for central and for peripheral stimuli.