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PubMed1993

Directional adaptation effects with contrast modulated stimuli.

In this study we investigated the directional selectivity of the mechanisms used to encode the motion of contrast modulated stimuli. Unlike luminance-domain stimuli, contrast-domain stimuli fail to generate convincing motion aftereffects. On the other hand, contrast-domain stimuli produce other direction specific adaptation effects that are as large as those produced by luminance-domain stimuli. The results of this study indicate that the dissociation between these two measures of directional selectivity might result from differences in the temporal nature of the test probe they employ. Specifically, it is suggested that the mechanisms which encode the motion of contrast modulated stimuli reveal their directional selectivity only when tested with a dynamic stimulus.

Adaptation, Ocular↗
PubMed1995

Local and global factors in spatially-contingent coloured aftereffects.

Dodwell and O'Shea's [(1987) Vision Research, 27, 569-580] conclusions that contingent coloured aftereffects (CAEs) depend on gobal pattern organization were investigated in four experiments. In Expt 1, we replicated findings that CAEs can be induced with complex patterns (concentric circles; radial spokes) under conditions of systematic eye movements. Contrary to Dodwell and O'Shea's argument that eye movements should uniformly cancel local orientation-colour contingencies, leaving only global effects, we reduced CAE magnitude by halving the diameter of the test stimuli. This suggests that cancellation did not occur uniformly over whole patterns, and that CAEs observed on these patterns are the residuals of uncancelled local orientation-colour contingencies. In Expt 2 we used central-fixation induction procedures to demonstrate that it is possible to induce CAEs with randomly-organized and locally-orthogonal orientation components. These findings are inconsistent with Dodwell and O'Shea's failure to observe CAEs under these conditions, and with their conclusion that global organization is necessary for CAE induction. However, CAEs induced with randomly-organized components were significantly weaker than those induced with globally-organized components. We examined the contribution of global organization in two additional experiments. In Expt 3 we induced CAEs with randomly-organized components under conditions in which the need for central fixation was removed, and found that CAE strength was directly related to the organization as well as the density of local-orientation components. In Expt 4, we found that the global organization of local-orientation components enhanced CAE strength only in regions away from the edges of these components: pattern organization did not affect the strength of CAEs at edges. We interpret these findings as evidence that CAEs may involve separate edge- and spread-colour components, and conclude that such components may account for observations previously attributed to global pattern geometry.

Afterimage↗
PubMed1995

Orientation and color processing for partially occluded objects.

The McCollough orientation-contingent color aftereffect could be equally well elicited by either a full test pattern of black and white stripes or a similar test pattern that was largely occluded by white surfaces, provided the latter stripes were made to appear as through continuing under the white surfaces--by means of stereo depth cues. The color aftereffect appeared concentrated around the edges of the stripes that protruded out from under the white surfaces; surfaces that themselves continued to appear a uniform white as shown by color matches. These results suggest that occluded, perceptually-continued edges can elicit the McCollough effect, which is generally thought to occur quite early in the visual pathway.

Adaptation, Ocular↗
PubMed1995

Motion aftereffect with flickering test patterns reveals higher stages of motion processing.

A series of experiments was conducted to clarify the distinction between motion aftereffects (MAEs) with static and counterphasing test patterns (static and flicker MAEs). It was found that while the motion of higher-order structure, such as areas defined by texture, flicker, or stereoscopic depth, induces little static MAE, such motion reliably generates flicker MAE. It was also found that static and flicker MAEs were induced in opposite directions for stimuli in which first- and second-order structures moved in opposite directions (compound graftings of 2f + 3f or 2f + 3f + 4f, shifting a half cycle of 2f). When the test was static, MAE was induced in the direction opposite to the first-order motion; but when the test was counterphasing, MAE was induced in the direction opposite to the second-order motion. This means that static MAE is predominantly induced by first-order motion, but that flicker MAE is affected strongly by second-order motion, along with first-order motion. The present results suggest that static MAE primarily reflects adaptation of a low-level motion mechanism, where first-order motion is processed, while flicker MAE reveals a high-level motion processing, where both first- and second-order motion signals are available.

Adaptation, Ocular↗
PubMed1995

Induced twinkle aftereffect as a probe of dynamic visual processing mechanisms.

After viewing a blank patch surrounded by a dynamic noise stimulus (a video "snowstorm"), viewers report the prolonged perception of twinkle in the unstimulated region of the blank patch. We compare this induced twinkle aftereffect to the filling-in phenomenon, which may be seen in a small blank region, under similar test conditions but during stimulation. We found that strong induced twinkle aftereffects were seen both centrally and peripherally for blank test regions from 0.5 deg to as large as 20 deg in diameter, whereas filling-in was seen centrally only for test patch diameters smaller than 0.75 deg, becoming stronger peripherally but still limited to test regions less than about 3 deg in diameter. Lower noise density and larger noise element size facilitated filling-in but had little effect on the induced twinkle aftereffect. Conversely, noise frame rate had little effect on filling-in but had to be faster than 10 frames/sec to produce a twinkle aftereffect. Induced twinkle showed binocular superiority but no interocular transfer. The binocular superiority was partially explained by monocular blankout of the dynamic noise by the blank field in the occluded eye. These results all imply a different mechanism for the induced twinkle aftereffect than for filling-in. We consider a model in which the induced twinkle aftereffect is produced by post-inhibitory rebound in complex cells.

Adaptation, Ocular↗
PubMed1995

Motion aftereffect after monocular adaptation to filled-in motion at the blind spot.

Although the blind spot encodes no visual information, one never perceives an odd blob or blank there, but sees a complete scene of the world even when viewing monocularly. This phenomenon called "filling-in" might be related to mechanisms essential to surface perception, but the neural representation has still been unclear. To determine at what stage the computation for filling-in is established in the visual system, whether prolonged observation of a filled-in motion including the blind spot of one eye could cause motion aftereffect at the corresponding visual field of the other eye was examined. The result was positive--interocular transfer of motion aftereffect was obtained at the tested eye. This finding suggests the possibility that real motion and filled-in motion share a common motion pathway in an early stage in the human visual system.

Adaptation, Ocular↗
PubMed1995

Modulation of motion aftereffect by surround motion and its dependence on stimulus size and eccentricity.

As a mechanism to detect differential motion, we have proposed a model of 'a motion contrast detector' and have shown that it can explain the perceptual change from motion capture to induced motion with increasing stimulus size and decreasing eccentricity. To further test the feasibility of the model, we examined the effect of surround motion on the motion aftereffect (MAE) elicited in the center. Using a drifting grating surrounded by another drifting grating, the duration of MAE in the center after adaptation was measured for various surround velocities (Expt 1). MAE was stronger when the surround moved oppositely to, than together with, the center. This finding was consistent with some previous reports. Using similar stimuli, MAE was measured at various stimulus sizes and eccentricities by the cancellation technique (Expt 2). The effect of surround modulation turned out to vary with both size and eccentricity. We examined if the apparent dependence on eccentricity could reflect a simpler effect of cortical size when the data were rescaled according to a linear scaling factor. We interpret our results in terms of motion contrast detectors, possibly located in the area MT.

Adaptation, Ocular↗
PubMed1995

Motion aftereffect with flickering test stimuli depends on adapting velocity.

Temporal tuning property of motion aftereffect (MAE) with flickering test stimuli (flicker MAE) was examined. Using sinusoidal gratings of several spatial frequencies (SF), MAE strength was measured for various adapting temporal frequencies (TF). Unlike the traditional MAE with static test field, the results indicated that flicker MAE did not depend on TF. Rather, when plotted as a function of velocity, the peaks had approximately the same adapting velocity of 5-8 deg/sec for all SF conditions tested, suggesting velocity dependence. This is further support of the idea that the two kinds of MAE are of different origin and suggests a higher origin of flicker MAE, perhaps in the area MT or MST.

Adaptation, Ocular↗
PubMed1995

A comparison of temporal integration in children with a specific reading disability and normal readers.

Previous research has suggested that whereas some techniques show that subjects with a specific reading disability (SRD) have greater visible persistence than controls, a temporal integration of form technique does not. It has been suggested that the failure of the temporal integration task to show a difference results from the spatial separation between stimuli used in the technique. In this study SRD and control subjects were compared on a new version of a temporal integration task, under two conditions varying the spatial separation of elements in the display. It was predicted that there would be no difference between groups when spatial separation was large, but that the SRD subjects would show greater visible persistence at small separations. Neither prediction was confirmed, denying previous explanations of why the temporal integration task does not discriminate between groups. Analysis of errors showed that the result was not due to inattention nor to a general deficit on the part of the SRD subjects.

Afterimage↗
PubMed1995

A target in real motion appears blurred in the absence of other proximal moving targets.

For exposure durations longer than about 40 msec, a field of dots in sampled motion has been reported to appear less smeared than predicted from the visual persistence of static displays. This reduction of perceived smear has been attributed to a motion "deblurring" mechanism. However, it has been long recognized that an isolated target moving continuously in a dark field appears to be extensively smeared. To reconcile these apparently contradictory observations, we investigated the effect of dot density on the extent of perceived smear for a single moving dot and for fields of dots with densities ranging from 0.75 to 7.5 dots/deg2. Bright targets were presented in continuous motion against a photopically illuminated background field. The results reconcile previous conflicting observations by showing that the length of perceived smear decreases systematically with dot density for exposure durations longer than about 50 msec. In three additional experiments, we arranged the spatial configuration of the targets to evaluate whether motion deblurring results primarily from a motion compensation mechanism (such as integration within the spatiotemporally oriented receptive fields of putative motion mechanisms) or from inhibition exerted by spatiotemporally adjacent targets. The results show that the activation of motion mechanisms is not a sufficient condition for motion deblurring and that the reduction of perceived smear requires the presence of spatiotemporally adjacent targets. Taken together, these findings suggest that motion deblurring results primarily from masking exerted by spatiotemporally proximal targets.

Afterimage↗
PubMed1994

The influence of contrast adaptation on color appearance.

Most models of color vision assume that signals from the three classes of cone receptor are recoded into only three independent post-receptoral channels: one that encodes luminance and two that encode color. Stimuli that are equated for their effects on two of the channels should be discriminable only to the remaining channel, and are thus assumed to isolate the responses of single channels. We used an asymmetric matching task to examine whether such models can account for changes in color appearance following adaptation to contrast--to temporal variations in luminance and chromaticity around a fixed mean luminance and chromaticity. The experiments extend to suprathreshold color appearance the threshold adaptation paradigm of Krauskopf, Williams and Heeley [(1982) Vision Research, 32, 1123-1131]. Adaptation changes the perceived color of chromatic test stimuli both by reducing their saturation (contrast) and by changing their hue (direction within the equiluminant plane). The saturation losses are largest for test stimuli that lie along the chromatic axis defining the adapting modulation, while the hue changes are rotations away from the adapting direction and toward an orthogonal direction within the S and L-M plane. Similar selective changes in both perceived color and perceived lightness occur following adaptation to stimuli that covary in luminance and chromaticity. The selectivity of the aftereffects for multiple directions within color-luminance space is inconsistent with sensitivity changes in only three independent channels. These aftereffects suggest instead that color appearance depends on channels that can be selectively tuned to any color-luminance direction, and that there are no directions that invariably isolate responses in only a single channel. We use the perceived color changes to examine the spectral sensitivities of the chromatic channels and to estimate the distribution of channels. We also examine how adaptation alters the contrast-response function, how it affects reaction times for luminance and chromatic contrast, the extent to which the aftereffects exhibit interocular transfer, and the way in which the perceived color changes differ from those induced by conventional light adaptation.

Adaptation, Ocular↗
PubMed1994

Adaptation to second-order motion results in a motion aftereffect for directionally-ambiguous test stimuli.

The magnitude of the motion aftereffect (MAE) obtained following adaptation to first- or second-order motion was measured in two experiments using a nulling method. The second-order motion adaptation stimulus was composed of contrast-modulated noise produced by multiplying two-dimensional random noise by a drifting, 1 c/deg, vertical sine grating. The first-order motion adaptation stimulus was composed of luminance-modulated noise produced by adding, rather than multiplying, the sine grating and noise field. The test stimuli were directionally-ambiguous first- or second-order motion patterns composed of either two oppositely drifting sine gratings added to static noise or its contrast-modulated equivalent. The amplitudes of the two drifting components were manipulated such that as one increased in amplitude the other decreased in amplitude by the same degree. This technique was employed to estimate the null point at which the test no longer appeared to drift in the direction opposite the adaptation direction. In the first experiment all stimuli were equated for visibility by presenting them at the same multiple of threshold and all possible combinations of first- and second-order motion adaptation and test stimuli were examined. The results were similar for all conditions: following adaptation the amplitude of the test component drifting in the same direction as adaptation needed to be approximately twice that of the oppositely drifting component in order to null the perception of unidirectional motion of the test. In a second experiment, the effects of manipulating the amplitude (visibility) of the first- and second-order motion adaptation stimuli on MAE magnitude were investigated. This revealed an approximately linear relationship between MAE magnitude and the amplitudes of the adaptation stimuli. The results demonstrate that, contrary to the findings of several previous studies, adaptation to second-order motion does produce a substantial movement aftereffect. Cross-adaptation between first- and second-order motion stimuli also occurs under appropriate conditions and produces aftereffects that are comparable in magnitude when the stimuli are equated for visibility.

Adaptation, Ocular↗
PubMed1994

Movement aftereffect of bi-vectorial transparent motion.

Two moving random-pixel arrays (RPAs) were presented simultaneously in the same target field. These RPAs are perceived as two superimposed transparent moving sheets. Although two directions are perceived simultaneously during stimulus presentation, the movement aftereffect (MAE) is unidirectional. The visual system averages both motion signals in the MAE. For motion vectors of equal magnitude and perpendicular direction the MAE direction is the inverse of the sum of both vectors. In the first experiment we measured perceived direction of the MAE of transparent motion for a range of speed combinations. Results indicate that vector summation only predicts the correct MAE direction for combinations of equal speeds. It is suggested that the direction of the MAE of transparent motion is a resultant of the weighted summation of the component inducing vectors. The question then arises what determines the weighting factors. Directional sensitivity and MAE duration of the individual vectors under transparent conditions were measured and used to weigh the vectors and predict the MAE direction of transparent motion. Statistical analyses showed that MAE duration is a better basis to determine the weighting factors predicting the direction of the MAE of transparent motion than component sensitivity. The direction of the MAE of transparent motion thus seems to be determined by the amount of adaptation to the component vectors as reflected by MAE duration. The results suggest that this gain control cannot be located in the individual motion detectors and must be situated at or after some subsequent cooperation stage of the human motion analysis system.

Adaptation, Ocular↗
PubMed1994

Complete interocular transfer of motion aftereffect with flickering test.

It has been suggested that motion aftereffect with static test patterns (static MAE) reflects activities at a lower level system that dominantly processes first-order motion, while MAE with a directionally ambiguous test (flicker MAE) reveals a higher level system where second-order motion signals as well as first-order signals are available. To test this hypothesis, we examined interocular transfer of static and flicker MAE. Flicker MAE should transfer more efficiently than static MAE if it occurs at a higher level system. In the first experiment, the adaptation stimulus was a drifting luminance grating (first-order motion), or a drifting grating defined by flicker or texture difference (second-order motion). The test stimulus was a luminance grating, either static or counterphasing. The results indicated that static MAE, which was induced only by first-order motion, transferred partially, as has been reported in previous studies, but the transfer of flicker MAE was nearly perfect with either first- or second-order adaptation stimuli. The second experiment with varied adaptation contrast indicated that this complete transfer was not due to a ceiling effect. These results supported the hypothesis that the underlying mechanism for flicker MAE is located at a level higher than the mechanism for static MAE.

Adaptation, Physiological↗
PubMed1994

The contribution of one-dimensional motion mechanisms to the perceived direction of drifting plaids and their after effects.

When motion aftereffects (MAEs) are measured by adapting to a drifting plaid (simultaneous adaptation) or by adapting to the plaid's component gratings in alternation (alternating adaptation), it has been shown that the velocity and duration of the MAE are smaller in the latter case [Wenderoth, P., Bray, R. & Johnstone, S. (1988) Perception, 17, 81-91; Burke, D. & Wenderoth, P. (1993) Vision Research, 33, 351-359]. However, Burke and Wenderoth additionally reported that the directions of MAEs induced by simultaneous and alternating adaptation were identical, an apparent inconsistency if the differences in duration and velocity were due to the presence of "blobs" at the component grating intersects in the simultaneous case. Presumably, the direction of the "blobs" should also affect perceived plaid direction during adaptation and, hence, the MAE direction. In five experiments, we have measured both perceived adapting plaid and MAE direction, tested with both alternating and simultaneous adaptation, measured interocular transfer of plaid-induced MAEs and obtained MAE and plaid direction judgments under monocular and binocular viewing conditions. All of the data indicate that there is a blob tracking mechanism which is preferentially stimulated by plaids whose component gratings have high spatial frequency, low temporal frequency and high contrast. Differences between simultaneous and alternating adaptation emerge only when more optimal blobs are used, thus accounting for Burke and Wenderoth's failure to find a difference. The data also support Burke and Wenderoth's claim that the blob tracking mechanism is monocular: alternating and simultaneous adaptation produce identical MAEs under interocular transfer conditions, even using plaids with more optimal blobs. We also report the unexpected finding that plaids with more- and less-optimal blobs appear to drift in directions 20 degrees apart yet their aftereffects differ in direction by only 3-5 degrees. That is, more optimal blob plaids--compared with less optimal blob plaids--change both perceived plaid direction during adaptation and subsequent perceived MAE direction but the latter change is much more modest. Possible explanations of this dissociation are considered.

Adaptation, Ocular↗
PubMed1994

Effect of the ISI on the visible persistence of a stimulus in apparent motion.

The persistence of briefly flashed stimuli undergoing a horizontal apparent motion is assessed as a function of the temporal interval (inter-stimulus interval or ISI) between successive locations. The main result is that the duration of persistence is increased when the ISI is reduced (within the range 1-15 msec). An increase of persistence also occurs when the spatial separation (delta chi) between successive presentations of the moving stimulus becomes larger, a well established result which is replicated here. In both cases, the elevation of persistence suggests that inhibitory processes, which are assumed to underlie the persistence-suppression, have become less efficient. According to the data, it seems that the spatio-temporal parameters of motion, and not the speed as such, are responsible for the strength of inhibition. Namely, optimal inhibition, and thus suppression, would need a minimum amount of time to take place, and would improve with proximity (i.e. with smaller delta chi). Finally, a persistence-suppression decrease is observed when the angular size of the flashed stimuli is reduced (i.e. when higher spatial frequencies become more predominant). A model of transient-on-sustained inhibition accounts well for these results.

Afterimage↗
PubMed1995

Mechanisms of purely subjective contour tilt aftereffects.

Neurones tuned for second-order stimuli--those which have edges defined by properties other than luminance and colour--have been frequently observed in prestriate cortex and in area V2 there are neurones which explicitly and unambiguously signal the orientation of purely subjective contours, i.e. contours with no Fourier components at the orientation of the perceived edge [von der Heydt, R. & Peterhans, (1989) Journal of Neuroscience, 9, 1731-1748]. No neurones in area V1 showed similar tuning characteristics. In addition, it has been demonstrated that like real contours, purely subjective test contours are subject to tilt aftereffects following prolonged viewing of an adapting stimulus. whether that stimulus is real or subjective [Paradiso, M. A., Shimojo, S. & Nakayama, K. (1989) Vision Research, 29, 1205-1213]. This result supports the assertion that the cortical processes responsible for real contour perception are also those giving rise to subjective contour perception. The data reported here further examined this hypothesis. Four experiments show that purely subjective contours exhibit both direct and indirect tilt aftereffects and tilt illusions like those observed with real contours. Further, they provide evidence that direct and indirect subjective contour effects, like direct and indirect real contour effects, arise via the operation of two mechanisms: a low level process, possibly lateral inhibition between orientation channels, and a second "higher-order" process. The data suggest that processing of orientation information beyond the striate cortex is similar to that which occurs in area V1 and the data are consistent with models of contour processing which assume that all perceived contours, both real and subjective, arise from a common mechanism.

Adaptation, Ocular↗
PubMed1996

Disparity tuning of the stereoscopic (cyclopean) motion aftereffect.

Across five experiments this study investigated the disparity tuning of the stereoscopic motion aftereffect (adaptation from moving retinal disparity). Adapting and test stimuli were moving and stationary stereoscopic grating patterns, respectively, created from dynamic random-dot stereograms. Observers adapted to moving stereoscopic grating patterns presented with a given disparity and viewed stationary test patterns presented with the same or differing disparity to examine whether the motion aftereffect is disparity contingent. Across experiments aftereffect duration was greatest when adapting motion and test pattern both were presented with zero disparity and in the plane of fixation. Aftereffect declined as disparity of adapting motion and/or test pattern increased away from fixation, even under conditions in which depth position of adapt and test was equal. This argues against a relative depth separation explanation of the decline, and instead suggests that the amount of adaptable substrate decreases away from fixation.

Adaptation, Physiological↗