Monocular occlusion cues alter the influence of terminator motion in the barber pole phenomenon.
The influence of monocular occlusion cues on the perceived direction of motion of barber pole patterns is examined. Unlike previous studies that have emphasized the importance of binocular disparity, we find that monocular cues strongly influence the perceived motion direction and can even override binocular depth cues. The difference in motion bias for occluders with and without disparity cues is relatively small. Additionally, although 'T-junctions' aligned with occluders are particularly important, they are not strictly necessary for creating a change in motion perception. Finally, the amount of motion bias differs for several stimulus configurations, suggesting that the extrinsic/intrinsic classification of terminators is not all-or-none. (+info)
Local velocity representation: evidence from motion adaptation.
Adaptation to a moving visual pattern induces shifts in the perceived motion of subsequently viewed moving patterns. Explanations of such effects are typically based on adaptation-induced sensitivity changes in spatio-temporal frequency tuned mechanisms (STFMs). An alternative hypothesis is that adaptation occurs in mechanisms that independently encode direction and speed (DSMs). Yet a third possibility is that adaptation occurs in mechanisms that encode 2D pattern velocity (VMs). We performed a series of psychophysical experiments to examine predictions made by each of the three hypotheses. The results indicate that: (1) adaptation-induced shifts are relatively independent of spatial pattern of both adapting and test stimuli; (2) the shift in perceived direction of motion of a plaid stimulus after adaptation to a grating indicates a shift in the motion of the plaid pattern, and not a shift in the motion of the plaid components; and (3) the 2D pattern of shift in perceived velocity radiates away from the adaptation velocity, and is inseparable in speed and direction of motion. Taken together, these results are most consistent with the VM adaptation hypothesis. (+info)
Quantitative depth for a phantom surface can be based on cyclopean occlusion cues alone.
Liu, L., Stevenson, S.B., and Schor, C.M. (1994, Nature, 367, 66-669) reported quantitative stereoscopic depth in a phantom rectangle which appeared to lack conventional matching elements. Later, Gillam, B.J. (1995, Nature, 373, 202-203) and Liu, L., Stevenson, S.B., and Schor, C.M. (1995, Nature, 373, 203) and Liu, L., Stevenson, S.B., and Schor, C.M. (1997, Vision Research, 37(5), 633-644) indicated that the varying depth of the phantom rectangle could be based on stereoscopic matching. To remove the contaminating effects of conventional stereopsis from the Liu et al. (1994) original example, we presented a pair of parallel vertical lines to each eye where there is a central gap in the right line for the left eye's view and in the left line for the right eye's view. Observers saw a phantom rectangle bounded by subjective contours whose depth increased with the thickness of the lines. We attribute the quantitative variation of depth to a purely cyclopean (binocular) process sensitive to the pattern of contour presence and absence in the two eye's view. (+info)
A computational model of selective deficits in first and second-order motion processing.
Recent neurological studies of selective impairments in first and second-order motion processing are of considerable relevance in elucidating the mechanisms of motion perception in normal human observers. We examine the stimuli which have been used to assess first and second-order motion processing capabilities in clinical subjects, and discuss the nature of the computations necessary to extract their motion. We find that a simple computational model of first and second-order motion processing is able to account for the data. The model consists of a first-order channel computing motion at coarse and fine scales, and a coarse scale second-order channel. The second-order channel is sensitive to motion information defined by variations in luminance, contrast, spatial frequency and flicker. When elements of the model are disabled, its performance on either first or second-order motion can be selectively impaired in line with the neurological data. (+info)
Orientation discrimination and tilt aftereffects with luminance and illusory contours.
Orientation discrimination and tilt aftereffects (TAEs) were measured to determine if the orientation of luminance and illusory contours are processed by separate mechanisms. The assumption was made that if a single mechanism supports the perception of both types of contours, then illusory and luminance contours that support the same level of orientation discrimination will be equally effective adapting patterns. Experiment I found that orientation discrimination psychometric functions for illusory and luminance contours are similar, confirming that performance could be matched. Experiment II measured orientation discrimination for a range of intensities for both contours. Experiment III measured TAEs following adaptation to illusory and luminance contours that supported a similar range of orientation discrimination. Similar TAEs were not observed, thus rejecting the single mechanism hypothesis. Experiments IV and V sought to validate the assumption that equivalent orientation discrimination predicts equivalent TAEs by using stimuli that seemed likely to be represented by the same visual mechanism. Luminance contours masked by randomly placed dots and unmasked luminance contours were used with the same procedures as experiments II and III. Equal TAEs were not observed for masked and unmasked contours matched on orientation discrimination, suggesting the assumption relating discriminability to adaptation was incorrect. (+info)
Figure ground segregation modulates perceived direction of ambiguous moving gratings and plaids.
A translating oriented grating viewed through a circular aperture with an occluding area in the middle appeared to move alternately in an oblique or in a vertical direction depending on the foreground/background assignment on the central occluding area. The effect occurred even when the central area was simply removed from the display, thus giving rise to a 'subjective' occluder. Parametric studies revealed that the probability of seeing oblique or vertical motion was affected by the size of the central area but not by its contrast relationships with the grating. Similar phenomena of ambiguous motion direction were observed using changes in colour along a translating grating that produced neon colour spreading effects, or using oriented edge discontinuities that collapsed into subjective plaids composed of two one-dimensional gratings. These results are discussed with respect to the hypothesis that surface segmentation mechanisms play a crucial part in the interpretation of motion signals. (+info)
Integration after adaptation to transparent motion: static and dynamic test patterns result in different aftereffect directions.
One of the many interesting questions in motion aftereffect (MAE) research is concerned with the location(s) along the pathway of visual processing at which certain perceptual manifestations of this illusory motion originate. One such manifestation is the unidirectionality of the MAE after adaptation to moving plaids or transparent motion. This unidirectionality has led to the suggestion that the origin of this MAE might be a single source (gain control) located at, or beyond areas that are believed to be responsible for the integration of motion signals. In this report we present evidence against this suggestion using a simple experiment. For the same adaptation pattern, which consisted of two orthogonally moving transparent patterns with different speeds, we show that the direction of the resulting unidirectional MAE depends on the nature of the test stimulus. We used two kinds of test patterns: static and dynamic. For exactly the same adaptation conditions, the difference in MAE direction between testing with static and dynamic patterns can be as large as 50 degrees. This finding suggests that this MAE is not just a perceptual manifestation of a passive recovery of adapted motion sensors but an active integrative process using the output of different gain controls. A process which takes place after adaptation. These findings are in line with the idea that there are several sites of adaptation along the pathway of visual motion processing and that the nature of the test pattern determines the fate of our perceptual experience of the MAE. (+info)
Second-order motion discrimination by feature-tracking.
When a plaid pattern (the sum of two high spatial frequency gratings oriented +/- 84 degrees from vertical) jumps horizontally by 3/8 of its spatial period its contrast envelope, a second-order pattern, moves in the opposite direction to its luminance waveform. Observers report that the pattern moves in the direction of the contrast envelope when the jumps are repeated at intervals of more than 125 ms and in the direction of the luminance profile when they are repeated at longer intervals. When a pedestal [Lu, Z.-L. & Sperling, G. (1995). Vision Research, 35, 2697-2722] is added to the moving plaid a higher contrast is required to see motion of the contrast envelope but not to see the motion of the luminance profile, suggesting that the motion of the contrast envelope is sensed by a mechanism that tracks features. Static plaids with different spatial parameters from the moving pattern are less effective at raising the contrast required to see the motion of the contrast envelope and simple gratings of low or high spatial frequency are almost completely ineffective, suggesting that the feature-tracking mechanism is selective for the type of pattern being tracked and rejects distortion products and zero-crossings. (+info)