Apparent motion produced by temporally modulated brightness contrast and assimilation.

When regions containing a counterphasing sine-wave grating are presented side by side and in spatial and temporal quadrature phase, a transparent perception of motion results. This occurs even though none of the stimulus parts is moving. The two percepts of motion in these displays are in opposite directions, one analogous to brightness contrast, the other to brightness assimilation. If the regions are separated by a gap, the contrast and assimilation motions remain visible for separations up to 0.5 and 1 period, respectively. Both motions occur at temporal frequencies from 1 to 16 Hz. The perceived motion analogous to brightness assimilation is easily modeled with elongated receptive fields that integrate flux along the long axis, such as simple cells. The perceived motion analogous to brightness contrast can be accounted for by receptive fields that subtract the flux in one region from the flux in another region. Examples are center-surround subunits such as are found in the elaborated Reichart model [W. Reichardt, in Sensory Communication, W. A. Rosenblith, ed. (MIT Press, Cambridge, Mass., 1961), pp. 303-317; J. P. H. van Santen and G. Sperling, J. Opt. Soc. Am. A 2, 300-321 (1985)]. The dual perceived motion suggests that more than one kind of motion channel (distinguished by the two-dimensional receptive field of the front-end filter) is present in the human visual system.

Parallel search for a conjunction of contrast polarity and shape.

When a target object embedded in an array of other objects can be distinguished along a single feature dimension (e.g. color or shape), it can be detected in parallel. When a target object is defined by a conjunction of stimulus features, at least some serial search is required, indicating that the visual system is less efficient in conducting a parallel search over two stimulus dimensions simultaneously. Some exceptions to this finding have been reported. The present results suggest another exception: search for a conjunction of contrast polarity and shape can be conducted in parallel while the same conjunction consisting of color and shape requires some serial search. The neurophysiological implications are discussed.

The effect of similarity and duration on spatial interaction in peripheral vision.

Spatial interactions are extensive in the peripheral visual field, extending up to about half the retinal eccentricity of the target (Toet and Levi, Vision Res. 32, 1349-1357, 1992). In the present study it is shown that the degree and extent of peripheral spatial interaction depends in large measure on the similarity between test and flanking stimuli. The stimulus consisted of a test T surrounded by four distracting flanking Ts, each randomly oriented. The task was to determine the orientation of the test T. The test and flanking Ts differed in contrast polarity, shape, depth, color, eye of origin, or contrast. When the target and flanks differed in contrast polarity, depth, or shape, performance improved markedly for all observers. A color difference enhanced the performance of most but not all observers. Eye-of-origin had no effect, that is, spatial interaction was identical when the target and flanks were presented to the same eye, or to opposite eyes. The role of stimulus duration in spatial interaction was examined in two additional experiments. In the first, the stimulus viewing duration was increased in order to allow the observer time to serially search for the test T. In the second experiment, a postmask was presented at the location of the test T. The results of these experiments showed that the influence of similarity was independent of stimulus duration and the postmask, and suggest that serial search does not play an important role in the spatial interaction effects reported here. The extent of spatial interaction is correlated with the ability to do parallel search.

Local direction of edge motion causes and abolishes the barberpole illusion.

The perceived direction of motion of a one-dimensional grating is measured in straight-edged rectangular and indented rectangular apertures. It is shown that the perceived direction of motion of the pattern is largely determined by the directions of motion at the edges, rather than by the aspect ratio or global shape of the aperture. The edge motion vectors appear to be calculated at a remarkably local scale. The barberpole illusion is abolished when indentation size equals or exceeds one-quarter of the grating period. This critical size is scale invariant with grating period and corresponds well with a quadrature model of motion perception.

Properties of the recombination of one-dimensional motion signals into a pattern motion signal.

We have examined the human ability to determine the direction of movement of a variety of plaid patterns. The plaids were composed of two orthogonal sine-wave gratings. When the plaid components are of unequal spatial frequency or sometimes of unequal contrast, observers judge the direction of movement incorrectly. In terms of the two-stage model of Adelson and Movshon (1982), these errors may result from either a misjudgment in the perceived speeds of each of the components or a failure in the combination of one-dimensional component movements into a coherent direction of motion of the two-dimensional plaid pattern, or both. A comparison of the perceived direction of motion of plaids with the relative perceived speeds of the plaid component gratings suggest that both failures occur, but in different circumstances. The relative perceived speed of the plaid components was measured with a spatial and a temporal forced-choice technique, the former leading to larger differences. Our results support the notion that the visual system decomposes a moving plaid into oriented components and subsequently recombines the component motions.

Higher-order factors influencing the perception of sliding and coherence of a plaid.

The effect of several new stimulus parameters on the perception of a moving plaid pattern (the sum of two sine-wave gratings) were tested. It was found that: (i) the degree of perceived sliding is strongly influenced by the aperture configuration through which the plaid is viewed; (ii) the chromaticity of the sinusoidal components affects coherence in that more sliding is observed when the plaid components differ in hue, and there is less sliding when they are of the same hue; (iii) equiluminant plaids made of components equal in color almost never show any sliding; and (iv) sliding increases with viewing time. The coherence-sliding percept must therefore be influenced by color, by global interactions, and by adaptation or learning effects, thus suggesting a higher-level influence. These results are most easily modelled by separating the decision to carry out recombination from the process of recombination.

Spatial localization across channels.

We have studied vernier acuity for patterns in which the stimuli to be aligned either are similar in their spatial and color characteristics or differ in these properties. The question which we address is whether spatial localization is independent of the channels being stimulated by the patterns to be aligned. We found that the precision of vernier alignment of Gabor patches was very similar irrespective of whether the patches were the same or different in spatial frequency, orientation, or color. It appears that the visual system extracts very precise location information independent of the similarity or dissimilarity of the spatio-chromatic selectivity of the channels carrying that information.