Reverse-phi Motion Research Articles

Sensitivity for reverse-phi motion

Low-level contrast information in the primary visual pathway is represented in two different channels. ON-center cells signal positive contrasts and OFF-center cells signal negative contrasts. In this study we address the question whether initial motion analysis is performed separately in these two channels, or also through combination of signals from ON and OFF cells. We quantitatively compared motion coherence detection for regular and for reverse-phi motion stimuli. In reverse-phi motion the contrast of a pattern flips during displacements. Sensitivity is therefore based on correlating positive and negative contrasts, whereas for regular motion it is based on correlating similar contrasts. We compared tuning curves for step size and temporal interval for stimuli in which motion information was limited to a single combination of step size and interval. Tuning for step size and temporal interval was highly similar for the two types of motion. Moreover, minimal coherence thresholds for both types of motion matched quantitatively, irrespective of dot density. We also measured sensitivity for so-called no-phi motion stimuli, in which the contrast of displaced dots was set to zero. Sensitivity for no-phi motion was low for stimuli containing only black or only white dots. When both dot polarities were present in the stimulus, sensitivity was absent. Thus, motion information based on separate contrasts was effectively cancelled by a component based on different contrasts. Together these results show equal efficiency in correlating dots of opposite contrast and of similar contrast, which strongly suggests efficient detection of correlations across ON and OFF channels.

The parallel between reverse-phi and motion aftereffects

Periodically flipping the contrast of a moving pattern causes a reversal of the perceived direction of motion. This direction reversal, known as reverse-phi motion, has been generally explained with the notion that flipping contrasts actually shifted the balance of motion energy toward the opposite direction. In this sense, the reversal is trivial because any suitable motion energy detector would be optimally excited in a direction opposite to that for regular motion. This notion, however, does not address the question how these two types of motion are initially detected. Here we show several perceptual phenomena indicating that low-level detection of the two types of motion is quite different. Reverse-phi motion percepts in many respects behave more like motion aftereffects than like regular motion. Motion adaptation causes reduced activity during a stationary test stimulus, which by means of directional opponency leads to motion perceived in the opposite direction. Our findings suggest that reverse-phi motion similarly reduces the activity of low-level motion detectors.

Explaining the footsteps, belly dancer, Wenceslas, and kickback illusions

The footsteps illusion (FI) demonstrates that an object's background can have a profound effect on the object's perceived speed. This illusion consists of a yellow bar and a blue bar that move over a black-and-white, striped background. Although the bars move at a constant rate, they appear to repeatedly accelerate and decelerate in antiphase with each other. Previously, this illusion has been explained in terms of the variations in contrast at the leading and trailing edges of the bars that occur as the bars traverse the striped background. Here, we show that this explanation is inadequate and instead propose that for each bar, the bar's leading edge, trailing edge, lateral edges, and the surrounding background edges all contribute to the bar's perceived speed and that the degree to which each edge contributes to the motion percept is determined by that edge's contrast. We show that this theory can explain all the data on the FI as well as the belly dancer and Wenceslas illusions. We conclude by presenting a new illusion, the kickback illusion, which, although geometrically similar to the FI, is mediated by a different mechanism, namely, reverse phi motion.

Contrast-reversing global-motion stimuli reveal local interactions between first- and second-order motion signals

Motion perception appears to be mediated by, at least, two systems: a first-order and a second-order system. To investigate the degree of interaction between these systems, we used a contrast-reversing global-motion stimulus in which the signal dots reverse their contrast polarity as they move. In response to such a stimulus, fullwave-rectifying second-order units would signal motion in the displacement direction and first-order units would signal motion in the opposite direction (reverse-phi motion). If these signals were of equal strength, then any inhibitory interaction between them would lead to motion nulling. Such a situation would account for the failure to perceive coherent motion with such a stimulus in a previous study [Vis. Res. 34 (1994) 2849]. In order to test for this possibility we manipulated the stimulus in order to reduce the strength of the second-order response relative to the first-order response. This was achieved by: decreasing dot contrast; increasing stimulus eccentricity; and increasing dot speed. These manipulations resulted in an increase in the perception of (first-order mediated) reverse-phi motion. We conclude that interaction between first- and second-order motion signals occur at the local-motion-pooling level.

Modeling reverse-phi motion-selective neurons in cortex: double synaptic-veto mechanism.

Reverse-phi motion is the illusory reversal of perceived direction of movement when the stimulus contrast is reversed in successive frames. Livingstone, Tsao, and Conway (2000) showed that direction-selective cells in striate cortex of the alert macaque monkey showed reversed excitatory and inhibitory regions when two different contrast bars were flashed sequentially during a two-bar interaction analysis. While correlation or motion energy models predict the reverse-phi response, it is unclear how neurons can accomplish this. We carried out detailed biophysical simulations of a direction-selective cell model implementing a synaptic shunting scheme. Our results suggest that a simple synaptic-veto mechanism with strong direction selectivity for normal motion cannot account for the observed reverse-phi motion effect. Given the nature of reverse-phi motion, a direct interaction between the ON and OFF pathway, missing in the original shunting-inhibition model, it is essential to account for the reversal of response. We here propose a double synaptic-veto mechanism in which ON excitatory synapses are gated by both delayed ON inhibition at their null side and delayed OFF inhibition at their preferred side. The converse applies to OFF excitatory synapses. Mapping this scheme onto the dendrites of a direction-selective neuron permits the model to respond best to normal motion in its preferred direction and to reverse-phi motion in its null direction. Two-bar interaction maps showed reversed excitation and inhibition regions when two different contrast bars are presented.

Reversed visual motion and self-sustaining eye oscillations.

A random-dot field undergoing counterphase flicker paradoxically appears to move in the same direction as head and eye movements, i.e. opposite to the optic-flow field. The effect is robust and occurs over a wide range of flicker rates and pixel sizes. The phenomenon can be explained by reversed phi motion caused by apparent pixel movement between successive retinal images. The reversed motion provides a positive feedback control of the display, whereas under normal conditions retinal signals provide a negative feedback. This altered polarity invokes self-sustaining eye movements akin to involuntary optokinetic nystagmus.

Precise Assessment of the Mean Effective Luminance of Texture Patches—An Approach Based on Reverse-phi Motion

In studying the response of mechanisms to contrast-defined texture stimuli, it is critical that the average effective luminance of these textures be equal to that of the background, to minimize net luminance-based signals. We present an efficient and accurate technique for constructing such equiluminant textures to isolate contrast-sensitive mechanisms for investigating their properties. The technique is based on the reverse-phi motion phenomenon, and the resulting settings agree closely with those obtained by photometric means for the class of textures studied. The method also allows one to explore the properties of contrast- and luminance-driven motion mechanisms and, in particular, to evaluate the contribution of putative second-order mechanisms to the motion percept. Results of applying the method are presented, and its advantages over the minimum-flicker and minimum-motion techniques are discussed. Copyright © 1996 Elsevier Science Ltd

Depth, motion, and static-flow perception at metaisoluminant color contrast.

Many experiments concerned with the role of color in depth and motion perception have applied isoluminant random-dot stereograms and cinematograms. The poor performance in the absence of luminance contrast has been associated with color-blindness of stereopsis and motion perception (Livingstone, M.S. & Hubel, D.H. (1987) J. Neurosci. 7, 3416-3468). Nevertheless, isoluminant stimuli are not fully accepted as appropriate tools in isolating central mechanisms (Logothetis, N.K., Schiller, P.H., Charles, E.R. & Hurlbert, A.C. (1990) Science 247, 214-217). In our experiments we use a broad luminance range to test whether color can contribute to a given mechanism when luminance contrast is present but has a strong "veto" effect from opposite luminance contrast, a condition we named "metaisoluminance." There is no fusion in stereopsis under polarity reversal, when only luminance information is given, and reversed-phi phenomenon is experienced for motion. As a third "matching" task, we included polarity-reversed random-dot Glass-patterns, which exhibit "static flow" and also show pattern reversal. We found that color can counteract the effects of polarity reversal by restoring stereoscopic fusion and reversed phi motion and does it with increased efficiency as the hue contrast increases. We found no such effect of color in Glass-patterns. Thus, we showed that the visual system for binocular depth and motion perception is not color-blind, although correlated hue information under metaisoluminance does not appear to yield shape perception.