Rumi Hisakata

The effects of eccentricity and retinal illuminance on the illusory motion seen in a stationary luminance gradient.

Kitaoka recently reported a novel illusion named the Rotating Snakes [Kitaoka, A., & Ashida, H. (2003). Phenomenal characteristics of the peripheral drift illusion. Vision, 15, 261-262], in which a stationary pattern appears to rotate constantly. In the first experiment, we attempted to quantify the anecdote that this illusion is better perceived in the periphery. The stimulus was a ring composed of stepwise luminance patterns and was presented in the left visual field. With increasing eccentricity up to 10-14deg, the cancellation velocity required to establish perceptual stationarity increased. In the next experiment, we examined the effect of retinal illuminance. Interestingly, the cancellation velocity decreased as retinal illuminance was decreased. We also estimated the human temporal impulse response at some retinal illuminances by using the double-pulse method to confirm that the shape of the impulse response actually changes from biphasic to monophasic, which indicates that the transient processing system has weaker activities at lower illuminances. We conclude that some transient temporal processing system is necessary for the illusion.

Illusory position shift induced by motion within a moving envelope during smooth-pursuit eye movements

The static envelope of a Gabor patch with a moving carrier appears to shift in the direction of the carrier motion; this phenomenon is known as the motion-induced position shift (De Valois & De Valois, 1991; Ramachandran & Anstis, 1990). This conventional stimulus configuration contains at least three covarying factors: the retinal carrier velocity, the environmental carrier velocity, and the carrier velocity relative to the envelope velocity, which happens to be zero. We manipulated these velocities independently to identify which is critical, and we measured the perceived position of the moving Gabor patch relative to a reference stimulus moving in the same direction at the same speed. In the first experiment, the position of the moving envelope observed with fixation appeared to shift in the direction of the carrier velocity relative to the envelope velocity. Furthermore, the illusion was more pronounced when the carrier moved in a direction opposite to that of the envelope. In the second and third experiments, we measured the illusion during smooth-pursuit eye movement in which the envelope was either static or moving, thereby dissociating retinal and environmental velocities. Under all conditions, the illusion occurred according to the envelope-relative velocity of the carrier. Additionally, the illusion was more pronounced when the carrier and envelope moved in opposite directions. We conclude that the carriers envelope-relative velocity is the primary determinant of the motion-induced position shift.

Adaptation to texture reveals a local metric underlying perceived size and distance.

How the visual system codes metric properties such as size and distance from the information present in the retinal image remains a puzzle. The degree of correlation in the firing of neurons can give some indication of receptive field separation but separation is not coded directly making judging the distance between points by integrating estimates of receptive field separation along a path between them problematic. We need a concept of local spatial scale to resolve this problem. Here we describe a novel and counterintuitive illusion that reveals an internal visual scale against which we determine the spatial properties of objects. In the experiment, observers adapted to dense dot texture and reported on the size of ring that was presented in the same location as the adapting texture as compared to a ring presented in an unadapted field. The perceived size of the ring shrank by approximately 15% after adaptation and the magnitude of the shrinkage depended on the density of texture. Furthermore, we found that this shrinkage not only occurred for geometric figures but also for the perceived distance between two dots. Counterintuitively, the shrinkage coincides with a reduction in apparent density of more sparse dot textures presented in the same adapted location. This new adaptation effect is difficult to explain on a texture or size channel model and shows that the human visual system has a malleable internal metric against which the spatial size or separation of objects is judged and this scale is influenced by adaptation to dense texture. As the underlying scale expands, as revealed though the expanding texture, the apparent size and distance of geometric objects appear compressed. Thus size is coded relative to the metric. This internal metric is an essential first step in processing the geometric properties objects. Meeting abstract presented at VSS 2015.

No motion-induced sensitivity modulation for chromatic gratings.

De Valois and De Valois (1991) showed a moving carrier within a static envelope induced a shift in the apparent position of the Gabor patch. It is known that many kinds of motion induce position shifts, however, the underlying mechanism remains unclear. Recently Roach, McGraw and Johnston (2012) showed that motion modulates the sensitivity to an abutting target depending on the relative phase between the target and inducer grating. Phase dependency was found at the leading position but not at the trailing position. However, the relationship between the position shifts and the sensitivity modulation effect remains a subject of debate. We explored this relationship by examining whether a chromatic grating, which can give rise to position shifts, also induces asymmetric phase dependent sensitivity modulation. There were two conditions, equiluminance and luminance. We used a red-green grating in the equiluminance condition and a yellow-black grating in the luminance condition. Equiluminance was measured by the minimum motion technique (Anstis & Cavanagh, 1983). The inducer size was 1 deg x 7 deg and the target size was 1 deg x 1 deg. For both the spatial frequency was 1cpd and the temporal frequency was 5Hz. The relative phase between the inducer and target was manipulated. We used 2AFC staircase to measure the contrast thresholds. We did not find asymmetric phase dependent sensitivity modulation in the equiluminance condition but the contrast thresholds with the motion inducer were higher than that in the no inducer condition, indicating a motion masking effect. We conclude that this modulation effect does not explain the position shifts due to chromatic motion and that sensitivity modulation does not affect position estimation in our visual system.Meeting abstract presented at VSS 2014

Motion-induced position shift in stereoscopic and dichoptic viewing

The static envelope of a Gabor patch with a moving sinusoidal carrier appears shifted in the direction of the carrier motion (De Valois & De Valois, 1991). This phenomenon is called motion-induced position shift. Although several motion-processing stages, ranging from low- to high-level processes, may contribute to position estimation, it is unknown whether a binocular matching stage or an even earlier stage exerts an influence. To elucidate this matter, we investigated the disparity tuning of this illusion by manipulating the binocular disparities of the carrier and the envelope. If the mechanisms underlying the illusion have disparity selectivity, the illusory shift should disappear when the carrier and envelope have sufficiently different disparities. We conducted an experiment in which a sinusoidal grating inside a Gaussian envelope had a crossed or uncrossed disparity and the background was filled with static random noise; each subject correctly judged whether the grating was in front of or behind the fixation plane. Position shift occurred even when the moving carrier had a vastly different disparity from that of the envelope, suggesting that one of the mechanisms responsible for the phenomenon exists at a monocular visual stage. To confirm this, in the next experiment we examined whether depth perception can be produced by an illusory disparity due to illusory position shifts in opposite directions between eyes. Two Gabor-like patches moving in opposite directions were presented at the same retinal position dichoptically. We found that when each monocular patch had a soft edge in its contrast envelope, the depth perception of such a patch was biased toward the depth consistent with the illusory crossed or uncrossed disparity, whereas depth perception of a stimulus with a hard edge was less biased. We suggest that the underlying mechanisms of motion-induced position shift are present at an early stage of monocular visual processing, and that the altered positions are represented in the left-eye and right-eye monocular pathways in a way that allows them to function as tokens of binocular matching.