Four-dot masking in monoptic and dichoptic viewing.

In visual backward masking paradigms, the visibility of a target is reduced using various kinds of mask stimuli presented immediately after the target. Four-dot masking is one such kind of backward masking, caused by four surrounding dots neither spatially adjacent nor similar to the target. Four-dot masking is often considered to involve object-level interferences. However, low-level contributions such as lateral inhibition and motion detection are also possible. To elucidate the loci of the underlying mechanism within the visual hierarchy, we compared the masking effect between monoptic and dichoptic viewing conditions. A target and a four-dot mask, which also served as a spatial cue to the target location, were presented to the same eye in monoptic viewing, whereas they were presented to different eyes in dichoptic viewing. Observers were then asked to discriminate the target shape. We found a significant decline in the correct response rate compared to the baseline condition in which the four-dot mask was not presented, and the masking effect was equivalent between the monoptic and dichoptic viewings. These results demonstrate that four-dot masking stems exclusively from processing within the binocular pathway.

Inhibition of return modulates the flash-lag effect.

Transient events are known to draw exogenous attention, and visual processing at the attended location is transiently facilitated, but after several hundred milliseconds, attentional processing at the cued location becomes poorer than processing elsewhere, resulting in a slower reaction to a target stimulus that subsequently appears at the cued location. Despite a number of previous studies on this effect, termed inhibition of return (IOR), it is still unclear whether a perceptual process related to the subjective onset time of the target stimulus is disrupted when IOR occurs. In the present study, we used a distinct visual phenomenon termed the flash-lag effect (FLE) as a tool to quantify IOR. The FLE is an illusion in which a flashed stimulus appears to lag behind a moving stimulus, despite being physically aligned. We used an identical stimulus configuration and asked observers to conduct two independent tasks in separate sessions. The first was a simple reaction task to measure the onset reaction time (RT) to an abruptly appearing target. The second was an orientation judgment task to measure the degree of the FLE. Both the RT and the FLE were found to be altered in accordance with IOR, and a significant correlation was demonstrated between the changes in the RT and those in the FLE. These results demonstrate that the perceptual process related to the stimulus onset can be compromised by IOR.

The Ebbinghaus illusion in contrast-defined and orientation-defined stimuli.

In the retinal image of the natural world, edges and shapes can be defined by first-order attributes, such as luminance, and second-order attributes, such as contrast and texture. Previous studies have suggested that, in the human visual system, these attributes are initially detected separately and integrated later. Thus, comparing the strength of different geometrical optical illusions in stimuli, in which different elements are defined by the same or different attributes, is helpful to investigate at which stage the underlying mechanism of the illusion is located. We investigated whether there is a single common mechanism underlying the Ebbinghaus illusion in stimuli defined by different attributes. We used the traditional Ebbinghaus (Titchener) illusion figure: a target disk surrounded by smaller or larger inducer disks. The background and stimuli consisted of sine-wave gratings. We manipulated the luminance, contrast, and grating orientations of the target disk and inducer disks to create stimuli defined by each of these attributes. We then examined whether the illusion occurred in stimuli defined by each single attribute and in compound stimuli, in which the target and inducers were defined by different attributes. We found that the Ebbinghaus illusion occurred with the same strength in stimuli defined by all three attributes. We also found an asymmetry, such as the second-order inducers affected the first-order target less than they affected the second-order targets, but the first-order inducers affected all targets similarly. Our findings suggest that different attributes are likely to be integrated into a cue-invariant shape representation prone to the Ebbinghaus illusion. However, first-order and second-order stimuli may differently contribute to the quantitative aspect of the illusion, resulting in the asymmetric illusion strength.