Aftereffect of perceived motion trajectories.

If our visual system has a distinct computational process for motion trajectories, such a process may minimize redundancy and emphasize variation in object trajectories by adapting to the current statistics. Our experiments show that after adaptation to multiple objects traveling along trajectories with a common tilt, the trajectory of an object was perceived as tilting on the repulsive side. This trajectory aftereffect occurred irrespective of whether the tilt of the adapting stimulus was physical or an illusion from motion-induced position shifts and did not differ in size across the physical and illusory conditions. Moreover, when the perceived and physical tilts competed during adaptation, the trajectory aftereffect depended on the perceived tilt. The trajectory aftereffect transferred between hemifields and was not explained by motion-insensitive orientation adaptation or attention. These findings provide evidence for a trajectory-specific adaptable process that depends on higher-order representations after the integration of position and motion signals.

Effects of binocular disparity on binocular luminance combination.

This study aimed to examine the effects of binocular disparity on binocular combination of brightness information coming from luminance increments and decrements. The point of subjective equality was determined by asking the observers to judge which stimulus appeared brighter-a bar stimulus with variable disparity or another stimulus with zero disparity. For the bar stimulus, the interocular luminance ratio was varied to trace an equal brightness curve. Binocular disparity had no effect on luminance increments presented on a gray or black background. In contrast, when luminance decrements were presented on a gray background, non-zero disparities elevated points of subjective equality for stimuli with interocular luminance differences. This means that the binocular brightness combination of the two monocular signals shifted from winner-take-all summation toward linear averaging. It has been argued that this effect may be caused by non-zero binocular disparities attenuating interocular suppression, which is deemed to operate normally with zero disparity.

The moment of awareness influences the content of awareness in orientation repulsion.

Through the neurally evolving process of dynamic contextual modulation of perceptual contents, it remains unclear how the content of awareness is determined. Here we quantified the visual illusion of orientation repulsion, wherein the target appears tilted against the surrounding's orientation, and examined whether its extent changed when the target awareness was quickened by a preceding flanker. Independently of spatial cueing, repulsion was reduced when the flanker preceded the target by 100 ms compared with when they appeared simultaneously. We confirmed that the preceding flanker quickened the awareness of a nearby target relative to distant ones by 40 ms. Furthermore, the preceding flanker that was greater than 7 degrees away from the target still evoked such reduction of repulsion. These findings imply that the content of awareness is determined by the temporal interaction of two distinct processes: one controls the moment of awareness, and the other represents the perceptual content.

Local motion signals silence the perceptual solution of global apparent motion.

Stimuli for apparent motion can have ambiguity in frame-to-frame correspondences among visual elements. This occurs when visual inputs cause a correspondence problem that allows multiple alternatives of perceptual solutions. Herein we examined the influence of local visual motions on a perceptual solution under such a multistable situation. We repeatedly alternated two frames of stimuli in a circular configuration in which discrete elements in two different colors alternated in space and switched their colors frame by frame. These stimuli were compatible with three perceptual solutions: globally consistent clockwise and counterclockwise rotations and color flickers at the same locations without such global apparent motion. We added a sinusoidal grating continuously drifting within each element to examine whether the perceptual solution for the global apparent motion was affected by the local continuous motions. We found that the local motions suppressed global apparent motion and promoted another perceptual solution that the local elements were only flickering between the two colors and drifting within static windows. It was concluded that local continuous motions as counterevidence against global apparent motion contributed to individuating visual objects and integrating visual features for maintaining object identity at the same location.

Temporal resolution and temporal extent of orientation repulsion.

A vertical target is perceived as tilted against a slightly tilted inducer surrounding it. To identify the temporal resolution and temporal extent of this phenomenon of orientation repulsion in the same paradigm, we used an alternating pair of inducer stimuli having complementary orientation distributions and quantified repulsion at various alternation frequencies. The duration of each inducer stimulus was inversely proportional to the frequency. When an orthogonal pair of D2 patterns, a type of grating whose luminance modulation in a particular orientation was the second-order partial derivative of an isotropic 2D-Gaussian, was used as the inducer, repulsion occurred when the duration exceeded 20 ms and leveled off at 30 ms and beyond. When a custom-made texture with a narrowband orientation distribution and another texture with a complementary orientation distribution were alternated as the inducer, repulsion gradually increased until the inducer duration reached 200 ms. The gradual increase in repulsion was observed regardless of whether the orientation of the inducer that appeared simultaneously with the target was discernible. These findings reveal that contextual modulation in orientation occurs at a high temporal resolution and continues to a long temporal extent under optimal conditions.

Search asymmetry in periodical changes of motion directions.

We often need to search for an object in a dynamic environment. However, there remains a limited understanding of search processes responding to dynamic inputs of stimuli, especially regarding the mid-level stages of our visual processing hierarchy. In this study, we investigated whether and to what extent search asymmetry is observed between a search for a directionally changing item among constantly drifting items and vice versa. We found a significant search asymmetry in which a search for a directionally changing target among constantly drifting distractors was fairly efficient, while a search for a constantly drifting target among directionally changing distractors was drastically inefficient. We compared these results with other cases of search for a color-changing item among color-constant items and vice versa. These results suggest that directional changes are not a guiding attribute, but they are processed differently depending on whether they are assigned as a target to be found or distractors to be rejected; in the latter case, observers have difficulty rejecting them as distractors. We propose that the significant search asymmetry reflects the period of time during which directional changes are temporarily inaccessible to the visual system when deciding between a distractor and a target.

Common-onset masking terminates the temporal evolution of orientation repulsion.

Our conscious awareness of visual events does not arise instantaneously. Previous studies on backward masking have investigated dynamic internal processes making targets visible or invisible subjectively. However, to understand the whole picture of our rich conscious experiences, the emergence of various phenomenal attributes of consciousness beyond visibility must be delineated. We quantified appearance as the strength of orientation repulsion during common-onset masking and found that masking reduced the repulsion in a near-vertical target grating surrounded by tilted inducers. Furthermore, this reduction was seen only when the inducers were presented together with or after the target. This demonstrates that orientation repulsion involves slow contextual modulation and that masking influences this modulation at a later period. Although appearance was altered as such, orientation discriminability was not reduced by masking in any of our experiments. We propose a process in which internal representations of objects spend a certain amount of time evolving before we become aware of them. Backward masking compulsorily terminates this temporal evolution of internal representations and allows premature representations to arise in our awareness.

Perceptual enhancement of suprathreshold luminance modulation in stereoscopic patterns.

Previous studies have shown that the binocular summation of luminance contrast signals depends on the parameters involved in stereopsis when the luminance contrast is at the detection threshold. However, less attention has been paid to the perception of luminance modulation in stereoscopic patterns at suprathreshold contrast. To address this issue, we determined the contrast of stereoscopic patterns at the perceptual match to a standard contrast as a function of binocular disparity. The matched contrast was close to the standard contrast at 0 degrees disparity, but decreased as disparity deviated from 0 degrees, suggesting that sufficient disparity perceptually enhances luminance contrast. The reduction of matched contrast was more evident for uncrossed disparities than for crossed disparities, which almost disappeared when the contrast was near the threshold and also occurred when vertical disparity was introduced. We argue that the perceptual enhancement of the luminance contrast is due to the weaker interocular suppression for stimuli with large disparities.

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.

Distinct mechanisms of temporal binding in generalized and cross-modal flash-lag effects.

It remains unknown how the brain temporally binds sensory data across different modalities and attributes to create coherent perceptual experiences. To address this question, we measured what we see at the time we experience an event using a generalized version of the flash-lag effect (FLE) for combinations of visual attribute (bar orientation, face orientation, or face identity) and probe modality (visual or auditory). We asked participants to judge the content of rapidly and serially presented images seen at the same time as a briefly presented visual (flash) or auditory (click) probe and estimated the "time windows" contributing to decisions using reverse correlation analysis. We also used displays in which the visual attribute of a stimulus continuously changed and measured FLEs around abrupt flip in change direction and at the initiation and termination of a sequence. We consistently found clear latency-difference effects, which depended on visual attribute for the visual probe but did not for the auditory probe. The intra-modal FLE can be explained in terms of differential latency and temporal integration, but the cross-modal FLE is suggested to operate via a distinct mechanism; the content of a successive visual stream experienced after the awareness of a click is interpreted as simultaneous with the click.

Feature-based attentional selection affects the perceived duration of a stimulus having two superposed patterns.

The perceived duration of a visual event is highly related to stimulus attributes. It is well known that a moving stimulus appears to last longer than a static one does. Previous studies have demonstrated that the time dilation in a moving stimulus can be influenced by perceived motion, rather than by mere physical motion, and that a faster motion appears to last longer than a slower one does. However, whether a top-down attentional set for the feature value can modulate the time dilation in a moving stimulus when two different visual patterns coexist within the same region of the visual field is still unknown. To test this, in Experiment 1, we presented a moving and a static random-dot pattern simultaneously within the same region, and instructed the observer to attend to one of these two patterns. The results demonstrate that perceived duration was longer when attention was directed to the moving, rather than static pattern, although both patterns physically coexisted at the same time and place and for the same duration. In Experiment 2, slow and/or fast moving patterns were presented at the same time and place, and again, feature-based attentional selection affected the perceived duration of the identical physical display. These results suggest that attention to a moving stimulus is an essential factor that determines the time dilation in a moving stimulus. This study revealed that feature-based attention, as opposed to location-based attention, plays an important role in motion-induced time dilation.

Preview benefit survives a three-dimensional rotation of the rigid configuration of search items.

When some distractors (old items) are presented prior to others (new items) during an inefficient visual search task, search becomes easier, as though observers were able to exclude the old items from the search. This phenomenon, called preview benefit, occurs when the locations of the old items are deprioritized relative to the locations of the new items, through a process of visual marking. It has been demonstrated that preview benefit persists when the old items move on the display, if the spatial relationships among the old items remain unchanged, suggesting that the memory template for visual marking represents the spatial configuration of the old items. One remaining question is whether the configuration is coded in two- or three-dimensional coordinates. In the present study, we examined whether preview benefit was preserved when all items were graphically rendered in three-dimensional coordinates and rigidly rotated around the vertical axis at a constant angular velocity. Preview benefit occurred in this situation, suggesting that the memory template for visual marking represents the spatial configuration in three-dimensional coordinates.

Spatial scaling of illusory motion perceived in a static figure.

In a phenomenon known as the Rotating Snakes illusion (Kitaoka & Ashida, 2003), illusory motion is perceived in a static figure with a specially designed luminance profile. It is known that the strength of this illusion increases with eccentricity, suggesting that the underlying mechanism of the illusion has a spatial property that changes with eccentricity. If a change in receptive-field size of responsible neurons causes the eccentricity dependence of the illusion, its strength should be spatially scalable using a scaling factor that increases with eccentricity, because the receptive field size of neurons in visual areas with retinotopy generally obeys quantitative dependence on eccentricity. For the luminance micropatterns comprising the figure for the Rotating Snakes illusion, we varied eccentricity from 9 to 15 deg and spatial frequency from 0.25 to 1.6 cycles/deg, and measured illusion strength. Illusion strength was found to increase with decreasing spatial frequency and with increasing eccentricity. Furthermore, the profiles of illusion strength at different eccentricities were spatially scalable into a single parabola as a function of the spatially scaled visual angle. The estimated scaling factors linearly increased with eccentricity with a slope similar to the eccentricity dependence of the receptive field size of V1 neurons, suggesting the involvement of early visual areas in the generation of the illusion.

Individual Variability in Simultaneous Contrast for Color and Brightness: Small Sample Factor Analyses Reveal Separate Induction Processes for Short and Long Flashes.

In classic simultaneous color contrast and simultaneous brightness contrast, the color or brightness of a stimulus appears to shift toward the complementary (opposite) color or brightness of its surrounding region. Kaneko and colleagues proposed that simultaneous contrast involves separate "fast" and "slow" mechanisms, with stronger induction effects for fast than slow. Support for the model came from a diverse series of experiments showing that induction by surrounds varying in luminance or color was stronger for brief than long presentation times (10-40 vs. 80-640 ms). Here, to further examine possible underlying processes, we reanalyzed 12 separate small data sets from these studies using correlational and factor analytic techniques. For each analysis, a principal component analysis of induction strength revealed two factors, with one Varimax-rotated factor accounting for brief and one for long durations. In simultaneous brightness experiments, separate factor pairs were obtained for luminance increments and decrements. Despite being based on small sample sizes, the two-factor consistency among 12 analyses would not be expected by chance. The results are consistent with separate fast and slow processes mediating simultaneous contrast for brief and long flashes.

Invisible light inside the natural blind spot alters brightness at a remote location.

The natural blind spot in the visual field has been known as a large oval region that cannot receive any optical input because it corresponds to the retinal optic disk containing no rod/cone-photoreceptors. Recently, stimulation inside the blind spot was found to enhance, but not trigger, the pupillary light reflex. However, it is unknown whether blind-spot stimulation also affects visual perception. We addressed this question using psychophysical brightness-matching experiments. We found that a test stimulus outside the blind spot was judged as darker when it was accompanied by a consciously unexperienced blue oval inside the blind spot; moreover, the pupillary light reflex was enhanced. These findings suggested that a photo-sensitive mechanism inside the optic disk, presumably involving the photopigment melanopsin, contributes to our image-forming vision and provides a 'reference' for calibrating the perceived brightness of visual objects.

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.

[Phenomenon of Perceived Duration in the Subjective Present].

Among phenomena involving the subjective present, duration perception is especially worth discussing from the viewpoints of psychophysics, cognitive neuroscience, and computational modeling of underlying mechanisms. First, several situations in which moving visual stimuli appear to last longer are reported. Second, the phenomenon of illusory time compression, which occurs in a moving stimulus after prolonged observation of another stimulus moving at a faster speed, is described. Third, how an attended visual stimulus appears to last longer and how a stimulus appears to last for a shorter duration when attention is directed away are explained. Finally, possible underlying mechanisms in relation to the stages of hierarchical visual information processing are discussed.

A Drastic Change in Background Luminance or Motion Degrades the Preview Benefit.

When some distractors (old items) precede some others (new items) in an inefficient visual search task, the search is restricted to new items, and yields a phenomenon termed the preview benefit. It has recently been demonstrated that, in this preview search task, the onset of repetitive changes in the background disrupts the preview benefit, whereas a single transient change in the background does not. In the present study, we explored this effect with dynamic background changes occurring in the context of realistic scenes, to examine the robustness and usefulness of visual marking. We examined whether preview benefit in a preview search task survived through task-irrelevant changes in the scene, namely a luminance change and the initiation of coherent motion, both occurring in the background. Luminance change of the background disrupted preview benefit if it was synchronized with the onset of the search display. Furthermore, although the presence of coherent background motion per se did not affect preview benefit, its synchronized initiation with the onset of the search display did disrupt preview benefit if the motion speed was sufficiently high. These results suggest that visual marking can be destroyed by a transient event in the scene if that event is sufficiently drastic.

Attentional Capture to a Singleton Distractor Degrades Visual Marking in Visual Search.

Visual search is easier after observing some distractors in advance; it is as if the previewed distractors were excluded from the search. This effect is referred to as the preview benefit, and a memory template that visually marks the old locations of the distractors is thought to help in prioritizing the locations of newly presented items. One remaining question is whether the presence of a conspicuous item during the sequential shift of attention within the new items reduces this preview benefit. To address this issue, we combined the above preview search and a conventional visual search paradigm using a singleton distractor and examined whether the search performance was affected by the presence of the singleton. The results showed that the slope of reaction time as a function of set size became steeper in the presence of a singleton, indicating that the singleton distractor reduced the preview benefit. Furthermore, this degradation effect was positively correlated with the degree of conventional attentional capture to a singleton measured in a separate experiment with simultaneous search. These findings suggest that the mechanism of visual marking shares common attentional resources with the search process.