Masahiko Terao

Awareness of Central Luminance Edge is Crucial for the Craik-O'Brien-Cornsweet Effect.

The Craik-O’Brien-Cornsweet (COC) effect demonstrates that perceived lightness depends not only on the retinal input at corresponding visual areas but also on distal retinal inputs. In the COC effect, the central edge of an opposing pair of luminance gradients (COC edge) makes adjoining regions with identical luminance appear to be different. To investigate the underlying mechanisms of the effect, we examined whether the subjective awareness of the COC edge is necessary for the generation of the effect. We manipulated the visibility of the COC edge using visual backward masking and continuous flash suppression while monitoring subjective reports regarding online percepts and aftereffects of adaptation. Psychophysical results showed that the online percept of the COC effect nearly vanishes in conditions where the COC edge is rendered invisible. On the other hand, the results of adaptation experiments showed that the COC edge is still processed at the early stage even under the perceptual suppression. These results suggest that processing of the COC edge at the early stage is not sufficient for generating the COC effect, and that subjective awareness of the COC edge is necessary.

White matter consequences of retinal receptor and ganglion cell damage.

PURPOSE Patients with Leber hereditary optic neuropathy (LHON) and cone-rod dystrophy (CRD) have central vision loss; but CRD damages the retinal photoreceptor layer, and LHON damages the retinal ganglion cell (RGC) layer. Using diffusion MRI, we measured how these two types of retinal damage affect the optic tract (ganglion cell axons) and optic radiation (geniculo-striate axons). METHODS Adult onset CRD (n = 5), LHON (n = 6), and healthy controls (n = 14) participated in the study. We used probabilistic fiber tractography to identify the optic tract and the optic radiation. We compared axial and radial diffusivity at many positions along the optic tract and the optic radiation. RESULTS In both types of patients, diffusion measures within the optic tract and the optic radiation differ from controls. The optic tract change is principally a decrease in axial diffusivity; the optic radiation change is principally an increase in radial diffusivity. CONCLUSIONS Both photoreceptor layer (CRD) and retinal ganglion cell (LHON) retinal disease causes substantial change in the visual white matter. These changes can be measured using diffusion MRI. The diffusion changes measured in the optic tract and the optic radiation differ, suggesting that they are caused by different biological mechanisms.

Smooth pursuit eye movements improve temporal resolution for color perception.

Human observers see a single mixed color (yellow) when different colors (red and green) rapidly alternate. Accumulating evidence suggests that the critical temporal frequency beyond which chromatic fusion occurs does not simply reflect the temporal limit of peripheral encoding. However, it remains poorly understood how the central processing controls the fusion frequency. Here we show that the fusion frequency can be elevated by extra-retinal signals during smooth pursuit. This eye movement can keep the image of a moving target in the fovea, but it also introduces a backward retinal sweep of the stationary background pattern. We found that the fusion frequency was higher when retinal color changes were generated by pursuit-induced background motions than when the same retinal color changes were generated by object motions during eye fixation. This temporal improvement cannot be ascribed to a general increase in contrast gain of specific neural mechanisms during pursuit, since the improvement was not observed with a pattern flickering without changing position on the retina or with a pattern moving in the direction opposite to the background motion during pursuit. Our findings indicate that chromatic fusion is controlled by a cortical mechanism that suppresses motion blur. A plausible mechanism is that eye-movement signals change spatiotemporal trajectories along which color signals are integrated so as to reduce chromatic integration at the same locations (i.e., along stationary trajectories) on the retina that normally causes retinal blur during fixation.

Human neural responses involved in spatial pooling of locally ambiguous motion signals

Early visual motion signals are local and one-dimensional (1-D). For specification of global two-dimensional (2-D) motion vectors, the visual system should appropriately integrate these signals across orientation and space. Previous neurophysiological studies have suggested that this integration process consists of two computational steps (estimation of local 2-D motion vectors, followed by their spatial pooling), both being identified in the area MT. Psychophysical findings, however, suggest that under certain stimulus conditions, the human visual system can also compute mathematically correct global motion vectors from direct pooling of spatially distributed 1-D motion signals. To study the neural mechanisms responsible for this novel 1-D motion pooling, we conducted human magnetoencephalography (MEG) and functional MRI experiments using a global motion stimulus comprising multiple moving Gabors (global-Gabor motion). In the first experiment, we measured MEG and blood oxygen level-dependent responses while changing motion coherence of global-Gabor motion. In the second experiment, we investigated cortical responses correlated with direction-selective adaptation to the global 2-D motion, not to local 1-D motions. We found that human MT complex (hMT+) responses show both coherence dependency and direction selectivity to global motion based on 1-D pooling. The results provide the first evidence that hMT+ is the locus of 1-D motion pooling, as well as that of conventional 2-D motion pooling.

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.

Compensation for equiluminant color motion during smooth pursuit eye movement

Motion perception is compromised at equiluminance. Because previous investigations have been primarily carried out under fixation conditions, it remains unknown whether and how equiluminant color motion comes into play in the velocity compensation for retinal image motion due to smooth pursuit eye movement. We measured the retinal image velocity required to reach subjective stationarity for a horizontally drifting sinusoidal grating in the presence of horizontal smooth pursuit. The grating was defined by luminance or chromatic modulation. When the subjective stationarity of the color motion was shifted toward environmental stationarity, compared with the subjective stationarity of luminance motion, that of color motion was farther from retinal stationarity, indicating that a slowing of color motion occurred before this factor contributed to the process by which retinal motion was integrated with a biological estimate of eye velocity during pursuit. The gain in the estimate of eye velocity per se was unchanged irrespective of whether the stimulus was defined by luminance or by color. Indeed, the subjective reduction in the speed of color motion during fixation was accounted for by the same amount of deterioration in speed. From these results, we conclude that the motion deterioration at equiluminance takes place prior to the velocity comparison.

A continuously lit stimulus is perceived to be shorter than a flickering stimulus during a saccade

When subjects made a saccade across a single-flashed dot, a flickering dot or a continuous dot, they perceived a dot, an array (phantom array), or a line (phantom line), respectively. We asked subjects to localize both endpoints of the phantom array or line and calculated the perceived lengths. Based on the findings of Matsumiya and Uchikawa (2001), we predicted that the apparent length of the phantom line would be larger than that of the phantom array. In Experiment 1 of the current study, contrary to the prediction, the phantom line was found to be shorter than the phantom array. In Experiment 2, we investigated whether the function underlying the filled-unfilled space illusion (Lewis, 1912) instead of the function underlying the saccadic compression could explain the results. Subjects were asked to localize both endpoints of a line or an array while keeping their eyes fixated. Although the results of Experiment 2 showed that the perceived length of a line was shorter than that of an array, the function underlying the filled-unfilled illusion could not fully account for the results of Experiment 1. To explain the present results, we proposed a model for the localization process and discussed its validity.

Enhancement of motion perception in the direction opposite to smooth pursuit eye movement

When eyes track a moving target, a stationary background environment moves in the direction opposite to the eye movement on the observers retina. Here, we report a novel effect in which smooth pursuit can enhance the retinal motion in the direction opposite to eye movement, under certain conditions. While performing smooth pursuit, the observers were presented with a counterphase grating on the retina. The counterphase grating consisted of two drifting component gratings: one drifting in the direction opposite to the eye movement and the other drifting in the same direction as the pursuit. Although the overall perceived motion direction should be ambiguous if only retinal information is considered, our results indicated that the stimulus almost always appeared to be moving in the direction opposite to the pursuit direction. This effect was ascribable to the perceptual dominance of the environmentally stationary component over the other. The effect was robust at suprathreshold contrasts, but it disappeared at lower overall contrasts. The effect was not associated with motion capture by a reference frame served by peripheral moving images. Our findings also indicate that the brain exploits eye-movement information not only for eye-contingent image motion suppression but also to develop an ecologically plausible interpretation of ambiguous retinal motion signals. Based on this biological assumption, we argue that visual processing has the functional consequence of reducing the apparent motion blur of a stationary background pattern during eye movements and that it does so through integration of the trajectories of pattern and color signals.

Time dilation in a perceptually jittering dot pattern

Although it is known that a moving stimulus appears to dilate in duration compared to a stationary stimulus, whether subjective motion devoid of stimulus motion is sufficient remains unknown. To elucidate this, we used a motion illusion in which an actually static stimulus clearly appears to move, a useful dissociation between actual and subjective motions. We used the jitter aftereffect resulting from adaptation to dynamic noise as such a tool and measured subjective durations of a static random-dot pattern in which illusory jitter was seen, an actually oscillating pattern mimicking the illusory jitter, and a static pattern without illusory jitter. Pattern oscillation as tiny as fixational eye movements robustly evoked time dilation, and time dilation to a similar extent was also induced by an actually static but subjectively jittering pattern. Taken together with the previous knowledge that this subjective jitter is related to a visually based compensation of spurious retinal image motions due to fixational eye movements, these findings demonstrate that visual duration computation is influenced by a representation at a high-level motion processing stage at which a stable visual world despite jittery retinal inputs has been established.

The aftereffect of a spatial offset between Gabor patches depends on carrier orientations

This study explored the orientation connectivity in a contrast modulation processing mechanism, modeled as two filtering stages with nonlinear processing in between, by investigating how a negative aftereffect of a contrast-defined spatial offset is influenced by carrier orientations in the adapting stimulus. After adaptation to multiple, globally presented pairs of Gabor patches with a specific horizontal offset, subjects perceived a vertically aligned test pair of patches as offset in the orientation opposite to that of the adaptor. Although the orientations of the carrier gratings in the adaptor pairs were irrelevant to the task, the aftereffect magnitude depended on them. A large aftereffect was observed when the carrier orientations were parallel and/or perpendicular to the contrast-defined orientation, supporting the notion that second-stage filters receive strong inputs from first-stage filters with parallel and perpendicular orientation preferences. Furthermore, the aftereffect was also large when the carrier for only one patch was parallel or perpendicular, and no significant difference in the aftereffect magnitude was observed whether the adaptor pair contained one or two such patches. These results suggest that connectivity is not strictly selective to parallel and perpendicular relationships. Spatially heterogeneous connectivity might explain the observed effect.