Yoshio Ohtani

Estimation of the Timing of Human Visual Perception from Magnetoencephalography

To explore the timing and the underlying neural dynamics of visual perception, we analyzed the relationship between the manual reaction time (RT) to the onset of a visual stimulus and the time course of the evoked neural response simultaneously measured by magnetoencephalography (MEG). The visual stimuli were a transition from incoherent to coherent motion of random dots and an onset of a chromatic grating from a uniform field, which evoke neural responses in different cortical sites. For both stimuli, changes in median RT with changing stimulus strength (motion coherence or chromatic contrast) were accurately predicted, with a stimulus-independent postdetection delay, from the time that the temporally integrated MEG response crossed a threshold (integrator model). In comparison, the prediction of RT was less accurate from the peak MEG latency, or from the time that the nonintegrated MEG response crossed a threshold (level detector model). The integrator model could also account for, at least partially, intertrial changes in RT or in perception (hit/miss) to identical stimuli. Although we examined MEG–RT relationships mainly for data averaged over trials, the integrator model could show some correlations even for single-trial data. The model predictions deteriorated when only early visual responses presumably originating from the striate cortex were used as the input to the integrator model. Our results suggest that the perceptions for visual stimulus appearances are established in extrastriate areas [around MT (middle temporal visual area) for motion and around V4 (fourth visual area) for color] ∼150–200 ms before subjects manually react to the stimulus.

Surround suppression in the human visual cortex: an analysis using magnetoencephalography

The responses of neurons in the primate and cat primary visual cortices (V1s) to the stimuli within their classical receptive fields (CRFs) are markedly suppressed by the surrounding stimuli outside CRFs. In the present study, we show that a similar suppressive effect occurs for visually evoked magnetic responses in the human visual cortex. The initial peak amplitude of the magnetic response (at a latency of around 90 ms) to a test grating accompanied by high-contrast surround gratings was smaller than that for the test without the surround. Current source localization with a single dipole model indicated that the initial response originated from cortical activity near the occipital pole in the contralateral hemisphere to the visual stimulation. The peak amplitude for the test decreased with increasing surround contrast, and increased with increasing test contrast. The contrast dependence and the early development of the surround suppression were in agreement with the results of the V1 single-cell studies of monkeys and cats. We suggest that the surround suppression of the initial peak amplitude of the magnetic response may be ascribed to the inhibition of the neural activity at the early processing stage(s), presumably at V1, in the human visual cortex.

Wiener filter-magnetoencephalography of visual cortical activity.

This paper reports a revised Wiener filter to resolve the inverse problem for magnetoencephalograms (MEGs) according to the structural and functional constraints based on magnetic resonance imaging (MRI) and functional magnetic resonance imaging (fMRI). Wiener filter-MEG imaging for half field stimulation with the chromatic stimulus resolved fast, slow and late responses in V1, V4 and the inferotemporal cortex, respectively. The time courses of these responses were roughly comparable with those reported by unit recording studies of the corresponding monkey visual cortical areas. Wiener filter-MEG imaging had comparable spatial resolution and better signal to noise ratio than fMRI. The background noise was robust in fMRI responses, but became virtually eliminated in Wiener filter responses. Wiener filter-MEG imaging with upper and lower quadrant field stimulation demonstrated V1 responses differentially distributed respectively in the lower and upper banks of the calcarine sulcus. These results demonstrate that responses in two cortical areas facing close to each other can be resolved by Wiener filter-MEG. The present method provides a way to image brain activities with millisecond- and millimeter-order spatiotemporal resolution.

Dependencies of Motion Assimilation and Motion Contrast on Spatial Properties of Stimuli: Spatial-frequency Nonselective and Selective Interactions Between Local Motion Detectors

Two sets of experiments were carried out to examine dependencies of two types of induced motion (motion assimilation and motion contrast) on spatial properties of stimuli in terms of spatial-frequency tuning of local motion detectors. In the first set, the magnitudes of motion assimilation and motion contrast for a sinusoidal grating were measured at a function of the spatial frequency of the inducing gratings, with the spatial frequency of the test grating as a parameter. In the second set, the magnitudes were measured as a function of the height of the inducing gratings with the spatial frequencies of the test and the inducing gratings as parameters. For motion assimilation, the magnitude was characterized by a low-pass function of the spatial frequency of the inducing gratings, and the critical height of the inducing gratings, which demarcates the extent of the spatial pooling, varied systematically depending on the spatial frequency of the inducing gratings. For motion contrast, on the other hand, the magnitude was characterized by a hand-pass function, and the critical height depended on the frequency of the test grating. These results suggest that motion assimilation is mediated by the spatial-frequency nonselective interaction between the local detectors, in which the motion signals of the detectors tuned to different spatial frequencies are integrated with each other. Motion contrast is mediated by the spatial-frequency selective interaction, in which the motion signals of the local detectors tuned to the same or similar spatial frequencies are compared and differentiated.

Magnetic responses of human visual cortex to illusory contours.

To examine the neural mechanism underlying illusory-contour perception, we measured the magnetic responses of the human visual cortex to an abutting-line grating inducing illusory contours (test stimulus) and a non-abutting-line grating (control stimulus) using the technique of magnetoencephalography (MEG). In the initial latency period of 60-80 ms, the MEG response to the test stimulus was nearly identical with that to the control stimulus, but in the subsequent period of 80-150 ms, the former was larger than the latter. The origin of the peak MEG response to the test stimulus was estimated to be in the vicinity of striate cortex/extrastriate visual cortex for two of the four subjects. These results suggest that, in accord with those of the previous electrophysiological and functional magnetic resonance imaging studies, illusory-contour signals are generated in the very early stage(s) of processing in the primate visual cortex.

Anisotropy for Direction Discrimination in a Two-frame Apparent Motion Display

Direction discrimination (upward/downward or left/right) for a Gabor patch in a two-frame motion display was measured as a function of the inter-frame displacement size of the component grating with the stimulus position (center, left, right, upper and lower visual fields) as a parameter. The results showed that, for vertical motion in the center, left, right and lower visual fields, the observers saw downward motion more frequently than upward motion, whereas for vertical motion in the upper field and for horizontal motion, no preference for one of the two opposite directions was obtained. Human motion vision is anisotropic in the lower half of the visual field.

Summation between nearby motion signals and facilitative/inhibitory interactions between distant motion signals.

To explain the finding that motion assimilation was dominant between nearby motion signals while motion contrast between distant ones, a center-surround antagonistic mechanism was proposed [Nawrot & Sekuler (1990). Vision Research, 30, 1439-1451]. However, motion assimilation occurred not only between nearby signals but also between distant ones, suggesting the existence of a center-surround non-antagonistic mechanism [Ido. Ohtani & Ejima (1997). Vision Research, 37, 1565-1574]. The present study was designed to provide direct evidence for the non-antagonistic mechanism, and to examine further the motion interactions which operate in different spatial scales. The nature of motion interaction between the test and the inducer was examined by varying the size, the number of frames, the frame duration and the inter-frame displacement of random-dot kinematograms. The results were consistent with the notion that there are three types of interactions in human motion processing; one is a summation process effective within nearby regions, and the other two are facilitative and inhibitory induction processes operating over larger spatial scales. Analysis of the results in terms of the Fourier components suggests that the facilitative and the inhibitory induction processes may be sensitive, respectively to the lower and the higher temporal frequency components of the stimulus.

Motion assimilation for expansion/contraction and rotation and its spatial properties

In a two-frame apparent motion display, a test grating was displaced horizontally or vertically in the presence of an inducer of which component gratings made up expanding/contracting or rotational motion as a whole. In the first experiment, we demonstrated that motion assimilation did occur for the test accompanied by the two-dimensional motion of the inducer. In the second experiment, we showed that the spatial limit of motion assimilation for expansion/contraction or rotation was large, extending over at least a visual angle of 14-21 deg in diameter, but spatial summation did not occur within the limit. The results were discussed in terms of the interaction between local motion detectors and higher-order detectors which monitor global motion of the whole stimulus pattern.

Temporal summation of magnetic response to chromatic stimulus in the human visual cortex.

The temporal-summation characteristics of the human visual cortex were investigated by recording the magnetic responses to isoluminant red–green gratings. In one condition, exposure duration (ED) of a single-pulse stimulus was varied between 16.7 ms and 200 ms, and in the other, stimulus-onset-asynchrony (SOA) of a double-pulse (presented for 16.7 ms each) stimulus was varied between 16.7 ms and 200 ms. The magnetic responses showed an initial peak at a latency of around 100 ms, the origin of which was estimated to be in the vicinity of the striate cortex. The peak amplitude increased with increasing ED and decreased with increasing SOA, showing a clear sign of temporal summation. The critical ED and SOA estimated from the peak amplitude vs. ED/SOA functions were about 50 ms. These values indicate the upper limit of temporal summation for chromatic stimuli in the human early visual cortex.

On the loss of apparent motion between isolated chromatic stimuli near isoluminance

Long range apparent motion (AM) between two isolated stimuli breaks down following prolonged inspection. Time-till-breakdown (TTB) for AM between random-dot squares (red or green) on a red random-dot background was measured as a function of luminance contrast of the stimuli against the background. For the same-color (red squares on the red background) and the different-color (green squares on the red background) conditions, TTB showed clear dependence on the luminance contrast, diminishing with decreasing the contrast. Near isoluminance (luminance contrast of approx. -14 to +14%), AM for the same-color condition disappeared, but AM for the different-color condition was clearly seen and persisted for 7-14 sec. These results show that AM can be produced by color alone. Previous controversial question on the loss of long range AM near isoluminance may be explained by taking into account the contrast dependence of the breakdown effect and the experimental procedures employed.