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Shin'ya Nishida

Publications and source records attributed to Shin'ya Nishida.

14 recordsLinked to original sources
PubMed2005

Temporal frequency characteristics of synchrony-asynchrony discrimination of audio-visual signals.

Temporal synchrony is a critical condition for integrating information presented in different sensory modalities. To gain insight into the mechanism underlying synchrony perception of audio-visual signals we examined temporal limits for human participants to detect synchronous audio-visual stimuli. Specifically, we measured the percentage correctness of synchrony-asynchrony discrimination as a function of audio-visual lag while changing the temporal frequency and/or modulation waveforms. Audio-visual stimuli were a luminance-modulated Gaussian blob and amplitude-modulated white noise. The results indicated that synchrony-asynchrony discrimination became nearly impossible for periodic pulse trains at temporal frequencies higher than 4 Hz, even when the lag was large enough for discrimination with single pulses (Experiment 1). This temporal limitation cannot be ascribed to peripheral low-pass filters in either vision or audition (Experiment 2), which suggests that the temporal limit reflects a property of a more central mechanism located at or before cross-modal signal comparison. We also found that the functional behaviour of this central mechanism could not be approximated by a linear low-pass filter (Experiment 3). These results are consistent with a hypothesis that the perception of audio-visual synchrony is based on comparison of salient temporal features individuated from within-modal signal streams.

Acoustic Stimulation↗
PubMed2005

MEG responses correlated with the visual perception of velocity change.

Magnetoencephalography (MEG) was used to find neural activities, in the human brain, involved in perception of velocity changes in visual motion. We recorded MEG responses evoked by the stimuli whose velocity increased by 40% or 80% of baseline velocities of 1.0, 2.0, 3.0, and 4.0 deg/s. The velocity increment threshold and the manual reaction time (RT) were also measured under similar stimulus conditions. To manipulate observer's sensitivity to velocity increments, the MEG responses and the psychophysical performances were measured after adaptation to motion in one direction (adapted condition) or alternating directions (control condition). MEG responses evoked by velocity increments peaked at 200-290 ms (M1), and the M1 amplitudes, especially those obtained for 40% increments, were correlated with the sensitivities, which are the reciprocal of velocity increment thresholds. Furthermore, motion adaptation enhanced sensitivity to velocity increments and increased the M1 amplitudes. These results suggest a close correlation between the perceptual velocity increment and the evoked MEG response. In other words, the results suggest that velocity increments are detectable when there is a constant increment in magnetic neural response. As for latencies, nearly constant value of M1 latency did not quantitatively match a large decrease in manual RT with the increase in the baseline velocity. Motion adaptation reduced neither the peak MEG latency nor the manual RT.

Adaptation, Psychological↗
PubMed2005

Large-field visual motion directly induces an involuntary rapid manual following response.

Recent neuroscience studies have been concerned with how aimed movements are generated on the basis of target localization. However, visual information from the surroundings as well as from the target can influence arm motor control, in a manner similar to known effects in postural and ocular motor control. Here, we show an ultra-fast manual motor response directly induced by a large-field visual motion. This rapid response aided reaction when the subject moved his hand in the direction of visual motion, suggesting assistive visually evoked manual control during postural movement. The latency of muscle activity generating this response was as short as that of the ocular following responses to the visual motion. Abrupt visual motion entrained arm movement without affecting perceptual target localization, and the degrees of motion coherence and speed of the visual stimulus modulated this arm response. This visuomotor behavior was still observed when the visual motion was confined to the "follow-through" phase of a hitting movement, in which no target existed. An analysis of the arm movements suggests that the hitting follow through made by the subject is not a part of a reaching movement. Moreover, the arm response was systematically modulated by hand bias forces, suggesting that it results from a reflexive control mechanism. We therefore propose that its mechanism is radically distinct from motor control for aimed movements to a target. Rather, in an analogy with reflexive eye movement stabilizing a retinal image, we consider that this mechanism regulates arm movements in parallel with voluntary motor control.

Adult↗
PubMed2005

Perisaccadic perception of continuous flickers.

To realize perceptual space constancy, the visual system compensates for the retinal displacement caused by eye movements. It has been reported that the compensation process does not function perfectly around the time of a saccade--a perisaccadic flash is systematically mislocalized. However, observations made with transient flash stimuli do not necessarily indicate a general perisaccadic failure of space constancy. To investigate how the visual system realizes perisaccadic space constancy for continuous stimuli, we examined the time course of localization for a perisaccadic 500 Hz flicker with systematic variation of the onset timing, the offset timing and the duration. If each flash in the flicker is localized individually in the same way as a single flash, the apparent position and length of the flicker should be predicted from the time course of mislocalization of a perisaccadic flash. However, the results did not support this prediction in many respects. A dot array (of half the length of the retinal image) was perceived when the flicker was presented during a saccade, while only a single dot was perceived when the flicker was presented only before or after the saccade. A flash in a flicker was localized at a different position, depending on the onset timing, the offset timing and the duration of the flicker, even if the flash was presented at the same timing to the saccade. In general, our results support a two-stage localization in which the local geometrical configuration is first generated primarily based on the retinal information, and then localized as a whole in the egocentric or exocentric space. The localization is based on the eye position signal sampled at a time temporally distant from the saccade, which enables precise localization and space constancy for continuous stimuli.

Adult↗
PubMed2004

Recalibration of audiovisual simultaneity.

To perceive the auditory and visual aspects of a physical event as occurring simultaneously, the brain must adjust for differences between the two modalities in both physical transmission time and sensory processing time. One possible strategy to overcome this difficulty is to adaptively recalibrate the simultaneity point from daily experience of audiovisual events. Here we report that after exposure to a fixed audiovisual time lag for several minutes, human participants showed shifts in their subjective simultaneity responses toward that particular lag. This 'lag adaptation' also altered the temporal tuning of an auditory-induced visual illusion, suggesting that adaptation occurred via changes in sensory processing, rather than as a result of a cognitive shift while making task responses. Our findings suggest that the brain attempts to adjust subjective simultaneity across different modalities by detecting and reducing time lags between inputs that likely arise from the same physical events.

Acoustic Stimulation↗
PubMed2004

Motion-based analysis of spatial patterns by the human visual system.

BACKGROUND: It is known that the visibility of patterns presented through stationary multiple slits is significantly improved by pattern movements. This study investigated whether this spatiotemporal pattern interpolation is supported by motion mechanisms, as opposed to the general belief that the human visual cortex initially analyses spatial patterns independent of their movements. RESULTS: Psychophysical experiments showed that multislit viewing could not be ascribed to such motion-irrelevant factors as retinal painting by tracking eye movements or an increase in the number of views by pattern movements. Pattern perception was more strongly impaired by the masking noise moving in the same direction than by the noise moving in the opposite direction, which indicates the direction selectivity of the pattern interpolation mechanism. A direction-selective impairment of pattern perception by motion adaptation also indicates the direction selectivity of the interpolation mechanism. Finally, the map of effective spatial frequencies, estimated by a reverse-correlation technique, indicates observers' perception of higher spatial frequencies, the recovery of which is theoretically impossible without the aid of motion information. CONCLUSIONS: These results provide clear evidence against the notion of separate analysis of pattern and motion. The visual system uses motion mechanisms to integrate spatial pattern information along the trajectory of pattern movement in order to obtain clear perception of moving patterns. The pattern integration mechanism is likely to be direction-selective filtering by V1 simple cells, but the integration of the local pattern information into a global figure should be guided by a higher-order motion mechanism such as MT pattern cells.

Adult↗
PubMed2004

Contrast-reversing global-motion stimuli reveal local interactions between first- and second-order motion signals.

Motion perception appears to be mediated by, at least, two systems: a first-order and a second-order system. To investigate the degree of interaction between these systems, we used a contrast-reversing global-motion stimulus in which the signal dots reverse their contrast polarity as they move. In response to such a stimulus, fullwave-rectifying second-order units would signal motion in the displacement direction and first-order units would signal motion in the opposite direction (reverse-phi motion). If these signals were of equal strength, then any inhibitory interaction between them would lead to motion nulling. Such a situation would account for the failure to perceive coherent motion with such a stimulus in a previous study [Vis. Res. 34 (1994) 2849]. In order to test for this possibility we manipulated the stimulus in order to reduce the strength of the second-order response relative to the first-order response. This was achieved by: decreasing dot contrast; increasing stimulus eccentricity; and increasing dot speed. These manipulations resulted in an increase in the perception of (first-order mediated) reverse-phi motion. We conclude that interaction between first- and second-order motion signals occur at the local-motion-pooling level.

Adaptation, Ocular↗
PubMed2004

Cross-orientation summation in texture segregation.

Human texture vision has been modeled as a filter-rectify-filter (FRF) process, in which '2nd-order' filters detect changes in the rectified outputs of luminance-based '1st-order' filters. This study tested the validity of the two basic assumptions of the standard FRF model, namely (a) that the 2nd-order filters are sensitive to spatial modulations in both contrast and orientation, and (b) that the 2nd-order filters are tuned to different 1st-order orientations. In the first experiment, we tested subthreshold summation between two orthogonal carrier orientations in detection of a texture region, which was defined by contrast modulations across regions in the two carrier orientations, while systematically varying the relative change magnitudes between the two orientations. The results showed that the detection thresholds were determined by spatial difference in the contrast integrated over the two orientations. Orientation difference did act as a segregation cue, but only when there was no differences in carrier contrast. This suggests that two mechanisms are involved in texture segregation; one that detects changes in luminance contrast and another that detects changes in orientation. To further analyze the latter mechanism, a second experiment measured cross-orientation summation in the detection of purely orientation-defined textures, using stimuli that were density modulations of two orientations presented among randomly-orientated distractors. Again, the relative modulation magnitudes between the two orientations was systematically varied. The results are consistent with the notions that (a) the dominant orientation is extracted from the 1st-order outputs before the 2nd-order process, and that (b) the 2nd-order, spatial comparison process integrates those dominant signals over different orientations.

Field Dependence-Independence↗
PubMed2004

Depth perception from second-order-motion stimuli yoked to head movement.

We examined whether depth perception was produced by the parallax of second-order motion (i.e., movement of non-luminance features, such as flicker, texture size modulation, or contrast modulation that moved in synchrony with lateral head movement). The results, obtained with second-order motion from a simple grating stimuli, showed that depth order was judged correctly with probabilities well above chance, but the reported depth magnitude did not co-vary with parallax magnitude. When we used a complex spatial pattern for which feature tracking was difficult, the accuracy of depth-order judgments descended to chance level. Our results suggest that the visual system (a) can detect the correct depth order by tracking a relative shift in the salient features of a stimulus pattern, but (b) cannot determine depth magnitude from a velocity field given by second-order-motion stimuli.

Cues↗
PubMed2003

Neuroimaging of direction-selective mechanisms for second-order motion.

Psychophysical findings have revealed a functional segregation of processing for 1st-order motion (movement of luminance modulation) and 2nd-order motion (e.g., movement of contrast modulation). However neural correlates of this psychophysical distinction remain controversial. To test for a corresponding anatomical segregation, we conducted a new functional magnetic resonance imaging (fMRI) study to localize direction-selective cortical mechanisms for 1st- and 2nd-order motion stimuli, by measuring direction-contingent response changes induced by motion adaptation, with deliberate control of attention. The 2nd-order motion stimulus generated direction-selective adaptation in a wide range of visual cortical areas, including areas V1, V2, V3, VP, V3A, V4v, and MT+. Moreover, the pattern of activity was similar to that obtained with 1st-order motion stimuli. Contrary to expectations from psychophysics, these results suggest that in the human visual cortex, the direction of 2nd-order motion is represented as early as V1. In addition, we found no obvious anatomical segregation in the neural substrates for 1st- and 2nd-order motion processing that can be resolved using standard fMRI.

Analysis of Variance↗
PubMed2003

Gaze modulation of visual aftereffects.

Physiological studies of non-human primates have suggested that the direction of gaze can modulate the gain of neuronal responses to visual stimuli in many cortical areas including V1. The neural gaze modulation is suggested to subserve the conversion from gaze-independent (eye-centered) to dependent (e.g., head-centered) representations. However, it has not been established whether the gaze modulation has significant influences on human visual perception. Here we show that gaze direction modestly but significantly modulates the magnitudes of the motion aftereffect, the tilt aftereffect and the size aftereffect. These aftereffects were stronger when the adaptation and test patterns were presented in the same gaze direction, than when they were presented in different gaze directions, even though the patterns always stimulated the same retinal location. The gaze modulation effect was not statistically significant for the post-adaptation elevation of contrast detection thresholds. The gaze modulation of visual aftereffects provides a useful psychophysical tool to analyze human cortical processes for coordinate transformations of visual space.

Figural Aftereffect↗
PubMed2002

Marker correspondence, not processing latency, determines temporal binding of visual attributes.

BACKGROUND: When simultaneous visual events appear to occur at different times, the discrepancy has generally been ascribed to time differences in neural transmission or cortical processing that lead to asynchronous awareness of the events. RESULTS: We found, however, that an apparent delay of changes in motion direction relative to synchronous color changes occurs only for rapid alternations, and this delay is not accompanied by a difference in reaction time. We also found that perceptual asynchrony depends on the temporal structure of the stimuli (transitions [first-order temporal change] versus turning points [second-order temporal change]) rather than the attribute type (color versus motion). CONCLUSIONS: We propose that the perception of the relative time of events is based on the relationship of representations of temporal pattern that we term time markers. We conclude that the perceptual asynchrony effects studied here do not reflect differential neural delays for different attributes; rather, they arise from a faulty correspondence match between color transitions and position transitions (motion), which in turn results from a difficulty in detecting turning points (direction reversals) and a preference for matching markers of the same type.

Color Perception↗
PubMed2002

Spatiotemporal interactions in detection of texture orientation modulations.

Previous studies have revealed spatial and temporal characteristics of texture orientation modulation detection. This study examined spatiotemporal interactions. We measured threshold amplitudes for detecting orientation modulations in various waveforms. The orientation modulations were presented in a dynamic texture display in which the spatial arrangement and mean orientation of elements were randomly updated at a given frame duration (17-900 ms). The results of three experiments all indicated significant spatiotemporal interactions. As the frame duration was decreased, the detection sensitivity declined more steeply for the sinusoidal orientation modulations than for the square and missing-fundamental waveforms (Expt 1), declined more steeply for low spatial-frequency sinusoidal modulations than for high frequency ones (Expt 2), and declined more steeply for sparse textures than for dense textures (Expt 3). These results indicate that the visual system loses its sensitivity more profoundly for long-range orientation modulations than for short-range modulations as the rate of orientation change increases, suggesting that the mechanism for detecting orientation modulation reduces its effective spatial range for rapid input changes.

Contrast Sensitivity↗