Biomedical subjects

Ikuya Murakami

Publications and source records attributed to Ikuya Murakami.

10 recordsLinked to original sources

PubMed2006

A positive correlation between fixation instability and the strength of illusory motion in a static display.

A stationary pattern with asymmetrical luminance gradients can appear to move. We hypothesized that the source signal of this illusion originates in retinal image motions due to fixational eye movements. We investigated the inter-subject correlation between fixation instability and illusion strength. First, we demonstrated that the strength of the illusion can be quantified by the nulling technique. Second, we concurrently measured cancellation velocity and fixation instability for each subject, and found a positive correlation between them. The same relationship was also found within a single observer when the visual stimulus was artificially moved in the simulation of fixation instability. Third, we confirmed the same correlation with eye movements for a wider variety of illusory displays. These results suggest that fixational eye movements indeed play a relevant role in generating this motion illusion.

Adult↗

PubMed2006

Fixational eye movements and motion perception.

Small eye movements are necessary for maintained visibility of the static scene, but at the same time they randomly oscillate the retinal image, so the visual system must compensate for such motions to yield the stable visual world. According to the theory of visual stabilization based on retinal motion signals, objects are perceived to move only if their retinal images make spatially differential motions with respect to some baseline movement probably due to eye movements. Motion illusions favoring this theory are demonstrated, and psychophysical as well as brain-imaging studies on the illusions are reviewed. It is argued that perceptual stability is established through interactions between motion-energy detection at an early stage and spatial differentiation of motion at a later stage. As such, image oscillations originating in fixational eye movements go unnoticed perceptually, and it is also shown that image oscillations are, though unnoticed, working as a limiting factor of motion detection. Finally, the functional importance of non-differential, global motion signals are discussed in relation to visual stability during large-scale eye movements as well as heading estimation.

Afterimage↗

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↗

PubMed2004

The aperture problem in egocentric motion.

When only a featureless straight contour of a moving object is visible, one cannot tell its true velocity and the object seems to be moving perpendicularly to its orientation. Using psychophysics and brain imaging, Goltz et al. have now demonstrated that this aperture problem also occurs in visual representations in egocentric coordinates. An afterimage of inclined lines that is perceived to move with smooth-pursuit eye movements appears to move perpendicularly to the lines rather than in the tracking direction.

Afterimage↗

PubMed2004

Correlations between fixation stability and visual motion sensitivity.

To assess influences of fixational drift eye movements on motion detection, lower thresholds for motion and drift amplitudes were measured in normal subjects. The threshold was higher without visible surrounds than with a surround, and had a positive correlation with drift amplitude. The same effect, but more pronounced, was found when the surround was visible but flickered synchronously. In contrast, the correlation disappeared in the threshold with a static surround. These results suggest that, while spurious image motions by eye drift can have a detrimental effect, a mechanism tuned for differential motions normally counteracts it.

Adult↗

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

Illusory jitter in a static stimulus surrounded by a synchronously flickering pattern.

The eyes are always moving even during fixation, making the retinal image move concomitantly. While these motions activate early visual stages, they are excluded from one's perception. A striking illusion reported here renders them visible: a static pattern surrounded by a synchronously flickering pattern appears to move coherently in random directions. There was a positive correlation between the illusion and fixational eye movements. A simulation revealed that motion computation artificially creates a motion difference between center and surround, which is usually a cue to object motion but now a wrong cue to seeing eye movements of oneself on-line. Therefore, this novel illusion indicates that the visual system normally counteracts shaky visual inputs due to small eye movements by using retinal, as opposed to extraretinal, motion signals. As long as they comprise common image motions over space, they are interpreted as coming from a static outer world viewed through moving eyes. Such visual stability fails in the condition of artificial flicker, because common image motions due to eye movements are registered differently between flickering and non-flickering regions.

Adult↗

PubMed2002

Human brain activity during illusory visual jitter as revealed by functional magnetic resonance imaging.

One central problem in vision is how to compensate for retinal slip. A novel illusion (visual jitter) suggests the compensation mechanism is based solely on retinal motion. Adaptation to visual noise attenuates the motion signals used by the compensation stage, producing illusory jitter due to the undercompensation of retinal slip. Here, we investigated the neural substrate of retinal slip compensation during this illusion using high-field fMRI and retinotopic mapping in flattened cortical format. When jitter perception occurred, MR signal decreased in lower stages of the visual system but increased prominently in area MT+. In conclusion, visual areas as early as V1 are responsible for the adaptation stage, and MT+ is involved in the compensation stage. The present finding suggests the pathway from V1 to MT+ has an important role in stabilizing the visual world.

Artifacts↗

PubMed2002

Neural responses in the primary visual cortex of the monkey during perceptual filling-in at the blind spot.

The phenomenon of perceptual filling-in demonstrates that physical stimuli presented on the retina do not necessarily correspond to surface perception, and that our visual system has mechanisms with which to interpolate missing information in order to construct continuous surfaces. Among its various forms, filling-in at the blind spot is one of the most remarkable. To study the neural mechanisms involved in filling-in at the blind spot, we recently conducted a recording experiment aimed at determining whether the neurons in the primary visual cortex (V1) that represent the visual field corresponding to the blind spot are activated when filling-in occurs. We found that neurons located in deep layers of the V1, particularly layer 6, respond to large stimuli that cover the blind spot and induce perceptual filling-in. These neurons tended to have very large receptive fields, which extended out of the blind spot, and preferred relatively large stimuli. We believe that neurons in the V1 region representing the blind spot encode information essential for perceptual filling-in at the blind spot.

Animals↗

PubMed2002

Response of MSTd neurons to simulated 3D orientation of rotating planes.

We studied whether the dorsal division of the medial superior temporal area (MSTd) in the macaque has activity related to structure-from-motion (SFM) processing. As the simplest form of three-dimensional (3D) structure, we chose a planar stimulus and examined the relation between the neural responses and the simulated 3D orientation of the plane defined by motion cues. We recorded from 114 MSTd neurons while monkeys were performing a visual fixation task. These neurons responded to a basic set of optic flow patterns such as translation, expansion/contraction, and rotation. Responses of these neurons to rotating plane stimuli were examined to see whether the MSTd neurons exhibited selectivity to the tilt and slant that characterize the 3D orientation of the plane. We found that most MSTd neurons tested (97 of 114) responded to the plane stimuli, and many neurons (65 of 97) exhibited selectivity to tilt and/or slant. Of 97 neurons, 18% (17/97) were selective only to tilt, 24% (23/97) only to slant, and 26% (25/97) to both. Control experiments rejected the possibility that the selectivity could be explained solely by the sensitivity to local stimulus components such as local translation, local speed, and local speed gradients. These results suggest that MSTd neurons are sensitive to stimulus features specific to the perceived 3D orientation of the rotating plane stimuli and suggest that area MSTd is involved in SFM processing.

Animals↗