Biomedical subjects

Hidehiko Komatsu

Publications and source records attributed to Hidehiko Komatsu.

11 recordsLinked to original sources

PubMed2006

Effects of task demands on the responses of color-selective neurons in the inferior temporal cortex.

Categorization and fine discrimination are two different functions in visual perception, and we can switch between these two functions depending on the situation or task demands. To explore how visual cortical neurons behave in such situations, we recorded the activities of color-selective neurons in the inferior temporal (IT) cortex of two monkeys trained to perform a color categorization task, a color discrimination task and a simple fixation task. Many IT neurons changed their activity depending upon the task, although color selectivity was well conserved. A majority of neurons showed stronger responses during the categorization task. Moreover, for the population of IT neurons as a whole, signals contributing to performing the categorization task were enhanced. These results imply that judgment of color category by color-selective IT neurons is facilitated during the categorization task and suppressed during the discrimination task as a consequence of task-dependent modulation of their activities.

Action Potentials↗

PubMed2006

Neuronal dynamics of bottom-up and top-down processes in area V4 of macaque monkeys performing a visual search.

Visual selection is thought to be guided by both bottom-up intrinsic visual saliency and top-down visual attention. We examined how the relative importance of each of these processes dynamically changes over the course of a visual search in area V4 of two macaque monkeys. The animals were trained to perform a multidimensional visual search task in which a search array that consisted of two singleton stimuli, unique in shape and color, and four nonsingleton stimuli was presented. The task was to select one of the singletons as the saccade target, depending on the instructed search dimension, which was switched between shape and color. The strengths of the neural modulations caused by the bottom-up and top-down factors were, respectively, evaluated by assessing the dependence of the visual responses on the singleton type and the behavioral significance of the receptive field stimulus. We found that the initial visual responses of V4 neurons predominantly specified the singleton type of the receptive field stimulus, while the late presaccadic response primarily specified its behavioral significance. For comparison, recordings were also made from the frontal eye field (FEF) of one monkey and we found that the predominant activity of FEF neurons signaled the behavioral significance of the receptive field stimulus over time. These findings suggest that area V4 might function as an intermediate stage where both sensory- and behavior-based signals are dynamically represented, depending on their online requirements during a visual search, whereas the FEF predominantly represents behavior-based signals for upcoming saccade responses.

Action Potentials↗

PubMed2006

The neural mechanisms of perceptual filling-in.

Filling-in is a perceptual phenomenon in which a visual attribute such as colour, brightness, texture or motion is perceived in a region of the visual field even though such an attribute exists only in the surround. Filling-in dramatically reveals the dissociation between the retinal input and the percept, and raises fundamental questions about how these two relate to each other. Filling-in is observed in various situations, and is an essential part of our normal surface perception. Here, I review recent experiments examining brain activities associated with filling-in, and discuss possible neural mechanisms underlying this remarkable perceptual phenomenon. The evidence shows that neuronal activities in early visual cortical areas are involved in filling-in, providing new insights into visual cortical functions.

Animals↗

PubMed2005

Air-driven eye shutter system for vision experiments using awake behaving animals.

When conducting physiological and behavioral experiments for vision research, we often need to present stimuli monocularly. In this report, we describe a new eye shutter system that was developed to occlude individual eyes during experiments with awake behaving animals. The eye of choice is occluded by an air pressure-driven opaque mask, the opening and closing of which is controlled by computer. This new eye shutter system has several advantages over other systems: (1) it can completely shield the eye from light using a physically opaque eye mask; (2) contamination of electrophysiological recordings by electrical noise is avoided because the actuator is not electrical; (3) the left and right eyes can be randomly occluded in each trial; (4) space for apparatus in front of the animal's face is not required. We used this eye shutter system to record neural activity from the primary visual cortex of awake monkeys and confirmed that this system is suitable for the monocular presentation of visual stimuli during electrophysiological recordings from awake animals.

Air↗

PubMed2005

Neural responses in the macaque v1 to bar stimuli with various lengths presented on the blind spot.

Although there is no retinal input within the blind spot, it is filled with the same visual attributes as its surround. Earlier studies showed that neural responses are evoked at the retinotopic representation of the blind spot in the primary visual cortex (V1) when perceptual filling-in of a surface or completion of a bar occurs. To determine whether these neural responses correlate with perception, we recorded from V1 neurons whose receptive fields overlapped the blind spot. Bar stimuli of various lengths were presented at the blind spots of monkeys while they performed a fixation task. One end of the bar was fixed at a position outside the blind spot, and the position of the other end was varied. Perceived bar length was measured using a similar set of bar stimuli in human subjects. As long as one end of the bar was inside the blind spot, the perceived bar length remained constant, and when the bar exceeded the blind spot, perceptual completion occurred, and the perceived bar length increased substantially. Some V1 neurons of the monkey exhibited a significant increase in their activity when the bar exceeded the blind spot, even though the amount of the retinal stimulation increased only slightly. These response increases coincided with perceptual completion observed in human subjects and were much larger than would be expected from simple spatial summation and could not be explained by contextual modulation. We conclude that the completed bar appearing on the part of the receptive field embedded within the blind spot gave rise to the observed increase in neuronal activity.

Action Potentials↗

PubMed2004

Target selection in area V4 during a multidimensional visual search task.

Natural scenes typically contain multiple objects that are unique in different stimulus dimensions so that an object with feature contrast to surrounding objects draws attention and pops out. Furthermore, if we have previous knowledge about the dimension in which a target object differs from the surrounding objects, we will attend to that dimension and more easily detect the target. Our aims here were to elucidate neural mechanisms underlying this type of attention by recording neuronal activities from area V4 and to investigate how visual signals encoding feature contrast between objects are modulated by attention specific to a particular dimension. To accomplish this, we trained monkeys to do a multidimensional visual search task in which two singleton stimuli, unique in the color or shape dimension, were presented with four other identical stimuli. The monkeys had to search for the singleton stimulus that was unique in the instructed dimension while the search dimension was switched between shape and color. We found that individual V4 neurons carry visual signals encoding feature contrast in either shape or color, and this signal is modulated depending on the search dimension. Population responses to the target singleton stimulus were significantly higher than to others, regardless of the search dimension. In most V4 neurons, however, significant response increases occurred only when one particular singleton stimulus was the target. These findings suggest that interaction between bottom-up signals encoding feature contrast between stimuli and top-down signals encoding search dimension occurs in V4 and facilitates adaptive selection of targets in a complex visual environment.

Animals↗

PubMed2004

Representation of angles embedded within contour stimuli in area V2 of macaque monkeys.

Angles and junctions embedded within contours are important features to represent the shape of objects. To study the neuronal basis to extract these features, we conducted extracellular recordings while two macaque monkeys performed a fixation task. Angle stimuli were the combination of two straight half-lines larger than the size of the classical receptive fields (CRFs). Each line was drawn from the center to outside the CRFs in 1 of 12 directions, so that the stimuli passed through the CRFs and formed angles at the center of the CRFs. Of 114 neurons recorded from the superficial layer of area V2, 91 neurons showed selective responses to these angle stimuli. Of these, 41 neurons (36.0%) showed selective responses to wide angles between 60 degrees and 150 degrees that were distinct from responses to straight lines or sharp angles (30 degrees ). Responses were highly selective to a particular angle in approximately one-fourth of neurons. When we tested the selectivity of the same neurons to individual half-lines, the preferred direction was more or less consistent with one or two components of the optimal angle stimuli. These results suggest that the selectivity of the neurons depends on both the combination of two components and the responses to individual components. Angle-selective V2 neurons are unlikely to be specific angle detectors, because the magnitude of their responses to the optimal angle was indistinguishable from that to the optimal half-lines. We suggest that the extraction of information of angles embedded within contour stimuli may start in area V2.

Animals↗

PubMed2003

Functional organization of the cat visual cortex in relation to the representation of a uniform surface.

Neuronal activity in the early visual cortex has been extensively studied from the standpoint of contour representation. On the other hand, representation of the interior of a surface surrounded by a contour is much less well understood. Several studies have identified neurons activated by a uniform surface covering their receptive fields, but their distribution within the cortex is not yet known. The aim of the present study was to obtain a better understanding of the distribution of such neurons in the visual cortex. Using optical imaging of intrinsic signals, we found that there are a group of surface-responsive regions located in area 18, along the area 17/18 border, that tend to overlap the singular points of the orientation-preference map. Extracellular recordings confirmed that neurons responsive to uniform plane stimuli are accumulated in these regions. Such neurons also existed outside the surface-responsive regions around the singular points. These results suggest that there exists a functional organization related to the representation of a uniform surface in the early visual cortex.

Animals↗

PubMed2002

Variations in long- and middle-wavelength-sensitive opsin gene loci in crab-eating monkeys.

We analyzed variations in long (L)- and middle (M)-wavelength-sensitive opsin gene loci in crab-eating monkeys. Unlike humans, most monkeys have a single L and a single M gene. Two variant genotypes, one with only one opsin gene (dichromatic) and one with tandemly arrayed multiple genes, were also found in the monkeys. However, the frequency of the former was 0.47%, and that of the latter was 5% in the monkeys, while 2% and 66%, respectively, in Caucasian males. The two variants were found only in Java Island, Indonesia, and South Thailand, respectively. The data suggest that the frequency of each genotype is different among Old World primates.

Animals↗

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↗