[Show abstract][Hide abstract] ABSTRACT: When a degraded two-tone image such as a "Mooney" image is seen for the first time, it is unrecognizable in the initial seconds. The recognition of such an image is facilitated by giving prior information on the object, which is known as top-down facilitation and has been intensively studied. Even in the absence of any prior information, however, we experience sudden perception of the emergence of a salient object after continued observation of the image, whose processes remain poorly understood. This emergent recognition is characterized by a comparatively long reaction time ranging from seconds to tens of seconds. In this study, to explore this time-consuming process of emergent recognition, we investigated the properties of the reaction times for recognition of degraded images of various objects. The results show that the time-consuming component of the reaction times follows a specific exponential function related to levels of image degradation and subject's capability. Because generally an exponential time is required for multiple stochastic events to co-occur, we constructed a descriptive mathematical model inspired by the neurophysiological idea of combination coding of visual objects. Our model assumed that the coincidence of stochastic events complement the information loss of a degraded image leading to the recognition of its hidden object, which could successfully explain the experimental results. Furthermore, to see whether the present results are specific to the task of emergent recognition, we also conducted a comparison experiment with the task of perceptual decision making of degraded images, which is well known to be modeled by the stochastic diffusion process. The results indicate that the exponential dependence on the level of image degradation is specific to emergent recognition. The present study suggests that emergent recognition is caused by the underlying stochastic process which is based on the coincidence of multiple stochastic events.
[Show abstract][Hide abstract] ABSTRACT: We investigated effects of noise inputs on synchronous firing in a simple model neuron system with electro-physiological heterogeneity. The system consists of two leaky integrate-and-fire (LIF) neurons with different membrane time constants. They are uncoupled and driven by Gaussian white noise inputs. We found that correlation between spike trains of two LIF neurons was maximized at input correlation slightly smaller than one. The same result was obtained for a more realistic system consisting of two non-identical Hodgkin-Huxley neurons. We also revealed the mechanism underlying the effect.
No preview · Article · Sep 2014 · Journal of the Physical Society of Japan
[Show abstract][Hide abstract] ABSTRACT: To understand neural mechanisms of object recognition, representation of visual object images has been studied in inferior temporal (IT) cortex, which is located at the final stage of the ventral object recognition pathway. Optical intrinsic signal imaging (OISI) is a powerful imaging technique to visualize spatial patterns of object representation in IT cortex. In this chapter, we will describe technical procedures of OISI particularly required when applying to IT cortex, and also recent advancement with OISI in understanding object representation and functional organization of IT cortex.
[Show abstract][Hide abstract] ABSTRACT: There are two dominant models for the functional organization of brain regions underlying object recognition. One model postulates category-specific modules while the other proposes a distributed representation of objects with generic visual features. Functional imaging techniques relying on metabolic signals, such as fMRI and optical intrinsic signal imaging (OISI), have been used to support both models, but due to the indirect nature of the measurements in these techniques, the existing data for one model cannot be used to support the other model. Here, we used large-scale multielectrode recordings over a large surface of anterior inferior temporal (IT) cortex, and densely mapped stimulus-evoked neuronal responses. We found that IT cortex is subdivided into distinct domains characterized by similar patterns of responses to the objects in our stimulus set. Each domain spanned several millimeters on the cortex. Some of these domains represented faces ("face" domains) or monkey bodies ("monkey-body" domains). We also identified domains with low responsiveness to faces ("anti-face" domains). Meanwhile, the recording sites within domains that displayed category selectivity showed heterogeneous tuning profiles to different exemplars within each category. This local heterogeneity was consistent with the stimulus-evoked feature columns revealed by OISI. Taken together, our study revealed that regions with common functional properties (domains) consist of a finer functional structure (columns) in anterior IT cortex. The "domains" and previously proposed "patches" are rather like "mosaics" where a whole mosaic is characterized by overall similarity in stimulus responses and pieces of the mosaic correspond to feature columns.
Full-text · Article · Oct 2013 · The Journal of Neuroscience : The Official Journal of the Society for Neuroscience
[Show abstract][Hide abstract] ABSTRACT: Purpose:
To investigate the properties and origin of retinal intrinsic signals by functional optical coherence tomography (fOCT) in macaque retinas.
We modified a spectral domain OCT system to be able to give short-duration flashes or continuous light stimulation to the retina of three adult macaque monkeys (Macaca mulatta) under general anesthesia. Changes in the intensities of the OCT signals following the stimulus were determined.
Stimulus-evoked decreases or increases in the OCT signals were observed in the photoreceptor inner segment ellipsoids and outer segments, respectively. Experiments with focal and colored stimuli confirmed that these fOCT signals originated from the photoreceptors. No diffuse changes in the OCT signals were detected in the inner retinal layers; however, there were slow changes in small discrete areas where the retinal vessels were located. The polarity of the fOCT signals in the inner retinal layer was dependent on each activated region, and one of the possible sources of the reflectance changes was the light-scattering changes of the retinal vessels.
The fOCT signals in the macaque retina consist of at least three components: light-scattering changes from the photoreceptor inner segment ellipsoids, light-scattering changes from the outer segments, and slow light-scattering changes from the blood vessels in the inner retina. This technique has the potential of mapping local neuronal activity three-dimensionally and may help in the diagnosis of retinal disorders of different retinal origins.
No preview · Article · Aug 2013 · Investigative ophthalmology & visual science
[Show abstract][Hide abstract] ABSTRACT: Higher mammals use different hierarchical levels of visual information to guide goal-oriented behavior. In the brain, visual features, objects and categories are encoded in inferotemporal (IT) cortex, but the neu- ronal organization linking these three levels of information is un- known. Using dense untargeted electrophysiological recordings and intrinsic signal imaging, we found that the exposed cortex is sub- divided into functionally distinct contiguous domains, each spanning several millimeters ; one of these domains represented the face cate- gory. Remarkably, we also identified domains with low responsiveness to the face category, revealing that anti-visual category preference coding participates in cortical processing. These face sensitive do- mains were observed to contain heterogeneous local activity for the face category corresponding to feature columns ; thus we term these domains mosaics. These findings demonstrate that hierarchical repre- sentation of facial features, faces, and face category are tightly coordi- nated in face mosaics.
[Show abstract][Hide abstract] ABSTRACT: Chronic multielectrode recording has become a widely used technique in the past twenty years, and there are multiple standardized methods. As for recording with high-density array, the most common method in macaque monkeys is to use a subdural array with fixed electrodes. In this study, we utilized the electrode array with independently maneuverable electrodes arranged in high-density, which was originally designed for use on small animals, and redesigned it for use on macaque monkeys while maintaining the virtues of maneuverability and high-density. We successfully recorded single and multiunit activities from up to 49 channels in the V1 and inferior temporal (IT) cortex of macaque monkeys. The main change in the surgical procedure was to remove a 5mm diameter area of dura mater. The main changes in the design were (1) to have a constricted layer of heavy silicone oil at the interface with the animal to isolate the electrical circuit from the cerebrospinal fluid, and (2) to have a fluid draining system that can shunt any potential postsurgical subcranial exudate to the extracranial space.
No preview · Article · Aug 2012 · Journal of Neuroscience Methods
[Show abstract][Hide abstract] ABSTRACT: Odor signals are conveyed from the olfactory bulb to the olfactory cortex (OC) by mitral cells (MCs) and tufted cells (TCs). However, whether and how the two types of projection neuron differ in function and axonal connectivity is still poorly understood. Odor responses and axonal projection patterns were compared between MCs and TCs in mice by visualizing axons of electrophysiologically identified single neurons. TCs demonstrated shorter onset latency for reliable responses than MCs. The shorter latency response of TCs was maintained in a wide range of odor concentrations, whereas MCs responded only to strong signals. Furthermore, individual TCs projected densely to focal targets only in anterior areas of the OC, whereas individual MCs dispersedly projected to all OC areas. Surprisingly, in anterior OC areas, the two cell types projected to segregated subareas. These results suggest that MCs and TCs transmit temporally distinct odor information to different OC targets.
Full-text · Article · Jun 2012 · The Journal of Neuroscience : The Official Journal of the Society for Neuroscience
[Show abstract][Hide abstract] ABSTRACT: The purpose of this study is to determine the topography of bleaching in rods, middle/long-wavelength (M/L) and short-wavelength (S) cones in the macaque retina by using a modified retinal densitometry technique.
A modified commercial digital fundus camera system was used to measure continuously the intensity of the light reflectance during bleaching with band pass lights in the ocular fundus of three adult Rhesus monkeys (Macaca mulatta) under general anesthesia. The topography of bleaching in rods, M/L-, and S-cones was obtained separately by considering the characteristic time course of the reflectance changes, depending on the wavelengths of light and retinal locations.
The distribution of M/L-cones response had a steep peak at the foveal center and was elongated horizontally. The distribution of rod responses was minimum at the foveal center and maximum along a circular region at the eccentricity of the optic disc. The distribution of S-cone responses was highest at the fovea and was excavated centrally. There was a circular region with the maximal responses at 0.38 to 1.0 degrees from the foveal center.
With the current imaging technique, not only the steep peak of the M/L-cone responses at the fovea, but the ring-shaped distribution of rod responses in the periphery and the central reduction of S-cone response could be determined with good resolution.
No preview · Article · Mar 2012 · Investigative ophthalmology & visual science
[Show abstract][Hide abstract] ABSTRACT: Binocular rivalry is a useful psychophysical tool to investigate neural correlates of visual consciousness because the alternation between awareness of the left and right eye images occurs without any accompanying change in visual input. The conventional experiments on binocular rivalry require participants to voluntarily report their perceptual state. Obtaining reliable reports from non-human primates about their subjective visual experience, however, requires long-term training, which has made electrophysiological experiments on binocular rivalry quite difficult. Here, we developed a new binocular rivalry stimulus that consists of two different object images that are phase-shifted to move in opposite directions from each other: One eye receives leftward motion while the other eye receives rightward motion, although both eyes' images are perceived to remain at the same position. Experiments on adult human participants showed that eye movements (optokinetic nystagmus, OKN) are involuntarily evoked during the observation of our stimulus. We also found that the evoked OKN can serve as a cue for accurate estimation about which object image was dominant during rivalry, since OKN follows the motion associated with the image in awareness at a given time. This novel visual presentation technique enables us to effectively explore the neural correlates of visual awareness using animal models.
No preview · Article · Mar 2012 · Journal of Vision
[Show abstract][Hide abstract] ABSTRACT: In studies of in vivo extracellular recording, we usually penetrate electrodes almost blindly into the neural tissue, in order to detect the neural activity from an expected target location at a certain depth. After the recording, it is necessary for us to determine the position of the electrodes precisely. Generally, to identify the position of the electrode, one method is to examine the postmortem tissue sample at micron resolution. The other method is using MRI and it does not have enough resolution to resolve the neural structures. To solve such problems, we propose swept source optical coherence tomography (SS-OCT) as a tool to visualize the cross-sectional image of the neural target structure along with the penetrating electrode. We focused on a rodent olfactory bulb (OB) as the target. We succeeded in imaging both the OB layer structure and the penetrating electrode, simultaneously. The method has the advantage of detecting the electrode shape and the position in real time, in vivo. These results indicate the possibility of using SS-OCT as a powerful tool for guiding the electrode into the target tissue precisely in real time and localizing the electrode tip during electrophysiological recordings.
Full-text · Article · Nov 2011 · Biomedical Optics Express
[Show abstract][Hide abstract] ABSTRACT: Here, we report in vivo 3-D visualization of the layered organization of a rat olfactory bulb (OB) by a swept source optical coherence tomography (SS-OCT). The SS-OCT operates at a wavelength of 1334 nm with respective theoretical depth and lateral resolutions of 6.7 μm and 15.4 μm in air and hence it is possible to get a 3D structural map of OB in vivo at the micron level resolution with millimeter-scale imaging depth. Up until now, with methods such as MRI, confocal microscopy, OB depth structure in vivo had not been clearly visualized as these do not satisfy the criterion of simultaneously providing micron-scale spatial resolution and imaging up to a few millimeter in depth. In order to confirm the OB's layered organization revealed by SS-OCT, we introduced the technique of electrocoagulation to make landmarks across the layered structure. To our knowledge this is such a first study that combines electrocoagulation and OCT in vivo of rat OB. Our results confirmed the layered organization of OB, and moreover the layers were clearly identified by electrocoagulation landmarks both in the OCT structural and anatomical slice images. We expect such a combined study is beneficial for both OCT and neuroscience fields.
Full-text · Article · Aug 2011 · Biomedical Optics Express