Yoshimichi Ejima

Effect of light adaptation on the perceptual red–green and yellow–blue opponent-color responses

Spectral sensitivities of the red-green and yellow-blue opponent-color responses were determined under broad-band light adaptation for the light-adaptation levels of 5 to 5000 Td. With changing light-adaptation level, the spectral-sensitivity functions of the opponent-color systems change in shape, especially in the short-wavelength region of the spectrum. The light-adaptation effect on the red-green responses can be ascribed to the changes at the cone receptor level, whereas the light-adaptation effect on the yellow-blue responses can be ascribed to the changes at two sites, i.e., at the cone receptor site and at the opponent site.

Topographic representation of an occluded object and the effects of spatiotemporal context in human early visual areas.

Occlusion is a primary challenge facing the visual system in perceiving object shapes in intricate natural scenes. Although behavior, neurophysiological, and modeling studies have shown that occluded portions of objects may be completed at the early stage of visual processing, we have little knowledge on how and where in the human brain the completion is realized. Here, we provide functional magnetic resonance imaging (fMRI) evidence that the occluded portion of an object is indeed represented topographically in human V1 and V2. Specifically, we find the topographic cortical responses corresponding to the invisible object rotation in V1 and V2. Furthermore, by investigating neural responses for the occluded target rotation within precisely defined cortical subregions, we could dissociate the topographic neural representation of the occluded portion from other types of neural processing such as object edge processing. We further demonstrate that the early topographic representation in V1 can be modulated by prior knowledge of a whole appearance of an object obtained before partial occlusion. These findings suggest that primary “visual” area V1 has the ability to process not only visible or virtually (illusorily) perceived objects but also “invisible” portions of objects without concurrent visual sensation such as luminance enhancement to these portions. The results also suggest that low-level image features and higher preceding cognitive context are integrated into a unified topographic representation of occluded portion in early areas.

Inconsistency and uncertainty of the human visual area loci following surface‐based registration: Probability and Entropy Maps

Here we created two different multisubject maps (16 subjects) to characterize interindividual variability in the positions of human visual areas (V1, dorsal and ventral parts of V2/3, V3A, V3B, V7, LOc, MT+, and hV4 [or V4v and V8]), which were localized using fMRI and coregistered using a surface‐based method. The first is a probability map representing the degree of alignment inconsistency for each area, in which each point in space is associated with the probability affiliated with a given area. The second, a novel map termed an entropy map in which each point is associated with Shannon entropy computed from the probabilities, represents the degree of uncertainty regarding the area that resides there, and is maximal when all areas are equally probable. The overall average probability and entropy values were about 0.27 and 1.15 bits, respectively, with dependencies on the visual areas. The probability and entropy maps generated here will benefit any application which requires predictions of areas that are most likely present at an anatomical point and know the uncertainty associated with such predictions. Hum Brain Mapp, 2012.

Chromatic induction as a function of wavelength of inducing stimulus

Induced chromatic effects were determined for monochromatic, equal-luminance inducing stimuli from 460 to 680 nm by using a hue-cancellation procedure. The observed red-green-and yellow-blue-induced chromatic-response functions, which were different from the prediction based on the opponent-color hypothesis, could accurately explain the characteristics of the simultaneous color contrast effect. Good linear fits were obtained for the red-green function with a linear combination of R and G cones and for the yellow-blue function with a linear combination of R and B cones. These findings suggest that the opponent mechanisms for color contrast may be different from those for homogeneous color.

Functional relationship between chromatic induction and luminance of the inducing stimulus

We determined the functional relationship between chromatic induction and luminance of the inducing stimulus for different spatial conditions and assessed whether the effects of luminance and spatial variables could be explained in terms of the total effective energy in the inducing field. The result showed that the relationship between chromatic induction and luminance of the inducing stimulus could be mathematically expressed by an exponential function of the luminance ratio between the test and inducing stimuli and that the coefficient of the exponent was independent of spatial variables, i.e., area and separation. This led to the conclusion that a luminance ratio between two fields, rather than a quantum energy of the inducing field, was a relevant determinant of the effect of luminance of the inducing stimulus on chromatic induction.

Visual Perception of Contextual Effect and Its Neural Correlates

We investigated contextual effect in the visual perception using fMRI measurements. First, we examined spatiotemporal pattern of response modulation in human V1, V2 and V3/VP during contextual modulation of perceptual contrast using fMRI. Analysis of the spatial distribution of the response modulation indicated that multiple neural processes underlie the contextual effects. A long-range interaction that is selective to relative orientation may contribute predominantly to the suppressive response modulation in V1 and V2. Second, we examined higher-level contextual effect in the visual perception. We investigated the neural mechanisms of meaning generation from visual inputs, including Rorschach inkblots, using fMRI. Significant activity was observed in the prefrontal cortex together with distributed regions in the parietal and occipital cortices. The activated brain regions included the memory system for visual information and the spatial processing in visually guided eye movement in the brain. The results provide a clue to identify the brain regions responsible for the thought disorders and eye movement abnormalities of schizophrenia.

Relationship between chromatic induction and spatial variables: an integrated explanation in terms of element-contribution function

Chromatic induction as a function of separation and as a function of area was determined by a hue-cancellation procedure. Both functions obtained were expressed by exponential functions with similar exponential coefficients. This led to the derivation of an element-contribution function, based on a linear summation model, that could explain both the relationship between chromatic induction and separation and that between chromatic induction and area. The effects of separation and area on chromatic induction could readily be determined in terms of an element-contribution function. In addition, the induction area that is due to a blue inducing stimulus was larger than those that are due to the other inducing stimuli, suggesting that the summation area of the blue response was larger than those of the other chromatic responses.

Neural correlates of the stereokinetic effect revealed by functional magnetic resonance imaging

The stereokinetic effect (SKE) refers to a visual phenomenon in which a two-dimensional figure rotating in the fronto-parallel plane about the visual axis can create the impression of a three-dimensional (3-D) object. Although several characteristics of SKE suggest that the perceptual mechanisms involved in SKE may differ from those of the kinetic depth effect (KDE), the differences between SKE and KDE in neural mechanisms have not yet been investigated. In order to determine the cortical areas involved in SKE, we presented a variety of SKE stimuli in a series of functional magnetic resonance imaging experiments, controlling for motion and contrast energies as well as stimulus presentation paradigm. Cortical activation associated with SKE was observed in the middle temporal complex (hMT+), lateral occipital area (LO), V3B, inferior temporal gyrus (ITG), fusiform gyrus (FG), and dorsal intraparietal sulcus anterior (DIPSA). On the other hand, ITG, FG, and DIPSA were also activated by the static versions of SKE stimuli. hMT+, LO, and V3B are also known to be activated in KDE. These findings suggest that general motion-dependent 3-D object processing may be performed in these areas.

Chromatic valence and hue sensation.

Red-green and yellow-blue chromaticnesses were scaled for various monochromatic lights by a just-noticeable-difference method. The just-noticeable difference of each chromaticness, i.e., redness, greenness, yellowness, or blueness, was defined by the change of the canceling light intensity that was required to produce a just-noticeable difference in the amount of the opponent-hue attribute of each monochromatic light. The results showed that an approximately logarithmic transformation took place at the two opponent-color coding systems and that there existed an interaction between red-green and yellow-blue opponent-color coding systems in such a manner that the effective contribution of one opponent-color response to the perceived opponent-hue attribute was reduced by increasing the magnitude of the other opponent-color response. This interaction is considered to be responsible for the well-known veiling effect.

Basic principles of visual functions: mathematical formalism of geometries of shape and space, and the architecture of visual systems

There is growing evidence for homologous mechanisms of recognition/perception and navigation in many species, from insects to humans. This leads to the notion that the core systems of recognition and navigation are shared across species and that the visual environment during motion and/or navigation molds the spatiotemporal properties of the nervous systems across widely separated phyla according to basic common principles. In this study, we propose a mathematical formalism for two distinct geometries of shape and space in the visual images on the retina. The formalism enunciates the relevance of the architecture of the visual system for processing the two geometries and for producing some sort of circulating memory in space-time, i.e., recognition of allocentric space. Correspondence to: Shigeko Takahashi, Psychology Laboratory, Kyoto City University of Arts, Ohe-Kutsukake-cho, 13-6, Nishikyo-ku, Kyoto 601-1197, Japan, Tel: +81-(0)75-334-2265; E-mail: sgtak@kcua.ac.jp