Neural Visual System Research Articles

Infant and Child Vision Research: Present Status and Future Directions

Infant and Child Vision Research: Present Status and Future Directions

Optical intrinsic signal mapping of rod- and cone-mediated visual cortex responses in mice

We used optical imaging of intrinsic signals to study visual cortex responses in three mouse strains: wild-type ( C57BL/ 6J), a strain with only rod function ( cpfl1), and a strain with only cone function ( rho −/−). A stationary flicker light stimulus with intensity ranging from 10 8.6 to 10 15.5 photons/cm 2/s was used. We found that the intrinsic signal patterns exhibited stimulus intensity-dependent changes. At a given stimulus intensity, the patterns of intrinsic signals were clearly different in the three strains. These results suggest that the lack of normal functions of certain photoreceptors induces significant reorganization in the visual neural systems in mice.

Urochordate βγ-Crystallin and the Evolutionary Origin of the Vertebrate Eye Lens

A refracting lens is a key component of our image-forming camera eye; however, its evolutionary origin is unknown because precursor structures appear absent in nonvertebrates. The vertebrate betagamma-crystallin genes encode abundant structural proteins critical for the function of the lens. We show that the urochordate Ciona intestinalis, which split from the vertebrate lineage before the evolution of the lens, has a single gene coding for a single domain monomeric betagamma-crystallin. The crystal structure of Ciona betagamma-crystallin is very similar to that of a vertebrate betagamma-crystallin domain, except for paired, occupied calcium binding sites. The Ciona betagamma-crystallin is only expressed in the palps and in the otolith, the pigmented sister cell of the light-sensing ocellus. The Ciona betagamma-crystallin promoter region targeted expression to the visual system, including lens, in transgenic Xenopus tadpoles. We conclude that the vertebrate betagamma-crystallins evolved from a single domain protein already expressed in the neuroectoderm of the prevertebrate ancestor. The conservation of the regulatory hierarchy controlling betagamma-crystallin expression between organisms with and without a lens shows that the evolutionary origin of the lens was based on co-option of pre-existing regulatory circuits controlling the expression of a key structural gene in a primitive light-sensing system.

Neural compensation for the eye's optical aberrations.

A fundamental problem facing sensory systems is to recover useful information about the external world from signals that are corrupted by the sensory process itself. Retinal images in the human eye are affected by optical aberrations that cannot be corrected with ordinary spectacles or contact lenses, and the specific pattern of these aberrations is different in every eye. Though these aberrations always blur the retinal image, our subjective impression is that the visual world is sharp and clear, suggesting that the brain might compensate for their subjective influence. The recent introduction of adaptive optics to control the eye's aberrations now makes it possible to directly test this idea. If the brain compensates for the eye's aberrations, vision should be clearest with the eye's own aberrations rather than with unfamiliar ones. We asked subjects to view a stimulus through an adaptive optics system that either recreated their own aberrations or a rotated version of them. For all five subjects tested, the stimulus seen with the subject's own aberrations was always sharper than when seen through the rotated version. This supports the hypothesis that the neural visual system is adapted to the eye's aberrations, thereby removing somehow the effects of blur generated by the sensory apparatus from visual experience. This result could have important implications for methods to correct higher order aberrations with customized refractive surgery because some benefits of optimizing the correction optically might be undone by the nervous system's compensation for the old aberrations.

Adaptive Optics for Vision: The Eye's Adaptation to Point Spread Function

Despite the fact that ocular aberrations blur retinal images, our subjective impression of the visual world is sharp, which suggests that the visual system compensates for subjective influence. If the brain adjusts for specific aberrations of the eye, vision should be clearest when looking through a subject's typical wave aberration rather than through an unfamiliar one. We used adaptive optics techniques to control the eye's aberrations in order to evaluate this hypothesis. We used adaptive optics to produce point spread functions (PSFs) that were rotated versions of the eye's typical PSF by angles in 45 degrees intervals. Five normal subjects were asked to view a stimulus with their own PSF or with a rotated version, and to adjust the magnitude of the aberrations in the rotated case to match the subjective blur of the stimulus to that seen when the wave aberration was in typical orientation. The magnitude of the rotated wave aberration required to match the blur with the typical wave aberration was 20% to 40% less, indicating that subjective blur for the stimulus increased significantly when the PSF was rotated. These results support the hypothesis that the neural visual system is adapted to an eye's aberrations and has important implications for correcting higher order aberrations with customized refractive surgery or contact lenses. The full visual benefit of optimizing optical correction requires that the nervous system compensate for the new correction.

Receptive Field Map Development by Anti-Hebbian Learning

Neurons selective to oriented visual stimuli participate in the early steps of visual signal processes in higher mammals. A highly ordered neural connectivity responds to such a selectivity. The information necessary to establish this connectivity is too large to be contained in a genome. Moreover, neurophysiological experiments show that the alteration of small parts of the developing visual neural system affects the normal development of the whole system. Self-organizing processes able to produce local order from general properties have already been proposed as the driving force of these development stages. In order to model these self-organizing processes we study a two layer neural network based on simple organizational rules such as (1) diffusion of neural signal in the same layer, (2) Hebbian and anti-Hebbian learning, and (3) individual restrictions to the growth of each neuronal connection. We simulate the development of neurons selective to orientation and size organized in a map, underscoring the importance of anti-Hebbian learning for normal neural visual system development. © 1997 Elsevier Science Ltd.

Color, contrast sensitivity, and the cone mosaic.

This paper evaluates the role of various stages in the human visual system in the detection of spatial patterns. Contrast sensitivity measurements were made for interference fringe stimuli in three directions in color space with a psychophysical technique that avoided blurring by the eye's optics including chromatic aberration. These measurements were compared with the performance of an ideal observer that incorporated optical factors, such as photon catch in the cone mosaic, that influence the detection of interference fringes. The comparison of human and ideal observer performance showed that neural factors influence the shape as well as the height of the foveal contrast sensitivity function for all color directions, including those that involve luminance modulation. Furthermore, when optical factors are taken into account, the neural visual system has the same contrast sensitivity for isoluminant stimuli seen by the middle-wavelength-sensitive (M) and long-wavelength-sensitive (L) cones and isoluminant stimuli seen by the short-wavelength-sensitive (S) cones. Though the cone submosaics that feed these chromatic mechanisms have very different spatial properties, the later neural stages apparently have similar spatial properties. Finally, we review the evidence that cone sampling can produce aliasing distortion for gratings with spatial frequencies exceeding the resolution limit. Aliasing can be observed with gratings modulated in any of the three directions in color space we used. We discuss mechanisms that prevent aliasing in most ordinary viewing conditions.

Efficiency in detection of isoluminant and isochromatic interference fringes

We examined the limitations imposed by neural factors on spatial contrast sensitivity for both isochromatic and isoluminant gratings. We used two strategies to isolate these neural factors. First, we eliminated the effect of blurring by the dioptrics of the eye by using interference fringes. Second, we corrected our data for additional sensitivity losses up to and including the site of photon absorption by applying an ideal-observer analysis described by Geisler [J. Opt. Soc. Am. A 1, 775 (1984)]. Our measurements indicate that the neural visual system modifies the shape of the contrast-sensitivity functions for both isochromatic and isoluminant stimuli at high spatial frequencies. If we assume that the high-spatial-frequency performance of the neural visual system is determined by a low-pass spatial filter followed by additive noise, then the visual system has a spatial bandwidth 1.8 times lower for isoluminant red-green than for isochromatic stimuli. On the other hand, we find no difference in bandwidth or sensitivity of the neural visual system for isoluminant red-green and S-cone-isolated stimuli.

Neural, optical, and search performance in prolonged viewing of chromatic displays.

The effect of prolonged cathode ray tube (CRT) work on the afferent visual neural and focusing system of the human eye was examined by visually evoked potentials (VEPs) and laser optometer techniques, respectively. These measures were compared with parallel measurements of search performance. Ten volunteers performed three 4-hr sessions of a search/decision task on a high-resolution-red/green/blue (RGB) monitor. Stimuli were numbers and symbols presented in a blue/black, red/black, or red/green foreground/background combination. Results showed minimal changes in the neural and focusing responses to each foreground/background combination over time on task. Weak correlations were found between neural-optical and search performance measurements. These data suggest that the use of chromatic stimuli in the color combinations examined do not have a detrimental effect on visual performance.

Model of neural visual system with self-organizing cells.

This paper describes a model of a neural visual system of a higher animal, in which the capability of pattern recognition develops adaptively. To produce the adaptability, we adopted "self-organizing cells," and with them modeled feature-detecting cells which were discovered by Hubel and Wiesel and whose plasticity was found by Blakemore and Cooper. Combining the "self-organizing cells" and the learning principle of a Perceptron-type system, we constructed a model of the whole visual system. The model is also equipped with an eye movement control mechanism for gazing, which reduces the number of "self-organizing cells" required for pattern recognition, thus contributing to their quick self-organization. Computer simulation and an experiment using a hardware simulator showed that "self-organizing cells" quickly become sensitive to the features often seen and that the resulted system can classify patterns with a rather small number of feature-detecting cells.

視覚神経系と視覚特性の年齢による変化

視覚神経系と視覚特性の年齢による変化

Optic nerve hypoplasia: changing perspectives.

Optic nerve hypoplasia, having once been regarded as exceedingly rare, is now regarded as one of the major causes of visual loss in infancy. Recognition of subtle forms and segmental forms of optic nerve hypoplasia are important in explaining the evaluation of subnormal vision, and have an added significance in the recognised associations of central nervous system anomalies including abnormalities of the endocrine system. These changing clinical perspectives are reviewed, together with a discussion of the pathogenesis of optic nerve hypoplasia in the light of cell death and nerve fibre degeneration seen in normal development of the neural visual system.

Visibility of interference fringes near the resolution limit.

The contrast sensitivity of the visual system to interference fringes has been measured in the range from 10 to 65 cycles/deg with a forced-choice psychophysical procedure. Masking produced by the spatial-noise characteristic of coherent fields was avoided by diluting the interferometric field with a fixed amount of uniform, incoherent light. The loss of contrast sensitivity between 10 and 60 cycles/deg ranged from 0.85 to 1.5 log units depending on the observer. Despite these individual differences, the mean contrast sensitivity for six observers at 60 cycles/deg was more than a factor of 8 higher than the most sensitive previous estimates, suggesting that the neural visual system is much more sensitive to fine detail than previously believed. The most sensitive observer required only 4% contrast to detect a 60-cycle/deg interference fringe. Even the shallow interferometric contrast-sensitivity functions reported here are too steep to be explained solely by scattered light at the retina. It is argued that the optical properties of the photoreceptor mosaic make a negligible contribution to the contrast-sensitivity loss between 0 and 60 cycles/deg, and neural factors must be implicated.

自己組織化細胞を含む視覚系のモデル

自己組織化細胞を含む視覚系のモデル

Synthesis of "On-Off" and "Off" Responses in a Visual-Neural System

Synthesis of "On-Off" and "Off" Responses in a Visual-Neural System

Synthesis of "On-Off" and "Off" Responses in a Visual-Neural System

Synthesis of "On-Off" and "Off" Responses in a Visual-Neural System

Visual Apparent Movement in the Absence of Neural Interaction

The literature concerned with the problem of visual apparent movement has been reviewed by Neff.' His paper describes Wertheimer's classical experiments defining the phi phenomenon,2 outlines later research, and considers the theoretical formulations which have grown up about the problem of apparent movement. Several hypotheses have been offered to explain visual apparent movement. Of all such hypotheses, those advanced by the Gestalt psychologists have been perhaps the best known and the most controversial.3 In general, the Gestalt theories contend that the experience of visual apparent movement is a direct one, correlated with appropriate neural stimulation. Not only is the initial stimulus presumed to activate one tract of the visual neural system, and the second stimulus to activate another; but by physiological interaction the two serially stimulated tracts are supposed to create between them a field of excitation whose path corresponds to the spatial path of the observed movement. It is this field which is held responsible for the experience of movement. Wertheimer's original formulation assigned the locus of interaction to the higher neural centers; later formulations have been much less specific.4