Biomedical subjects

W H Bosking

Publications and source records attributed to W H Bosking.

5 recordsLinked to original sources

PubMed2000

Functional specificity of callosal connections in tree shrew striate cortex.

Although callosal connections have been shown to link extensive regions of primary visual cortex, the distribution of these connections with respect to the map of visual space and the map of orientation preference remains unclear. Here we combine optical imaging of intrinsic signals with injection of fluorescent microspheres to assess the functional specificity of callosal connections in the tree shrew. By imaging both hemispheres simultaneously while presenting a series of spatially restricted stimuli, we find that a substantial region of visual space is represented bilaterally. Each hemisphere includes a representation of the ipsilateral visual field that is highly compressed relative to that of the contralateral visual field and is most extensive in the lower visual field, where approximately 30(o) of central visual space are represented bilaterally. Callosal connections extend throughout the region of bilateral representation but terminate in a spatially restricted manner that links visuotopically corresponding sites in the two hemispheres. In contrast, callosal connections appear to terminate without regard for the map of orientation preference, showing little sign of the orientation-specific modular and axial specificity that is characteristic of long-range horizontal connections. By coordinating the activity in the two hemispheres in a way that preserves nearest neighbor relationships, callosal connections may best be viewed as elements of local circuits that operate within a single bilateral representation of visual space.

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PubMed1999

Maps of central visual space in ferret V1 and V2 lack matching inputs from the two eyes.

In the visual cortex, the representation of central visual space is supplied by matching geniculate inputs that are driven exclusively by one eye or the other. In layer 4 of early visual areas (V1 in primates and V1 and V2 in cat), these inputs form a nearly uniform array of small ocular dominance domains, while preserving overall topographic order within the cortical map. In ferret, however, ocular dominance domains in different regions of the visual cortex are strikingly irregular in size and shape. The exceptionally large size of domains in some regions implies a departure from the usual visuotopic matching of inputs from the two eyes. Using optical-imaging, electrophysiological, and anatomical techniques, we show that this regional variation is attributable to exclusively monocular maps of the central portions of the ipsilateral visual field in V1 and the contralateral visual field in V2. In addition, we document a complex interdigitation of V1 and V2 that entails a discontinuity in the mapping of visual space and fragmentation of V2 into isolated cortical territories. We suggest that both the monocularity of these cortical maps and the visuotopic discontinuity along the V1-V2 border derive from asymmetries in the crossed and uncrossed retinal pathways.

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PubMed1997

Orientation selectivity and the arrangement of horizontal connections in tree shrew striate cortex.

Horizontal connections, formed primarily by the axon collaterals of pyramidal neurons in layer 2/3 of visual cortex, extend for millimeters parallel to the cortical surface and form patchy terminations. Previous studies have provided evidence that the patches formed by horizontal connections exhibit modular specificity, preferentially linking columns of neurons with similar response characteristics, such as preferred orientation. The issue of how these connections are distributed with respect to the topographic map of visual space, however, has not been resolved. Here we combine optical imaging of intrinsic signals with small extracellular injections of biocytin to assess quantitatively the specificity of horizontal connections with respect to both the map of orientation preference and the map of visual space in tree shrew V1. Our results indicate that horizontal connections outside a radius of 500 microm from the injection site exhibit not only modular specificity, but also specificity for axis of projection. Labeled axons extend for longer distances, and give off more terminal boutons, along an axis in the map of visual space that corresponds to the preferred orientation of the injection site. Inside of 500 microm, the pattern of connections is much less specific, with boutons found along every axis, contacting sites with a wide range of preferred orientations. The system of long-range horizontal connections can be summarized as preferentially linking neurons with co-oriented, co-axially aligned receptive fields. These observations suggest specific ways that horizontal circuits contribute to the response properties of layer 2/3 neurons and to mechanisms of visual perception.

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PubMed1996

A systematic map of direction preference in primary visual cortex.

Neurons in the primary visual cortex respond selectively to the orientation of edges and their direction of motion. Orientation preference is mapped in a systematic fashion across the cortical surface, such that neurons in adjacent columns have similar but slightly shifted preferred orientations. Microelectrode studies have suggested that direction preference is also arranged in a systematic fashion, but exactly how this response property is mapped remains unclear. Here we show by optical imaging of intrinsic signals in ferret cortical area 17 that there is a mosaic-like map of direction preference. This map consists of numerous regions within which direction preference changes in a slow, continuous fashion. These regions are separated by winding boundaries (fractures) across which direction preference shifts abruptly, often by 180 degrees. Comparison of direction and orientation preference maps shows that these fractures subdivide iso-orientation domains into regions selective for opposite directions of motion.

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PubMedDate not supplied

Consistent mapping of orientation preference across irregular functional domains in ferret visual cortex.

The mammalian visual cortex harbors a number of functional maps that represent distinct attributes of stimuli in the visual environment. How different functional maps are accommodated within the same cortical space, especially in species that show marked irregularities in one or more functional maps, remains poorly understood. We used optical imaging of intrinsic signals and electrophysiological techniques to investigate the organization of the maps of orientation preference, ocular dominance, and visual space in ferret. This species shows striking nonuniformity in the arrangement of ocular dominance domains and disruption of the mapping of visual space along the V1/V2 border. We asked whether these irregularities would be reflected in the organization of the map of orientation preference. The results show that orientation preference is mapped consistently within both V1 and V2, and across the interareal boundary, with no reflection of the irregularities in the other maps. These observations demonstrate the accommodation of multiple functional maps within the same cortical space without systematic geometrical relationships that necessarily constrain the organization of each representation. Furthermore, they imply that the structure of the map of orientation preference reflects the architecture and activity patterns of cortical circuits that are independent of other columnar systems established in layer 4.

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