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D Fitzpatrick

Publications and source records attributed to D Fitzpatrick.

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The sublaminar organization of corticogeniculate neurons in layer 6 of macaque striate cortex.

We examined the laminar distribution of corticogeniculate neurons in the macaque striate cortex labeled by axonal transport following injections of retrograde tracers into the lateral geniculate nucleus (LGN). Large injections of retrograde tracers involving all layers of the LGN resulted in a distinctive bilaminar distribution of labeled cells in cortical layer 6. One tier of labeled neurons was located along the layer 5-6 border and a second was located near the bottom of the layer, leaving the middle of layer 6 largely free of labeled neurons. Following injections of tracers that were restricted to the magnocellular layers of the LGN, almost all of the labeled neurons were located in the lower tier. In contrast, following injections of retrograde tracers confined to the parvocellular layers of the LGN, labeled cells were found in both tiers, with the greatest number in the upper tier. Thus, layer 6 of macaque striate cortex consists of three distinct sublayers only two of which are the source of descending projections to the LGN: an upper tier that projects exclusively to the parvocellular layers and a lower tier that projects to both magnocellular and parvocellular layers.

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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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The functional organization of local circuits in visual cortex: insights from the study of tree shrew striate cortex.

We have used a combination of anatomical and physiological techniques to explore the functional organization of vertical and horizontal connections in tree shrew striate cortex. Our studies of vertical connections reveal a remarkable specificity in the laminar arrangement of the projections from layer IV to layer III that establishes three parallel intracortical pathways. The pathways that emerge from layer IV are not simple continuations of parallel thalamocortical pathways. Layer IV and its connections with layer II/III restructure the inputs from the LGN, combining the activity from ON and OFF channels and from the left and right eye and transmit the products of this synthesis to separate strata within the overlying layers. In addition, studies of two other prominent vertical connection pathways, the projections from layer VI to layer IV and from layer II/III to layer V suggest that the parallel nature of these systems is perpetuated throughout the cortical depth. Our studies of horizontal connections have revealed a systematic relationship between a neuron's orientation preference and the distribution of its axon arbor across the cortical map of visual space. Horizontal connections in layer II/III extend for greater distances and give rise to a greater number of terminals along an axis of the visual field map that corresponds to the neuron's preferred orientation. These findings suggest that the contribution of horizontal inputs to the response properties of layer II/III neurons is likely to be greater in regions of visual space that lie along the axis of preferred orientation (endzones) than along the orthogonal axis (side zones). Topographically aligned horizontal connections may contribute to the orientation preference of layer II/III neurons and could account for the axial specificity of some receptive field surround effects. Together, these results emphasize that specificity in the spatial arrangement of local circuit axon arbors plays an important role in shaping the response properties of neurons in visual cortex.

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