Spatial integration and cortical dynamics.

Abstract

Cells in adult primary visual cortex are capable of integrating information over much larger portions of the visual field than was originally thought. Moreover, their receptive field properties can be altered by the context within which local features are presented and by changes in visual experience. The substrate for both spatial integration and cortical plasticity is likely to be found in a plexus of long-range horizontal connections, formed by cortical pyramidal cells, which link cells within each cortical area over distances of 6-8 mm. The relationship between horizontal connections and cortical functional architecture suggests a role in visual segmentation and spatial integration. The distribution of lateral interactions within striate cortex was visualized with optical recording, and their functional consequences were explored by using comparable stimuli in human psychophysical experiments and in recordings from alert monkeys. They may represent the substrate for perceptual phenomena such as illusory contours, surface fill-in, and contour saliency. The dynamic nature of receptive field properties and cortical architecture has been seen over time scales ranging from seconds to months. One can induce a remapping of the topography of visual cortex by making focal binocular retinal lesions. Shorter-term plasticity of cortical receptive fields was observed following brief periods of visual stimulation. The mechanisms involved entailed, for the short-term changes, altering the effectiveness of existing cortical connections, and for the long-term changes, sprouting of axon collaterals and synaptogenesis. The mutability of cortical function implies a continual process of calibration and normalization of the perception of visual attributes that is dependent on sensory experience throughout adulthood and might further represent the mechanism of perceptual learning.

Oriented Axon Projections in Primary Visual Cortex of the Monkey

“…In squirrel monkeys, this is equivalent to no more than 0.5°beyond the borders of the receptive field (at 1°e ccentricity), similar to what has been found in macaques (Yoshioka et al, 1996). The range of influence is therefore no more than one receptive field diameter on any side, creating an "integrating field" that is ϳ10 times the area of the receptive field, comparable with what has been reported in the cat (Gilbert et al, 1996). Such an integrative field has been demonstrated directly in cat V1, where depolarizing inputs to a neuron were recorded from a region approximately three times the diameter of the minimum response field (Bringuier et al, 1999).…”

Remapping for visual stability

“…The idea is that corollary discharge signals from these FEF projections could trigger remapping based on local transmission of visual signals within each area and between homologous regions across hemispheres. Horizontal connections are ubiquitous in visual cortex [49][50][51] and they serve to link more distant parts of the field at higher levels of the hierarchy [52]. New experiments are needed to constrain hypotheses about the neural circuits that give rise to remapping in extrastriate cortex.…”

Mapping behavioural evolution onto brain evolution: the strategic roles of conserved organization in individuals and species

“…Dynamic reallocation of function according to relative activity, however, both during development and at adulthood (e.g. [98]) is probably the normal state of affairs, and should be studied more systematically in non-manipulated brain, and as an emergent property in species adaptations. The massive re-use of brain tissue implied by imaging studies, showing the same tissue lighting up repeatedly in diverse contexts and tasks (e.g.…”