GABA-induced inactivation of Cebus apella V2 neurons: effects on orientation tuning and direction selectivity

We investigated the GABA-induced inactivation of V2 neurons and terminals on the receptive field properties of this area in an anesthetized and paralyzed Cebus apella monkey. Extracellular single-unit activity was recorded using tungsten microelectrodes in a monkey before and after pressure-injection of a 0.25 or 0.5 M GABA solution. The visual stimulus consisted of a bar moving in 8 possible directions. In total, 24 V2 neurons were studied before and after blocker injections in 4 experimental sessions following GABA injection into area V2. A group of 10 neurons were studied over a short period. An additional 6 neurons were investigated over a long period after the GABA injection. A third group of 8 neurons were studied over a very long period. Overall, these 24 neurons displayed an early (1-20 min) significant general decrease in excitability with concomitant changes in orientation or direction selectivity. GABA inactivation in area V2 produced robust inhibition in 80% and a significant change in directional selectivity in 60% of the neurons examined. These GABA projections are capable of modulating not only levels of spontaneous and driven activity of V2 neurons but also receptive field properties such as direction selectivity.

Changes of ongoing activity in Cebus monkey perirhinal cortex correlate with behavioral performance

A Cebus apella monkey weighing 4 kg was trained in a saccadic eye movement task and while the animal performed the task we recorded the extracellular activity of perirhinal cortical neurons. Although the task was very simple and maintained at a constant level of difficulty, we observed considerable changes in the performance of the monkey within each experimental session. The behavioral states responsible for such variation may be related to arousal, motivation or attention of the animal while engaged in the task. In approximately 20 percent (16/82) of the units recorded, long-term direct or inverse correlations could be demonstrated between the monkey's behavioral state and the cells' ongoing activity (independent of the visual stimulation or of the specific behavior along a trial). The perirhinal cortex and other medial temporal structures have long been associated with normal memory function. The data presented here were interpreted in terms of recent reports focusing on the subcortical afferents to temporal lobe structures and their possible role in controlling arousal, motivation, or attention.

Filling-in topographically organized distributed networks

We propose a framework for understanding visual perception based on a topographically organized, functionally distributed network. In this proposal the extraction of shape boundaries starts at retinal ganglion cells with concentric receptive fields. This information, relayed through the lateral geniculate necleus, creates a neural representation of negative and positive boundaries in a set of topographically connected and organized visual areas. After boundary extraction, several processes involving contrast, brightness, texture and motion extraction take place in subsequent visual areas in different cortical modules. Following these steps of processing, filling-in processes at different levels, within each area, and in separate channels, propagate locally to transform boundary representations onto surfaces representations. These partial representations of the image propagate back and forth in the network, yielding a neural representation of the original image. We propose that completion takes places in a wide cortical circuit that heavily relies on V1, where long-range information helps determine contour responses at specific topographically organized locations. Neural representations of illusory contours would emerge in circuits involving primarily area V2. The neural representation of filling-in of a peripheral stimulus in a dynamic surround (such as in texture filling-in) would depend on circuits involving primarily cells in areas V2 and V3, and would include competitive mechanisms required for figure to ground segregation. Finally, we suggest that multiple representations of the stimulus engage competitive mechanisms that select the "most likely hypothesis". Such choice behavior would rely on winner-take-all mechanisms capable of constructing a single neural representation of perceived objects.