Functionally linked neuronal assemblies: fMRI adaptation studies in monkeys

A great deal is known about the properties of single neurons processing visual information. In contrast, less is known about the collective properties of contiguous or distributed neuronal assemblies that may underlie sensory or perceptual capacities of animals. We are studying the activation properties of functionally linked neural populations using fMRI adaptation experiments. Using this paradigm we have investigated the spatial distribution of motion-induced neural activity in cortical and subcortical visual structures in anesthetized monkeys. A black and white foveally-centered rotating polar pattern was used, which consistently activated the LGN, V1 and extrastriate areas including area MT (Logothetis et al., 1999). During the first presentation phase, lasting a few minutes, the polar rotated in one direction (i.e. clockwise). Then, abruptly, the direction of rotation was reversed (second phase). The first phase was long enough for the fMRI signal to show adaptation. We hypothesize that activity in brain areas carrying direction of motion information increases immediately following the reversal due to release from adaptation to the opposite direction of motion, occurring prior to the reversal. Initial experiments, reveal that the time course of adaptation to rotating polar patterns can be monitored in several visual areas using fMRI. Preliminary results suggest that activity in several areas (cortical and subcortical) increased after the direction of motion reversal. These initial results suggest that the global-network properties of brain areas carry stimulus specific information beyond that typically measured in single unit recordings