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The goal of these experiments is to locate areas of the macaque brain involved in processing global form. For this purpose, natural objects are not ideal stimuli because their form-defining characteristics are difficult to isolate, parameterize, or control. Glass patterns, created by pairing each dot in a random texture with another at a specified spatial offset (Glass 1969, Nature), are useful because form is defined by the global statistics of dot placement with spatial correlations that are purely local. Recently, Wilson et al (1997, Vision Research) showed that concentric Glass patterns are processed more efficiently than other patterns with the same local but differing global statistics, suggesting that there exist higher-order ‘grouping‘ filters tuned to particular patterns of activation among local filters.
We generated static and dynamic Glass patterns with different global forms, and used these to activate visual cortex in anesthetized monkeys. We measured the resulting BOLD fMRI signals in a 4.7T/40cm Biospec vertical scanner (Bruker, Inc), with 50mT/m gradients, using quadrature transmit/receive RF coils. Voxel volume was 0.5x0.5x2mm, TE=20ms, TR=750ms, and FA=40deg. We collected multi-shot, multi-slices T2* weighted images (13 horizontal slices) of the entire brain using gradient-recalled EPI fMRI sequences.
We found significant signal differences between coherent Glass patterns and randomly oriented dot pairs in V2 and other areas. This suggests that grouping mechanisms and filters tuned to global form relationships among dots are located in these areas, but not in V1.
