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We have used functional imaging methods 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 and 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 in patterns with identical local statistics purely by the global arrangement of dot pairs. Wilson et al (1997, VR) showed that concentric Glass patterns are processed more efficiently than other patterns, suggesting that there exist higher-order ‘grouping’ filters tuned to particular patterns of activation among local filters.
We generated different Glass patterns (concentric, radial, translational, and random) and used these to activate visual cortex in anaesthetized monkeys. We measured the resulting BOLD fMRI signals in a 4.7T scanner; voxel volume was 0.5×0.5×2mm. We collected 13 horizontal slices of the entire brain using multi-shot T2* weighted gradient-recalled EPI sequences.
All Glass patterns produced substantial BOLD activity in V1 and V2, and in favorable cases also in more anterior areas including V4. Further analysis suggested that despite their identical local statistics, different patterns produced different degrees of activation in some cortical areas, with concentric patterns usually producing greater activation than other patterns. This difference was most marked in anterior extrastriate cortical areas including V4, and suggests that these areas contain neurons selectively sensitive to different global forms.
