The representation of shape in the context of visual object categorizationtasks

To investigate the role of human fusiform gyrus in shape processing, we det ermined the effect of shape degradation on BOLD contrast in this region wit h fMRI during three tasks requiring subjects to determine either whether tw o successively presented nonsense shapes had the same global orientation (O R task); whether two successively presented meaningful objects belonged to the same basic level category (CAT task); or whether two successively prese nted objects represented the same exemplar of a category (EX task). On the behavioral level, shape degradation by locally shifting the pixels constitu ting the lines of stimuli had no effect on performance in the OR task, whil e it was detrimental to performance in the CAT and EX tasks. In comparison to the OR task, both the CAT and EX tasks were associated with activations in the occipitotemporal and parietal cortex. When shape degradation was app lied, activation in the middle fusiform gyrus was reduced in all tasks. The occurrence of this effect in the OR task indicates that it is independent of memory representations. The persistence of the effect in both tasks that showed a behavioral effect of degradation suggests that it does not reflec t the amount of shape processing performed on the stimuli, but rather the s pecificity of the final perceptual representation that can be built from th e shape information that is available. Other studies have shown effects of stimulus familiarity and task requirements in the fusiform gyrus, suggestin g that there is no need to assume different modules for perceptual represen tation and representation in memory. (C) 2000 Academic Press.