Effect of spatial attention on stimulus-specific haemodynamic repetition effects.

The aim of this fMRI study was to investigate whether spatial attention to the initial and/or repeated presentation of a stimulus is necessary to observe repetition-related modulations of the neural responses evoked by that stimulus. During each trial, two stimuli were presented simultaneously, one left and one right of fixation. During each block, participants were instructed to attend covertly to stimuli in one of the two hemifields and respond whether each was a face or house, ignoring the contralateral stimulus. Regions that preferred one stimulus category over the other, such as the fusiform face area and parahippocampal place area, showed evidence of some processing of the ignored stimuli. However, a reduced response to repeated stimuli (repetition suppression) was only reliable for preferred stimuli when both their initial and repeated presentations were attended. This suggests that attention is necessary for both the acquisition and expression of the neural mechanisms that underlie repetition suppression, at least over the lags of 2-16 intervening trials used here.