Dissociating task performance from fMRI repetition attenuation in ventral visual cortex.

Repeated visual stimuli elicit reduced neural responses compared with novel stimuli in various brain regions (repetition attenuation). This effect has become a powerful tool in fMRI research, allowing researchers to investigate the stimulus-specific neuronal representations underlying perception and cognition. Repetition attenuation is also commonly associated with behavioral priming, whereby response accuracy and speed increase with repetition. This raises the possibility that repetition attenuation merely reflects decreased processing time. Here, we report a full dissociation between repetition attenuation and behavioral performance by varying the task performed on identical visual stimuli. In the scene task, observers judged whether two photographs came from the same scene, and in the image task, they judged whether the two photographs were identical pixel for pixel. The two tasks produced opposite patterns of behavioral performance: in the scene task, responses were faster and more accurate when the photographs were very similar, whereas, in the image task, responses were faster and more accurate when the photographs were less similar. However, in the parahippocampal place area (PPA), a scene-selective region of ventral cortex, identical repetition attenuation was observed in both tasks: lower neural responses for the very similar pairs relative to the less similar pairs. Whereas the PPA was impervious to task modulation, responses from two frontal regions mirrored behavioral performance, consistent with their role in decision-making. Thus, although repetition attenuation and performance are often correlated, they can be dissociated, suggesting that attenuation in ventral visual areas reflects stimulus-specific processing independent of task demands.