fMRI adaptation: stimulus specific or processing load specific?

Editor's Note: These short reviews of a recent paper in the Journal, written exclusively by graduate students or postdoctoral fellows, are intended to mimic the journal clubs that exist in your own departments or institutions. For more information on the format and purpose of the Journal Club, please see Review of Xu et al. Repetition suppression, or fMRI-adaptation, refers to decreased neural activity for repeated versus novel stimuli as measured with functional magnetic resonance imaging (fMRI). Recently , this technique has been widely used, especially in probing the plasticity of the nervous system and the functional properties of neural ensembles (Grill-Spector et al., 2006). However, stimulus repetition also leads to performance facilitation [i.e., repetition priming (Schacter and Buckner, 1998)]. Thus, it remains unclear whether fMRI-adaptation is stimulus-specific or simply reflects the stimulus processing that is necessary to reach a task decision (i.e., processing load-specific). One way to examine the correlation between fMRI-adaptation and visual repetition priming is to dissociate behavioral performance from neural activity. For example , Henson et al. (2000) used familiar and unfamiliar stimuli to manipulate stimulus familiarity. They found attenuated responses to the repetition of familiar stimuli but enhanced responses to the repetition of unfamiliar stimuli, which excludes a simple one-to-one correspondence between adaptation and repetition priming. A recent paper by Xu et al. (2007) in The Journal of Neuroscience reports a full dissociation between adaptation and repetition priming in the scene-specific region in the ventral visual cortex, the para-hippocampal place area (PPA). Observers viewed pairs of very similar and less similar scene photographs [Xu et al. tasks were used to induce opposite behav-ioral patterns to identical stimuli. In the scene task, observers judged whether two photographs originated from the same scene and, thus, needed to attend to the photos as a whole. As such, behavioral responses were faster and more accurate when the two photographs were very similar than when they were less similar. In the image task, observers judged whether the two photographs were identical pixel by pixel and, thus, needed to focus on feature analysis. This, however, resulted in faster and more accurate behavioral responses when the two photographs were less similar than when they were very similar [Xu et al. Because overall reaction time and accuracy showed no difference between the two tasks, processing load was matched. This was further supported by the lack of difference in peak amplitude and latency of the …