Stimulus repetition probability does not affect repetition suppression in macaque inferior temporal cortex.

Recent human functional magnetic resonance imaging studies (Summerfield C, Trittschuh EH, Monti JM, Mesulam MM, Egner T. 2008. Neural repetition suppression reflects fulfilled perceptual expectations. Nat Neurosci. 11:1004-1006.) showed that adaptation or repetition suppression is affected by contextual factors related to perceptual expectations, suggesting that adaptation results from a fulfillment of perceptual expectation or a reduction in prediction error. This view contrasts with the bottom-up fatigue or sharpening mechanisms of adaptation proposed in single-cell studies. We examined whether stimulus repetition probability affects adaptation of spiking activity and local field potentials (LFPs) in macaque inferior temporal (IT) cortex, using a protocol similar to that of Summerfield et al. Monkeys were exposed to 2 randomly interleaved trials, each consisting of either 2 identical (rep trial) or 2 different stimuli (alt trial). Trials were presented in repetition (rep) blocks consisting of 75% of rep trials and 25% of alt trials or in alternation (alt) blocks having opposite repetition probabilities. For both spiking and LFP activities, the stimulus-selective adaptation did not differ significantly between rep and alt blocks. The number of preceding rep or alt trials and the trial position within a block did not affect adaptation. This absence of any effect of stimulus repetition probability on adaptation suggests that adaptation in IT is not caused by contextual factors related to perceptual expectation.