Effects of adaptation on the stimulus selectivity of macaque inferior temporal spiking activity and local field potentials.

Stimulus repetition reduces neural response in cortical areas. Such adaptation is used in functional magnetic resonance imaging to infer the selectivity of neuronal populations; however, the mechanisms of adaptation remain elusive, especially in higher areas. We measured adaptation of spiking activity and local field potentials (LFPs) in macaque inferior temporal (IT) cortex for parameterized shapes by comparing tuning for test stimuli following a brief adaptation with predictions derived from different models of adaptation. Adaptation was similar during passive fixation or an attention-demanding task. We found consistent adaptation of spiking activity and LFP power in high- (gamma) but not low-frequency bands when repeating shapes. Contrary to sharpening models, repetition did not affect shape selectivity. The degree of similarity between adapter and test shapes was a stronger determinant of adaptation than was the response to the adapter. Adaptation still occurred when adapter and test stimuli did not spatially overlap, but adaptation was stronger for same, compared with different, adapters and test stimulus positions. These adaptation effects were similar for spiking and for gamma activity. In conclusion, adaptation of IT spiking activity and LFPs in IT is strongly dependent on feature similarities in the adapter and test stimuli, in agreement with input, but not firing-rate fatigue models.