Stimulus repetition affects both strength and synchrony of macaque inferior temporal cortical activity.

Repetition of a visual stimulus reduces the firing rate of macaque inferior temporal (IT) neurons. The neural mechanisms underlying this adaptation or repetition suppression are still unclear. In particular, we do not know how the IT circuit is affected by stimulus repetition. To address this, we measured local field potentials (LFPs) and multiunit spiking activity (MUA) simultaneously at 16 sites with a laminar electrode in IT while repeating visual images. Stimulus exposures and interstimulus intervals were each 500 ms. The rhesus monkeys were performing a passive fixation task during the recordings. Induced LFP power decreased with repetition for spectral frequencies above 60 Hz but increased with repetition for lower frequencies, the latter because of a delayed decrease in power when repeating a stimulus. LFP-LFP and MUA-LFP coherences decreased with repetition for frequencies above 60 Hz. This repetition suppression of the MUA-LFP coherence was not due to differences in firing rate since it was present when spike counts were equated for the adapter and repeated stimuli. For frequencies between 15 and 40 Hz, the effect of repetition on synchronization depended on the electrode depth: For the putative superficial layers synchronization was enhanced with repetition, while the LFPs of the putative deep layers decreased their synchrony across layers. The between-site, trial-to-trial covariations in MUA ("noise correlations") decreased with repetition, but this might have reflected repetition suppression of the firing rate. This work demonstrates that short-term stimulus repetition affects the synchronized activity, in addition to response strength, in IT cortex.