Stimulus Repetition Induces a Two-Stage Learning Process in the Primary Visual Cortex.

Repeated stimulus exposure alters the brain's response to the stimulus. Recording fMRI data from both men and women viewing 120 presentations of two Gabor patches (each Gabor repeating 60 times), we evaluated support for two prominent models of stimulus repetition, the fatigue model and the sharpening model. Our results uncovered a two-stage learning process in the primary visual cortex. In Stage 1, univariate BOLD activation in V1 decreased over the first 14 repetitions of the stimuli, replicating the well known effect of repetition suppression. Applying moving-window multivoxel pattern analysis decoding, we found that (1) the decoding accuracy between the two Gabors decreased from the above-chance level (∼60 to ∼70%) at the beginning of the stage to the chance level at the end of the stage (∼50%). This result, together with the accompanying weight map analysis, suggested that the learning dynamics in Stage 1 were consistent with the predictions of the fatigue model. In Stage 2, univariate BOLD activation for the remaining 46 repetitions of the two stimuli exhibited significant fluctuations but no systematic trend. The moving-window decoding accuracy between the two Gabor patches was at the chance level initially and became progressively higher as stimulus repetition continued, rising above and staying above the chance level starting at the ∼36th repetition. Thus, results from the second stage supported the notion that sustained and prolonged stimulus repetition prompts sharpened representations. Additional analyses addressed (1) whether the neural patterns within each learning stage remained stable and (2) whether new neural patterns were evoked in Stage 2 relative to Stage 1.