Spatial frequency adaptation modulates spatial tuning in human visual cortex

Neurons in the early visual areas are selectively sensitive to spatial frequency (SF). However, the relationship between SF and the spatial tuning of neuronal populations has not been directly studied in humans. Here, we explored the interplay between SF adaptation and its effect on the size of population receptive fields (pRFs) in the human visual cortex. We reasoned that prolonged exposure to high and low SFs would lead to a selective decrease in the sensitivity of neurons with small and large receptive fields, thereby affecting overall pRF sizes, as measured via functional magnetic resonance imaging (fMRI). We first conducted a psychophysical experiment to quantify the subjective perceptual changes after adaptation to bandpass-filtered isotropic noise stimuli with SFs of 0.5 and 3.5 cpd. Next, we performed a fMRI experiment that integrated the SF adaptation paradigm into a standard pRF mapping procedure. This enabled the measurement of pRF size changes after adaptation to two noise stimuli with relatively high and low SFs. The perceptual aftereffect confirmed significant over- and underestimations of SF after adaptation to low and high SFs. Most importantly, our fMRI results showed that adaptation to a certain SF modified the spatial tuning of neuronal populations. As predicted, low and high SF adaptation resulted in smaller and larger pRF sizes, respectively. Our results provide the most direct evidence to date that the spatial tuning of the visual cortex, as measured by pRF mapping, is directly linked to the spatial frequency selectivity of visual neural populations. Our study has implications for our understanding of size perception, visual acuity, and sensitivity to blur.