Metaplasticity: Dark exposure boosts excitability and visual plasticity in adult human cortex

An interlude of dark exposure for about one week is known to shift the excitatory/inhibitory (E/I) balance of the mammalian visual cortex, promoting cortical plasticity across species and accelerating visual recovery in animals that have experienced cortical lesions during development. However, the translational impact of our understanding of dark exposure from animal studies on humans remains elusive. Here, we use magnetic resonance spectroscopy as a probe for E/I balance in the primary visual cortex (V1) to determine the effect of 60-min dark exposure, and measure binocular combination as a behavioral assay to assess visual plasticity in 18 normally sighted human adults (13 females). To induce neuroplastic changes in the observers, 60-min monocular deprivation was performed, which is known to shift sensory eye balance in favor of the previously deprived eye. We report that prior dark exposure strengthens cortical excitability in V1 and boosts visually plasticity in normal adults. Our findings are surprising, given the fact that the interlude is very brief. We present direct evidence that an environmental manipulation that reduces intracortical inhibition can act as a metaplastic facilitator for visual plasticity in adult humans. ### Competing Interest Statement The authors have declared no competing interest.