Perceptual and physiological consequences of dark adaptation: a TMS-EEG study

In this study we assessed changes in phosphene perception and transcranial magnetic stimulation (TMS)-evoked potentials (TEPs) after light and dark adaptation. The primary aim was to link a direct neurophysiological correlate of cortical excitability (i.e., TEPs) to the subjective report (i.e., phosphene perception) in order to evaluate whether environmental light changing induced modifications in visual cortex excitability or only at retinal level, as previously reported. Ten young healthy participants underwent three conditions: baseline (environmental light), 30 minutes of light adaptation (LA) and 30 minutes of and dark adaptation (DA) in counterbalanced order. After each condition, we assessed the TMS-induced phosphenes perception by single pulse TMS over the visual cortex (V1) while 32-channels electroencephalogram (EEG) was recorded. Coil position was maintained stable across conditions through a stereotaxic neuronavigation system. Phosphenes threshold was calculated through the method of constant stimuli, in which TMS pulses were delivered at different TMS intensities ranging from 46 to 76% of the maximal stimulator output. The participants were asked to report whether they perceived or not a phosphene after each TMS pulse. A Weibull function was fitted to phosphenes report for each condition and participant separately, in order to estimate the threshold and slope of the psychometric functions. Results confirmed that dark adaptation modulated visual perception: the slope of the psychometric function after dark adaptation significantly increased (p<0.05) compared to light adaptation and baseline, whilst the estimated threshold for phosphene perception did not change across conditions. A preliminary analysis of TMS-EEG data shows a reduction of TEPs amplitude after DA compared to LA, thus suggesting that a period of prolonged dark adaptation is able to decrease cortical excitability. This modulation could explain the reduction of sensitivity indexed by the increased slope of the psychometric function