Enhanced excitability of the human visual cortex induced by short-term light deprivation

Long term deprivation of visual input for several days or weeks leads to ma rked changes in the excitability and function of the occipital cortex. The time course of these changes is poorly understood. In this study, we addres sed the question whether a short period of light deprivation (minutes to a few hours) can elicit such changes in humans. Noninvasive transcranial magn etic stimulation (TMS) of the human occipital cortex can evoke the percepti on of flashes or spots of light (phosphenes). To assess changes in visual c ortex excitability following light deprivation, we measured the minimum int ensity of stimulation required to elicit phosphenes (phosphene threshold) a nd the number of phosphenes elicited by different TMS stimulus intensities (stimulus-response curves). A reduced phosphene threshold was detected 45 m in after the onset of light deprivation and persisted for the entire depriv ation period (180 min). Following re-exposure to light, phosphene threshold s returned to predeprivation values over 120 min. Stimulus-response curves were significantly enhanced in association with this intervention. In a sec ond experiment, we studied the effects of light deprivation on functional m agnetic resonance imaging (fMRI) signals elicited by photic stimulation. fM RI results showed increased visual cortex activation after 60 min of light deprivation that persisted following 30 min of re-exposure to light. Our re sults demonstrated a substantial increase in visual cortex excitability. Th ese changes may underlie behavioral gains reported in humans and animals as sociated with light deprivation.