Seeing in the dark: Phosphene thresholds with eyes open versus closed in the absence of visual inputs

BACKGROUND: Voluntarily opening or closing our eyes results in fundamentally different input patterns and expectancies. Yet it remains unclear how our brains and visual systems adapt to these ocular states.

OBJECTIVE/HYPOTHESIS: We here used transcranial magnetic stimulation (TMS) to probe the excitability of the human visual system with eyes open or closed, in the complete absence of visual inputs.

METHODS: Combining Bayesian staircase procedures with computer control of TMS pulse intensity allowed interleaved determination of phosphene thresholds (PT) in both conditions. We measured parieto-occipital EEG baseline activity in several stages to track oscillatory power in the alpha (8-12 Hz) frequency-band, which has previously been shown to be inversely related to phosphene perception.

RESULTS: Since closing the eyes generally increases alpha power, one might have expected a decrease in excitability (higher PT). While we confirmed a rise in alpha power with eyes closed, visual excitability was actually increased (PT was lower) with eyes closed.

CONCLUSIONS: This suggests that, aside from oscillatory alpha power, additional neuronal mechanisms influence the excitability of early visual cortex. One of these may involve a more internally oriented mode of brain operation, engaged by closing the eyes. In this state, visual cortex may be more susceptible to top-down inputs, to facilitate for example multisensory integration or imagery/working memory, although alternative explanations remain possible.