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

- We measured phosphene thresholds (PT) with eyes closed or opened, in complete darkness.
- PT staircases were interleaved, a staircase algorithm controlled TMS intensity.
- PT was lower with eyes closed than with eyes open.
- We also measured parieto-occipital alpha power with EEG.
- EEG alpha power was higher with eyes closed versus eyes open.

BackgroundVoluntarily 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.MethodsCombining 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.ResultsSince 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.ConclusionsThis 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.