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

• Main Authors: T.A. de Graaf, F. Duecker, Y. Stankevich, S. ten Oever, A.T. Sack
• Format: Article
• Language: English
• Published: Elsevier 2017-07-01
• Series: Brain Stimulation
• Subjects: Phosphene; Threshold; Alpha; Oscillations; Excitability; Transcranial magnetic stimulation
• Online Access: http://www.sciencedirect.com/science/article/pii/S1935861X1730788X

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.