Distinct Oscillatory Frequencies Underlie Excitability of Human Occipital and Parietal Cortex

Transcranial magnetic stimulation (TMS) of occipital and posterior parietal cortex can produce visual sensations called phosphenes. Using near-threshold TMS with concurrent EEG, the authors found that prestimulus power and phase in the alpha band (8–13 Hz) predicted occipital TMS phosphenes, while higher-frequency beta-band (13–20 Hz) power (but not phase) predicted parietal TMS phosphenes. TMS-evoked responses related to phosphene perception were similar across sites, showing an early posterior negativity and later parietal positivity, plus low-frequency power increase followed by broadband alpha/beta power decrease. These correlates resemble those of conscious perception of near-threshold visual stimuli. The regionally differential prestimulus predictors suggest distinct frequencies reflect cortical excitability in occipital versus parietal cortex, challenging the assumption that alpha rhythm serves as a general index of cortical excitability.

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Abstract

Transcranial magnetic stimulation (TMS) of human occipital and posterior parietal cortex can give rise to visual sensations called phosphenes. We used near-threshold TMS with concurrent EEG recordings to measure how oscillatory brain dynamics covary, on single trials, with the perception of phosphenes after occipital and parietal TMS. Prestimulus power and phase, predominantly in the alpha band (8–13 Hz), predicted occipital TMS phosphenes, whereas higher-frequency beta-band (13–20 Hz) power (but not phase) predicted parietal TMS phosphenes. TMS-evoked responses related to phosphene perception were similar across stimulation sites and were characterized by an early (200 ms) posterior negativity and a later (>300 ms) parietal positivity in the time domain and an increase in low-frequency (∼5–7 Hz) power followed by a broadband decrease in alpha/beta power in the time–frequency domain. These correlates of phosphene perception closely resemble known electrophysiological correlates of conscious perception of near-threshold visual stimuli. The regionally differential pattern of prestimulus predictors of phosphene perception suggests that distinct frequencies may reflect cortical excitability in occipital versus posterior parietal cortex, calling into question the broader assumption that the alpha rhythm may serve as a general index of cortical excitability. SIGNIFICANCE STATEMENT Alpha-band oscillations are thought to reflect cortical excitability and are therefore ascribed an important role in gating information transmission across cortex. We probed cortical excitability directly in human occipital and parietal cortex and observed that, whereas alpha-band dynamics indeed reflect excitability of occipital areas, beta-band activity was most predictive of parietal cortex excitability. Differences in the state of cortical excitability predicted perceptual outcomes (phosphenes), which were manifest in both early and late patterns of evoked activity, revealing the time course of phosphene perception. Our findings prompt revision of the notion that alpha activity reflects excitability across all of cortex and suggest instead that excitability in different regions is reflected in distinct frequency bands.