Dissociable Spatial and Feature Tuning of Gamma and Alpha Rhythms in Human Visual Cortex
Visual stimulation in humans reliably induces narrowband gamma (40-80 Hz) increases and alpha (8-12 Hz) suppression in EEG, but the relationship between these rhythms is not fixed. Using high-density EEG, we mapped gamma and alpha responses across a broad set of retinotopic, orientation, and motion conditions. Gamma was strongly retinotopically tuned, with linear summation of subfield responses accurately predicting full-field responses. In contrast, alpha showed little spatial tuning, substantial responses even in the absence of visual input (anticipatory suppression), and subadditive summation consistent with divisive normalization. Across most retinotopic configurations, higher gamma coincided with stronger alpha suppression, yet systematic dissociations emerged, with full-field and foveal-centered gratings evoking stronger than expected gamma relative to alpha suppression. Orientation tuning was robust for gamma but weak for alpha, with oblique gratings producing high gamma yet weak alpha suppression, reversing the usual inverse relationship. These patterns indicate that EEG gamma power primarily reflects large-scale synchronous activity that matches the global LFP observed in macaque V1, whose spatial and feature tuning properties are independent of both single-neuron selectivity and feedback-driven alpha dynamics. The results establish a mechanistic dissociation between gamma and alpha rhythms, highlighting distinct circuit origins and tuning principles for these canonical visual responses.