Abstract
Neuronal oscillations at about 10 Hz, called alpha oscillations, are often thought to arise from synchronous activity across large regions of occipital cortex, reflecting general cognitive states such as attention, arousal, and alertness. However, there is also evidence that modulation of alpha oscillations in visual cortex can be spatially specific. Here, we used intracranial electrodes in human patients to measure alpha oscillations in multiple visual areas in response to visual stimuli whose location varied systematically across the visual field. We used a model-based approach to separate the alpha oscillation from other signals, and show that without this separation, alpha power estimates are inaccurate due to spectral contamination from broadband power changes. After quantifying alpha oscillations for each stimulus and each electrode, we used a 2D symmetric Gaussian population receptive field (pRF) model to explain the pattern of the alpha signal. We find that the alpha pRF centers are highly similar to the pRF centers estimated from broadband (70–180 Hz) time series, but are several times larger. The results demonstrate that alpha suppression in human visual cortex can be precisely tuned, likely influencing stimulus-triggered neural responses. Our interpretation is that stimulus onset leads to a reduction in alpha oscillations in the regions of cortical maps representing locations near the stimulus, and that this in turn increases cortical excitability of that region, analogous to the spatial spread of stimulus-cued attention. Finally, in separate experiments, we measured the ECoG responses to a wide range of gray scale images varying in contrast and pattern. We find that the suppression of the alpha oscillation depends systematically on image properties such as contrast and the number of orientations (e.g., gratings vs plaids). These findings confirm our conclusion that alpha suppression reflects part of the computations in generating neural responses to visual stimulation.