The occipital alpha rhythm (∼10 Hz) is the most prominent electrophysiological activity in the awake human brain, yet its functional role and relation to visual perception are little understood. Transient stimuli normally elicit a short series of positive and negative deflections lasting between 300 and 500 ms: the visual-evoked potential (VEP). Alpha oscillations, on the other hand, are generally suppressed by transient visual input; they only augment in response to periodic ("steady-state") inputs around 10 Hz. Here, we applied reverse-correlation techniques to the visual presentation of random, nonperiodic dynamic stimulation sequences and found that the brain response to each stimulus transient was not merely a short-lived VEP but also included a strong ∼10 Hz oscillation that lasted for more than 1 s. In other words, the alpha rhythm implements an "echo" or reverberation of the input sequence. These echoes are correlated in magnitude and frequency with the observer's occipital alpha rhythm, are enhanced by visual attention, and can be rendered perceptually apparent in the form of ∼10 Hz flicker. These findings suggest a role for the alpha rhythm in the maintenance of sensory representations over time.
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