Perceptual Echoes at 10 Hz in the Human Brain

The occipital alpha rhythm (∼10 Hz) is the most prominent electrophysiological activity in the awake human brain [1Berger H. Über das Elektroenkephalogramm des Menschen.Arch. Psychiatr. Nervenkr. 1929; 87: 527-570Crossref Scopus (2366) Google Scholar], yet its functional role [2Klimesch W. EEG alpha and theta oscillations reflect cognitive and memory performance: a review and analysis.Brain Res. Brain Res. Rev. 1999; 29: 169-195Crossref PubMed Scopus (4478) Google Scholar, 3Palva S. Palva J.M. New vistas for alpha-frequency band oscillations.Trends Neurosci. 2007; 30: 150-158Abstract Full Text Full Text PDF PubMed Scopus (953) Google Scholar, 4Buzsaki G. Rhythms of the Brain. Oxford University Press, New York2006Crossref Scopus (3339) Google Scholar] and relation to visual perception [5Makeig S. Westerfield M. Jung T.P. Enghoff S. Townsend J. Courchesne E. Sejnowski T.J. Dynamic brain sources of visual evoked responses.Science. 2002; 295: 690-694Crossref PubMed Scopus (1278) Google Scholar] are little understood. Transient stimuli normally elicit a short series of positive and negative deflections lasting between 300 and 500 ms [6Cigánek L. Variability of the human visual evoked potential: normative data.Electroencephalogr. Clin. Neurophysiol. 1969; 27: 35-42Abstract Full Text PDF PubMed Scopus (60) Google Scholar, 7Hillyard S.A. Teder-Sälejärvi W.A. Münte T.F. Temporal dynamics of early perceptual processing.Curr. Opin. Neurobiol. 1998; 8: 202-210Crossref PubMed Scopus (186) Google Scholar, 8Nogawa T. Katayama K. Tabata Y. Ohshio T. Kawahara T. Changes in amplitude of the EEG induced by a photic stimulus.Electroencephalogr. Clin. Neurophysiol. 1976; 40: 78-88Abstract Full Text PDF PubMed Scopus (32) Google Scholar]: the visual-evoked potential (VEP). Alpha oscillations, on the other hand, are generally suppressed by transient visual input [1Berger H. Über das Elektroenkephalogramm des Menschen.Arch. Psychiatr. Nervenkr. 1929; 87: 527-570Crossref Scopus (2366) Google Scholar, 8Nogawa T. Katayama K. Tabata Y. Ohshio T. Kawahara T. Changes in amplitude of the EEG induced by a photic stimulus.Electroencephalogr. Clin. Neurophysiol. 1976; 40: 78-88Abstract Full Text PDF PubMed Scopus (32) Google Scholar]; they only augment in response to periodic ("steady-state") inputs around 10 Hz [9Regan D. Some characteristics of average steady-state and transient responses evoked by modulated light.Electroencephalogr. Clin. Neurophysiol. 1966; 20: 238-248Abstract Full Text PDF PubMed Scopus (368) Google Scholar, 10Herrmann C.S. Human EEG responses to 1-100 Hz flicker: resonance phenomena in visual cortex and their potential correlation to cognitive phenomena.Exp. Brain Res. 2001; 137: 346-353Crossref PubMed Scopus (734) Google Scholar]. 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.