Eskandar, E. | bioRxiv Science

bioRxiv2017

The Generation and Propagation of the Human Alpha Rhythm

IntroductionThe alpha rhythm (7-13 Hz) is the longest studied brain oscillation and has been theorized to play a key role in cognition. Still, substantial uncertainty remains over its physiology. In this study, we used micro and macro electrodes in patients undergoing surgery for epilepsy to measure the intracortical and thalamic generators of the human alpha rhythm. We first found that alpha propagates from higher-order anterosuperior cortex towards the lower-order occipital poles, consistent with alpha effecting top-down processing. This cortical alpha drives thalamic alpha, reversing prevailing theories of a thalamic alpha pacemaker. Finally, alpha is dominated by currents and firing in supragranular cortex, contravening the popular conception of alpha as an infragranular rhythm. Together, these results demonstrate that the alpha rhythm reflects short-range supragranular feedback which propagates from higher-order to lower order cortex and cortex to thalamus. These physiological insights explain how alpha could mediate feedback throughout the thalamocortical system.

neuroscience↗

bioRxiv2017

Replay of large-scale spatio-temporal patterns from waking during subsequent slow-wave sleep in human cortex

Animal studies support the hypothesis that in slow-wave sleep, replay of waking neocortical activity under hippocampal guidance leads to memory consolidation. However, no intracranial electrophysiological evidence for replay exists in humans. We identified consistent sequences of population firing peaks across widespread cortical regions during complete waking periods. The occurrence of these Motifs were compared between sleeps preceding the waking period (Sleep-Pre) when the Motifs were identified, and those following (Sleep-Post). In all subjects, the majority of waking Motifs (most of which were novel) had more matches in Sleep-Post than in Sleep-Pre. In rodents, hippocampal replay occurs during local sharp-wave ripples, and the associated neocortical replay tends to occur during local sleep spindles and down-to-up transitions. These waves may facilitate consolidation by sequencing cell-firing and encouraging plasticity. Similarly, we found that Motifs were coupled to neocortical spindles, down-to-up transitions, theta bursts, and hippocampal sharp-wave ripples. While Motifs occurring during cognitive task performance were more likely to have more matches in subsequent sleep, our studies provide no direct demonstration that the replay of Motifs contributes to consolidation. Nonetheless, these results confirm a core prediction of the dominant neurobiological theory of human memory consolidation.

neuroscience↗