Simultaneous desynchronization and synchronization of different alpha responses in the human electroencephalograph: a neglected paradox?

This study aims to resolve a paradox. Experiments measuring alpha band powe r report an event related decrease (desynchronization) in alpha activity, w hereas those measuring evoked alpha report synchronization. During a recogn ition memory task with human subjects, we measured the evoked (phase locked ) and induced (not phase locked) alpha response. The findings reveal that e voked alpha is due to a transient phase locking (at about 100-200 ms postst imulus) of three alpha sub-bands which can be observed only at parieto-occi pital sites. In contrast, induced alpha shows a widespread pattern of desyn chronization at most recording sites. Thus, opposite alpha responses occur within similar time windows. Evoked alpha synchronization may reflect corti cal inhibition which serves to increase the signal to noise ratio for activ ation processes following immediately later. (C) 2000 Elsevier Science Irel and Ltd. All rights reserved.