Oscillatory synchrony between human extrastriate areas during visual short-term memory maintenance

How do we keep an object in mind? Based on evidence from animal electrophys iology and human brain-imaging techniques, it is commonly held that short-t erm memory relies on sustained activity in a network distributed over senso ry and prefrontal cortices. How does neural firing persist in such a distri buted network in the absence of visual input? Hebb's influential but so far unproved proposal, developed more than 50 years ago, is that sustained act ivation in short-term memory networks is maintained by reverberating activi ty in neuronal loops. We hypothesized that synchronized oscillatory activit y, proposed to provide a dynamic link between distributed areas, could not only coordinate activity in the network but also establish reentrant loops in the system to enable both sustained firing and temporal coincidence of i nputs. We show in human intracranial recordings that limited regions of ext rastriate visual areas, separated by several centimeters, become synchroniz ed in an oscillatory mode during the rehearsal of an object in visual short -term memory. Synchrony occurs specifically in the beta range (15-25 Hz) an d disappears in a control condition. These findings thus confirm experiment ally the hypothesis of a functional role of synchronized oscillatory activi ty in the coordination of distributed neural activity in humans, and suppor t Hebb's popular but unproved concept of short-term memory maintenance by r eentrant activity within the activated network.