No abstract

We studied the existence, localization and attentional modulation of gamma-band oscillatory activity (30-130 Hz) in the human intracranial region. Two areas known to play a key role in visual object processing: the lateral occipital (LO) cortex and the fusiform gyrus. These areas consistently displayed large gamma oscillations during visual stimulus encoding, while other extrastriate areas remained systematically silent, across 14 patients and 291 recording sites scattered throughout extrastriate visual cortex. The lateral extent of the responsive regions was small, in the range of 5 mm. Induced gamma oscillations and evoked potentials were not systematically co-localized. LO and the fusiform gyrus displayed markedly different patterns of attentional modulation. In the fusiform gyrus, attention enhanced stimulus-driven gamma oscillations. In LO, attention increased the baseline level of gamma oscillations during the expectation period preceding the stimulus. Subsequent gamma oscillations produced by attended stimuli were smaller than those produced by unattended, irrelevant stimuli. Attentional modulations of gamma oscillations in LO and the fusiform gyrus were thus very different, both in their time-course (preparatory period and/or stimulus processing) and direction of modulation (increase or decrease). Our results thus suggest that the functional role of gamma oscillations depends on the area in which they occur.

In contrast to patients in VS, a third of patients in MCS improved more than 1 year after coma onset. This emphasizes the need to define reliable boundaries between VS and MCS using repeated clinical evaluations and all imaging and neurophysiologic tools available today.

How do we keep an object in mind? Based on evidence from animal electrophysiology and human brain-imaging techniques, it is commonly held that short-term memory relies on sustained activity in a network distributed over sensory and prefrontal cortices. How does neural firing persist in such a distributed network in the absence of visual input? Hebb's influential but so far unproved proposal, developed more than 50 years ago, is that sustained activation in short-term memory networks is maintained by reverberating activity in neuronal loops. We hypothesized that synchronized oscillatory activity, 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 inputs. We show in human intracranial recordings that limited regions of extrastriate visual areas, separated by several centimeters, become synchronized 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) and disappears in a control condition. These findings thus confirm experimentally the hypothesis of a functional role of synchronized oscillatory activity in the coordination of distributed neural activity in humans, and support Hebb's popular but unproved concept of short-term memory maintenance by reentrant activity within the activated network.