Gamma Synchrony

Synchrony dynamics are still a hotly debated topic in neuroscience, and the role of synchrony in cortical processing is unclear. Much of this confusion stems from the fact that EEG activity can be interepreted as the result of phasic bursts of activity or of phase-changes resulting in synchrony. Adding fuel to the fire is some new evidence from yesterday's issue of Nature, showing that increased gamma-band (40-70 Hz) synchrony in the V4 layer of visual cortex, as measured by intracranial electrodes, is predictive of certain behavioral reaction times.

Two monkeys were trained to detect changes in visual stimuli and ignore distractors while the researchers recorded from mulitple electrode sites in V4. Those trials which resulted in the fastest reaction time to detect a change in a target item could be predicted on the basis of increased gamma-band power and spike-field coherence. As in other studies showing the effects of synchrony on vision and attention, the earliest changes could be observed before stimulus onset, such that gamma-band differences occured in faster RT trials as much as 350ms prior to the change.

These effects were detected only in those neurons whose receptive fields overlap with the target, suggesting that increased gamma-band coherence is not purely a result of globally increased arousal or alertness. In fact, there was a reversal of the trend such that gamma coherence in other receptive fields anti-correlated with change detection RT. Previous work on synchrony dynamics in the attentional blink paradigm has likewise shown that increased visual cortex synchrony appears to result in shorter attentional blink times as well. How synchrony in visual cortex is successfully engaged by "attentional processes" remains to be proven.

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