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要旨:
Oscillatory activity in cortical networks is believed to index the neural mechanism that underlie perception and cognition. It reflects changes in the local excitation-inhibition balance and provide insights about network state changes that correlate with the behavioral aspects of sensory information processing. In particular, the precise temporal structure (phase) of oscillations contains significant information about the state of the brain, which can be used as forward indicators of perceptual performance, predicting subjects' response time and accuracy in the immediate future. This effect of pre-stimulus phase can be further enhanced by expectation or attention, which may thereby partition sensation into discrete 'perceptual moments', indexed by specific oscillatory states.
However the periodic nature of oscillatory activity also suggests that periods of enhanced perception should go in-hand with periods of reduced perceptual ability. Low frequency oscillations of the early sensory cortices are known to be naturally entrained by dynamic natural scenes. In this situation, the detection of a brief target may thus depends critically on its relative timing to the entrained rhythm. We tested this hypothesis by implementing a cocktail-party like scenario requiring subjects to detect an acoustic target embedded in a cacophony of naturalistic background sounds. Using EEG to measure background-entrained auditory cortical oscillations we find that the chance of target detection systematically depends on both power and phase of low frequency (2-6Hz) oscillations prior to target. Detection rates were higher and responses were faster when oscillatory power was low and both detection rate and response speed were modulated by phase. Intriguingly, the phase dependency was stronger for miss than for hit trials, suggesting that phase has a inhibiting but not ensuring role for detection. These results demonstrate that the ubiquitous entrainment of slow oscillations by complex sensory environments may have negative effects on target detection and further promote the notion that specific phase ranges of cortical oscillations act as gatekeepers for perception by providing 'windows of opportunity' that permit (but do not guarantee) the entry of sensory stimuli to perception.
