Brain Mapping of Language Using High Frequency Gamma Oscillations in the Electrocorticography.

Electrocorticographic (ECoG) recordings, necessary for clinical intervention in epileptic patients, provide a unique opportunity to study the electrophysiological correlates of functional brain activation in more detail than noninvasive recordings. The proximity of ECoG electrodes to EEG sources enhances their spatial resolution, as well as their sensitivity and signal-to-noise ratio, particularly for high frequency EEG activity. ECoG recordings have therefore been used to study the event-related dynamics of brain oscillations in a variety of frequency ranges, and in a variety of functional-neuroanatomic systems, including cortical networks responsible for language. Synchronized gamma oscillations particularly in the high gamma range (>60Hz) was suggested to play a role in binding between cortical regions, essential part of efficient semantic network. The functional response properties of high gamma activity (HGA) are distinct from similar phenomena in lower frequencies. The HGA findings are consistent with its proposed binding role in models of neural computation. Preliminary studies suggest that interactions between gamma oscillations in macroscopic local field potentials (LFP) generated by different large-scale populations of neurons engaged by the same functional task occur in gamma frequencies at latencies consistent with the timing of task performance. The neuronal mechanisms underlying high gamma activity and its distinctive response properties in humans are still largely unknown, but their research through invasive means is expected to support and expand their potential clinical and research applications, including functional brain mapping, braincomputer interfaces, and neurophysiological studies of human cognition. In our lab studies we aim to asses HGA and compare it to event related potentials (ERP's) at different stages of linguistic processing from early sensory to late lexical and semantic stages of processing. The results of these studies indicate that HGA which is related to the semantic network activation distinguished spatially and temporally from the early sensory registration. HGA specific to semantic processing occurs mainly in late stages of processing and its cortical distribution is restricted in comparison to ERP's response of the same process. [ABSTRACT FROM AUTHOR]

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