Subdural grid recordings of distributed neocortical networks involved with somatosensory discrimination.

Previous studies suggest that evidence for the sub-second activation of distributed neural networks can be obtained by computing the covariance between segments of the scalp-recorded evoked potential. However, the cortical representation of such potentials is not known. Here we report a case study where the evoked potential covariance (EPC) measure was applied to data recorded from a 58-channel subdural grid implanted in an epilepsy patient. Recordings were made while the patient performed a task that required judging the somatosensory intensities of electrical stimuli and executing precise finger flexion responses in response to a subset of those stimuli. Post-stimulus EPC patterns involved covariances between somatosensory, motor, and temporal regions. Pre-stimulus EPC patterns involved these same regions, but only when it could be anticipated that the upcoming stimulus would likely require a response. The majority of the observed EPCs occurred with non-zero time-lags, and these EPCs often involved non-adjacent electrode pairs. Thus, the observed EPCs were unlikely to arise solely from volume conduction. Rather, they appeared to reflect the transient integration of activity across distinct cortical processing nodes.