Abstracts

Rationale: Posteromedial cortex (PMC) collectively defines an anatomical region in the primate brain that encompasses the posterior cingulate cortex, retrosplenial cortex and precuneus. Uniquely, the PMC is one of the most richly connected and metabolically active regions of the cerebral cortex. Moreover, the PMC is thought to be involved in higher order self-referential cognition and plays a key role in correlated brain network activity observed during the resting state. The current intracranial electrophysilogical study addressed the resting and task-dependent patterns of activity within the PMC and lateral parietal cortex (LPC). Methods: Electrocorticography (ECoG) recordings were performed in five patients with refractory epilepsy implanted with multiple strips and grids over the medial cortical surface. Prior to implantation of electrodes, all subjects had undergone routine fMRI studies during the resting state. Intracranial electrophysiological data were acquired during a cognitive task that included math equations (MATH task) or statements about themselves (SELF condition) or about a distant other person (NON-SELF condition). Subjects were instructed to respond via button presses as to whether each equation or statement was true or false. Math and Self/Non-Self stimuli were separated randomly from each other by the REST condition where subjects passively viewed a fixation cross that appeared on the screen for 5 seconds. Subjects were instructed to let their mind wander during the REST condition. Conditions were presented randomly and were counterbalanced. The total time of the experiment varied by the reaction time of the subject during MATH, SELF, and NON-SELF conditions. Results: Our findings can be summarized as follows: 1) The PMC showed a significant increase of broadband gamma activity (40-180Hz) during the resting state while the activity in the same frequency range was decreased significantly during arithmetic task; 2) the LPC showed a significant increase of gamma activity during the arithmetic task while the activity in the same frequency range was not seen during the resting state; 3) in the PMC or LPC, when there was an increase in the gamma power during a task, there was a parallel decrease of activity in the lower frequency range (e.g, alpha band activity) and vice versa; 4) there was a significant increase of activity in the gamma range within the retrosplenial cortex during autobiographical statements; 5) electrical brain stimulation within the PMC did not cause any perceptual or behavioral change. Conclusions: The superior spatial and temporal resolution of the ECOG method allowed us study some of the remaining questions about the functional role of the PMC and the temporal dynamics and the exact anatomical specificity of resting or task dependent activity within the PMC.