Abstracts

Rationale: As a core node of the human default mode network (DMN), the posteromedial cortex (PMC) has been endowed with a wide variety of functional roles, but its unique contribution to cognition still remains unclear. Clinically, the PMC shows sensitive changes in metabolic and hemodynamic activity, as revealed by neuroimaging, during the loss or restoration of consciousness (e.g. generalized seizures, anesthesia, sleep and coma). Functional neuroimaging has also revealed the PMC, and DMN more generally, to be activated during tasks of putative ‘self-referential' processing, such as autobiographical memory. Methods: We studied the effects of direct electrical stimulation of the human PMC via intracranial electrodes over the mesial parietal and occipital cortices in 15 human subjects. All stimulations were delivered through a bipolar pair of proximal electrodes; which were classified as being within (PMC) or outside the PMC (non-PMC), based on formal anatomical boundaries within each subject using high-resolution electrode localization. Results: From a total of 552 stimulations, we observed 277 reported effects (patient report, or observed behavior). Interestingly, the bulk of these responses could be characterized as either motor behavior (e.g. leg movement), or sensory phenomena reported by the patients as visual (e.g. phosphenes) or tactile (e.g. somatosensory tingling) sensation. Furthermore, the anatomical distribution of stimulations producing these effects conformed to an interesting functional configuration. Visual effects were observed for sites surrounding and inferior to the parietal-occipital sulcus (POS), whereas motor and sensorimotor effects were elicited from sites surrounding or superior to the marginal branch of the cingulate sulcus (mbCS). Consequently, this left a large number of stimulations (n = 128), which occurred within the PMC (i.e. in between POS and mbCS), which produced no reported or observed changes in perception or behavior. Whereby only 13.7% of the stimulations within the PMC, produced any reported or observed effects, however all of these were highly similar to the motor or sensory effects typically noted for regions outside of the PMC or on its boundary (indicative of charge spread). Consistent with these observations a non-parametric test of association revealed a significant difference in the occurrence of responsive stimulations between PMC and non-PMC sites (Chi-square(2) = 48.6, p<.001). Conclusions: Although the PMC has been implicated in a host of higher order cognitive functions, including consciousness itself, direct causal perturbation of this region produced no profound changes in conscious state. Importantly, these stimulations included sites where clear functional responses were observed for autobiographical memory retrieval during experimental paradigms. Together such data promotes the notion that associative cortices, not involved in basic sensory transduction and detection, can only be functionally mapped under conditions of task related perturbation (i.e. during their associative/integration functional engagement).