The role of posterior parietal activity in multisensory integration of episodic contexts

This thesis endeavoured to investigate the nature of the relationship of the posterior parietal cortex (PPC) with richness of episodic memory, and particularly binding of multisensory contexts in retrieved episodes. Electrophysiological signatures (ERPs) of memory retrieval were examined in order to determine the association of posterior parietal signals with increased recollection of multisensory episodic contexts. In two studies of the Old/New recognition ERP measures the increased electrophysiological response over PPC sites was found to be significantly associated with the fine amount of multisensory details retrieved in an extended source memory paradigm. Parietal ERPs were shown to directly vary across 4 levels of increasing multisensory source memory performance. Subsequent examination of recognition ERPs over the PPC further specified that this was a recollection enhancement which was distinguished from similar familiarity-related signals. In order to evaluate the causal influence of this PPC electrophysiological enhancement on retrieval, transcranial direct current stimulation (tDCS) was employed as a source of cortical neuromodulation. In two experiments tDCS was applied before participants performed source retrievals in the same source memory task significantly associated with enhanced PPC recollection –based activity. Anodal and cathodal tDCS to the PPC did not affect recognition performance, however anodal stimulation lead to an enhancement in source memory performance above sham performance. Conversely, anodal stimulation of the M1 did lead to an enhancement of recognition accuracy, but no effect on source memory performance. Taken together, it can be concluded from these studies that PPC activity distinctly influences the integration of multi-sensory episodic details.