Electrocorticography reveals the temporal dynamics of posterior parietal cortical activity during recognition memory decisions

Abstract

Theories of the neurobiology of episodic memory predominantly focus on the contributions of medial temporal lobe structures, based on extensive lesion, electrophysiological, and imaging evidence. Against this backdrop, functional neuroimaging data have unexpectedly implicated left posterior parietal cortex (PPC) in episodic retrieval, revealing distinct activation patterns in PPC subregions as humans make memory-related decisions. To date, theorizing about the functional contributions of PPC has been hampered by the absence of information about the temporal dynamics of PPC activity as retrieval unfolds. Here, we leveraged electrocorticography to examine the temporal profile of high gamma power (HGP) in dorsal PPC subregions as participants made old/new recognition memory decisions. A double dissociation in memory-related HGP was observed, with activity in left intraparietal sulcus (IPS) and left superior parietal lobule (SPL) differing in time and sign for recognized old items (Hits) and correctly rejected novel items (CRs). Specifically, HGP in left IPS increased for Hits 300-700 ms poststimulus onset, and decayed to baseline ∼200 ms preresponse. By contrast, HGP in left SPL increased for CRs early after stimulus onset (200−300 ms) and late in the memory decision (from 700 ms to response). These memory-related effects were unique to left PPC, as they were not observed in right PPC. Finally, memory-related HGP in left IPS and SPL was sufficiently reliable to enable brain-based decoding of the participant's memory state at the single-trial level, using multivariate pattern classification. Collectively, these data provide insights into left PPC temporal dynamics as humans make recognition memory decisions.intracranial EEG | declarative memory | decision making | multivariate pattern analysis | ECoG T he ability to remember past events-episodic memory-is known to critically depend on medial temporal lobe (MTL) structures (1, 2) and their interaction with prefrontal cortex (3). Neuroimaging studies of humans making memory-based decisions, although advancing understanding of MTL and prefrontal mnemonic function (4-8), consistently and unexpectedly demonstrate that activity in left lateral posterior parietal cortex (PPC) also varies with episodic memory outcomes (9-14). In particular, functional MRI (fMRI) data reveal dissociable effects of memory on activity in left intraparietal sulcus (IPS), superior parietal lobule (SPL), and angular gyrus (AG), wherein activity tracks perceived memory strength, retrieval decision uncertainty, and episodic recollection, respectively (9-14). For example, during recognition memory decisions, activity in left lateral IPS is greater during higher confidence hits and monotonically decreases across lower confidence hits to lower confidence correct rejections (CRs) to higher confidence CRs (15-17). By contrast, activity in left SPL is greater during lower confidence recognition decisions (for both hits and correct rejections) relative to higher confidence decisions (11,16,17).Studies of ...

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“…101 ). Recently, an electrocorticography study 102 reported greater sustained high-frequency gamma power, a proxy signal for multiunit activity, for old versus new correct decisions in the IPS; this increase in gamma power decayed only 200 ms before the motor response, consistent with an accumulator hypothesis.…”

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