The medial temporal lobe (MTL) has traditionally received credit as supporting episodic memory, a type of declarative memory that enables access to one’s past experiences. However, converging evidence suggests that parietal areas may also contribute to episodic retrieval–in particular, the retrosplenial (RSC) and posterior cingulate cortices (PCC) in the left medial parietal cortex (MPC) and the angular gyrus (AG) in the left lateral parietal cortex (LPC) (Cabeza et al., 2008; Wagner et al., 2005). As nodes of the default mode network (Greicius et al., 2003; Raichle et al., 2001)–a cluster of interacting areas implicated in self-referential thinking–the RSC/PCC and AG would seem prime candidates for promoting retrieval of autobiographical episodes. Indeed, functional imaging has corroborated these intuitions (Cabeza et al., 2008; Wagner et al., 2005). Nevertheless, fMRI has its limitations, and a technique with both high spatial and temporal resolution would potentially provide further insight into the dynamics  of the parietal lobe in episodic memory. Fortunately, Josef Parvizi’s Laboratory of Behavioral and Cognitive Neuroscience at Stanford University recognizes the shortcomings of functional imaging, frequently incorporating intracranial electrocorticography and/or intracranial electrical brain stimulation to investigate the neurological underpinnings of human behavior and cognition. In a recent study, Foster et al. (2015)  exploited the spatiotemporal precision of ECoG in three human subjects during conditions of task performance, rest, and sleep. Electrode coverage of the MPC and LPC in these epileptic patients (Figure 1b) offered the valuable opportunity to obtain electrophysiological recordings in these areas and to observe parietal activity and connectivity.

Foster et al. (2015) employed a task that required participants to judge a visually presented statement as true or false, with some statements involving episodic or semantic memory, self- or other-judgments, or arithmetic (Figure 1c). Analysis of the electrophysiological data recorded under these different task conditions most apparently demonstrated greater high-frequency broadband amplitude (HFB, 70-180 Hz) both in the RSC/PCC and AG for the episodic condition relative to the other task conditions. HFB has been suggested as an index of neuronal population response and thus supposedly reflects increased activity in these regions during episodic retrieval. In addition, the HFB response profiles of the RSC/PCC and AG across task conditions were remarkably similar; activity in other parietal regions failed to so strongly resemble that of the RSC/PCC and AG (see Figure 1d/e).

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Figure 1. Parietal Subregions, Electrode Locations, Experimental Task, and Task Responses

Other analyses confirmed the notable similarity in responses between the RSC/PCC and AG: a strong positive correlation was found between the trial-level mean HFB responses in these areas with the strongest correlations arising in the episodic and semantic conditions. On a temporal scale, these two sites showed essentially identical response onset latencies for the episodic condition (Figure 2). Cumulatively, these findings suggest that the RSC/PCC and AG may receive simultaneous inputs–perhaps from the MTL–and work in parallel to process different aspects of these inputs and therefore different components of episodic memory.

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Figure 2. Correlated HFB Trial Responses between PCC and AG

The study conducted by Foster et al. (2015) also examined parietal activity profiles and connectivity patterns during rest and sleep to delve deeper into the dynamics of MPC and LPC. Resting-state ECoG analysis extracting slow (<1 Hz) ongoing fluctuations in HFB amplitude displayed strong correlations between RSC/PCC and AG; a similar correlation pattern held for the low beta range as well. Resting-state fMRI yielded comparable correlations. Additionally, these correlation patterns persisted in ECoG data collected during stage-2 and stage-3 sleep, reinforcing the similarity of parietal connectivity and activity across three quite behaviorally distinct states (Figure 3). The notion that resting-state activity recapitulates task-driven activity is not novel, and some assert that such spontaneous activity reflects the strength and organization of new connections shaped by recent experience and neuronal activation (Harmelech and Malach, 2013).

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Figure 3. Similarity of ECoG Correlation Patterns across Task, Rest, and Sleep States

Overall, the study by Foster et al. (2015) clearly demonstrates the utility of ECoG recordings in probing the spatiotemporal dynamics of the parietal lobe–not only during episodic retrieval per se, but also during rest and sleep. The current findings support and expand upon those of past fMRI experiments (Cabeza et al., 2008; Wagner et al., 2005), revealing highly similar neuronal responses in the RSC/PCC and AG, as well as simultaneity in these responses. Nonetheless, while Foster et al. (2015) have pioneered electrophysiological investigation of parietal activity related to memory, their study is far from exhaustive and demands future research to establish a more comprehensive framework for understanding the precise roles of the MPC and LPC in episodic retrieval.

To hear Dr. Josef Parvizi elaborate on this research, attend his talk on Tuesday, October 11, 2016, at 4:00 pm in the CNCB’s Marilyn G. Farquhar Seminar Room.

Cabeza, R., Ciaramelli, E., Olson, I.R., and Moscovitch, M. (2008). The parietal cortex and episodic memory: an attentional account. Nat. Rev. Neurosci. 9, 613–625.

Foster, B. L., Rangarajan, V., Shirer, W. R., & Parvizi, J. (2015). Intrinsic and Task-Dependent Coupling of Neuronal Population Activity in Human Parietal Cortex. Neuron, 86(2), 578–590.

Greicius, M.D., Krasnow, B., Reiss, A.L., and Menon, V. (2003). Functional connectivity in the resting brain: a network analysis of the default mode hypothesis. Proc. Natl. Acad. Sci. USA 100, 253–258.

Harmelech, T., and Malach, R. (2013). Neurocognitive biases and the patterns of spontaneous correlations in the human cortex. Trends Cogn. Sci. 17, 606–615.

Raichle, M.E., MacLeod, A.M., Snyder, A.Z., Powers, W.J., Gusnard, D.A., and Shulman, G.L. (2001). A default mode of brain function. Proc. Natl. Acad. Sci. USA 98, 676–682.

Wagner, A.D., Shannon, B.J., Kahn, I., and Buckner, R.L. (2005). Parietal lobe contributions to episodic memory retrieval. Trends Cogn. Sci. 9, 445–453.

Gina D’Andrea-Penna is a first-year student in the neurosciences graduate program rotating in Dr. Bradley Voytek’s lab. Her strongest interests lie in the field of cognitive neuroscience, and she aspires to investigate and, one day, comprehend consciousness.

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