Theta-gamma Coupling Dysfunction: Using Neurological and Psychiatric Disorder Case Comparisons to Develop a Proposed Model For Episodic Memory Retrieval Failure Under High Cognitive Load

Abstract

The study of episodic memory is an intricate yet fundamental aspect of human cognition. This idea is supported by individuals suffering from a system malfunction–either due to physical damage as a result of neuropathology (e.g. medial temporal lobe damage) or a circuit level disruption of the system in healthy individuals (episodic retrieval inhibition under high cognitive load). The data underlying the former instance (memory inhibition resulting from physical damage) is more clear cut, however, understanding the breakdown among individuals with no apparent damage has been more of a challenge. In the existing literature, the breakdown of episodic memory retrieval often coincides with situations that induce high cognitive load–moments that exhaust our limited amount of cognitive resources (e.g. police interrogation, student exams, etc.), however, it is unknown as to why this occurs. In this literature review, we argue that the mechanistic breakdown of episodic memory retrieval under high cognitive load may come as a result of a theta-gamma oscillatory coupling dysfunction–a known mechanism for episodic memory retrieval. In support, individuals suffering from neurological or psychiatric diseases with known episodic memory retrieval deficits (e.g. Alzheimer’s disease, schizophrenia, temporal lobe epilepsy), provide evidence of a theta-gamma coupling dysfunction. Cognitive load also seems to have a direct effect on the inhibition of theta-gamma coupling, as an increase in load has been shown to cause shifts in phase coherence, an excitatory-inhibitory circuit imbalance, and an initial increase in coupling that is followed by a decrease after the load reaches a threshold. While there is evidence to suggest a relationship between high cognitive load and theta-gamma coupling dysfunction, future studies are necessary to examine the relationship between cognitive load and theta-gamma coupling directly. Key words: episodic memory, theta-gamma coupling, cognitive load, phase coherence