Common and distinct neural mechanisms of episodic and semantic memory: Evidence from EEG oscillations

It is essential to examine brain structures and processes involved in retrieval tasks in semantic and episodic memory, two key components of the declarative (explicit) memory system, particularly in terms of regional activity differences and functional connectivity. The objective of this study was to investigate the neural basis of semantic and episodic memory retrieval processes in healthy adults using EEG oscillatory activity. The sample consisted of 28 participants between 18 and 30 years of age. Episodic memory was assessed using the Öktem Verbal Memory Processes Test, while semantic memory was evaluated using the Turkish Emotional Norm Words List. EEG recordings were obtained with a 32-channel active EEG system. Data collected during the memory tasks were analyzed by comparing mean activity across predefined ROIs. The findings revealed both shared and distinct oscillatory dynamics between episodic and semantic memory processes. Episodic memory retrieval was found to be associated with significantly increased delta power in bilateral parietal regions and elevated alpha power in the left parietal region in comparison with semantic memory. Despite these differences, correlation analyses demonstrated significant positive associations between episodic and semantic memory tasks across multiple frequency bands, particularly in parietal regions (delta, alpha, and gamma), as well as in the left temporal region (gamma). These findings indicate substantial overlap in the neural mechanisms underlying the two memory systems. Furthermore, hemispheric analyses revealed a predominance of left parietal activation in both tasks, contrary to expectations of right-lateralized episodic processing.