Functional characterization of memory-encoding networks after medial temporal lobe resection

Abstract

Considering the central position of the hippocampus as a densely interconnected hub in brain networks and its role in episodic memory, medial temporal lobe resection, including the hippocampus, should modify recruitment and strength of connectivity of functional memory network. However, functional memory encoding network in patients with medial temporal lobe resection has not been well characterized, which could provide a clue for new therapeutic targets for people with memory impairment. The aim of present study is to understand how brain supports normal episodic memory function without unilateral medial temporal lobe structures in a new perspective of functional interactions of brain network. Thirty-seven patients who underwent unilateral medial temporal lobe resection for the treatment of medically intractable temporal lobe epilepsy (17 left, 20 right

median age 34 years) and 24 healthy controls (median age 32 years) were studied. To understand stable and an effective memory network, patients who underwent resective surgery at least 1 year before fMRI scanning and who have normal range of postoperative memory capacity were recruited. All subjects performed functional MRI memory encoding paradigm of words and figures. Hippocampal regions of interest analysis revealed that greater activation of hippocampus contralateral to the resection was related to higher memory scores in both patient groups. Whole-brain functional activation analysis revealed that well-known task-negative areas including the medial prefrontal cortex were less deactivated in patient groups than healthy controls. Task-based functional connectivity analysis revealed that the right medial prefrontal cortex showed stronger interactions with widespread brain areas including hippocampus contralateral to the resection during successful word encoding in left surgery group and during successful figure encoding in right surgery group. Furthermore, the strengths of right medial prefrontal cortex functional connectivity predict individual memory capacity of patients. The results of present study suggest that hyper-connectivity of medial prefrontal cortex may play a pivotal role in episodic memory function with the absence of functional connections of medial temporal lobe. These results, therefore, further implicated in the studies of brain stimulation toward enhancing memory for people who suffer from medial temporal lobe-dysfunction-related memory disturbances by providing possible new target area of medial prefrontal cortex.