The Hippocampus: Myth, Memory & Magnetic Resonance Imaging

Purpose The hippocampi are complex, convoluted regions of archicortex in the temporal lobes of the brain critical to explicit memory function. Over time our view of the structural anatomy of this region has evolved, yet how this relates to underlying functions remains poorly understood. Advances in functional neuroimaging and general anaesthetic pharmacology provide the potential for new insights into the mechanisms of this ancient structure. Methods Twenty-five healthy volunteers were sedated with a target-controlled infusion of the general anaesthetic agent, propofol, to two sedation levels. Structural and resting-state Blood Oxygen Level Dependent (BOLD) functional magnetic resonance imaging (fMRI) was performed. Volunteers completed a semantic decision-making task to auditory stimuli at baseline and each sedation level. Memory performance was assessed and related to the functional imaging of the hippocampus by microanatomical sub-region. Results Baseline hippocampal functional connectivity patterns were retained during sedation and recovery. Despite a progressive decrease in memory performance across sedation levels, there was a multiphasic pattern of functional connectivity changes between sedation levels and reciprocal changes during the recovery phase. Adjusting for memory performance identified asymmetric changes which were partially sustained across incremental sedation depths. Conclusion This analysis characterises the intra-hippocampal functional connectivity signatures associated with pharmacologically induced amnesia in healthy volunteers at low levels of sedation. Results suggest that functional connectivity changes due to sedation may be multiphasic with an underlying core amnesic signature.