Modulating the efficiency of memory formation: Insights from temporal lobe epilepsy and nociceptive arousal

Abstract

The efficiency of memory formation, i.e. encoding and consolidation, can be modulated by various factors. While some of these factors exert a constant influence on memory processing, others act temporarily. In the present thesis, the effects of two examples of modulating factors were investigated. Study I focused on a neurological disorder constantly affecting the neural correlates of memory formation, and Study II focused on the temporary modulation of consolidation due to arousal. In Study I, different magnetic resonance imaging techniques and memory tasks were implemented to investigate potential reasons for memory deficits in patients with temporal lobe epilepsy of unknown cause. Despite the lack of overt morphological lesions, functional imaging revealed increased hippocampal activity during encoding, but decreased associative memory during recognition for patients compared to healthy controls. The findings suggest that subtle alterations of neuronal microcircuits due to epilepsy exist which impair the efficiency of encoding. Thus, the increase of activity is assumed to reflect a compensatory process for successful encoding within less efficient hippocampal cell assemblies. In Study II, event-related effects of a temporary modulator were investigated. Electrical shocks were applied to healthy participants in order to induce arousal after the initial processing of stimuli probed for memory. Importantly, this paradigm could disentangle effects of arousal from effects of cognitive factors, which usually accompany emotionally arousing stimuli during encoding. Enhanced memory for stimuli followed by electrical shocks in Study II was only found after a retention interval, representing a more efficient consolidation. Since cognitive factors could not account for this effect, the increase of efficiency is most likely due to an enhanced noradrenergic innervation and thus facilitation of neuronal responsiveness in those temporal lobe areas relevant for stimuli processing.