The Amygdala, Arousal and Memory: From Lesions to Neuroimaging

Emotional events are better remembered than neutral events. But what are the mechanisms behind this memory enhancing effect? It seems that they depend on the arousal level at the moment we experience the event to be remembered. The first study of the present thesis mapped the brain areas that changed their activity in a highly arousing situation in subjects with snake or spider phobia. Looking at pictures of their feared object engaged the amygdala, situated in the medial temporal lobe. This area has previously been demonstrated to be necessary for fear reactions. Here, the novel question was what other brain areas the amygdala engages when the brain is in a state of high arousal. Results suggest that the amygdala recruits other limbic and cortical areas known to be involved in motor behavior and object recognition. In contrast, when subjects watched fear-relevant but non-phobic pictures, amygdala activity was negatively correlated to the anterior cingulate cortex suggesting cortical inhibition. The final two studies aimed at explaining the physiological brain mechanisms behind arousal enhancement of memory. In the first one, epileptic patients with medial temporal lobe resections including the amygdala were compared to healthy controls on a recognition memory task where the pictures to be remembered varied in arousal intensities. Results suggested that the anterior medial temporal lobe including the amygdala is necessary for arousal enhancement of memory because the enhancement effect was abolished in resectioned patients. The last study related inter-individual differences in bodily arousal to amygdala-parahippocampal interaction. Results suggest that the beneficial effects of emotion on memory depend on arousal regulating mechanisms of the amygdala that in turn affects parahippocampal activity. Collectively, results suggest that the amygdala is regulating changes in arousal states of the brain and body during distressful situations. Further, arousal in turn determines memory strength through gating amygdala influences on the parahippocampal cortex. Thus, the amygdala is a node both in a fear and a memory network and arousal influences the amygdala to prepare for action and to enhance memory. This seems evolutionary sound.