Stress affects memory acquisition, but the direction of the effect varies. Some studies report that stress enhances memory, and others report that stress impairs memory. The relation between the effect of pleasant stimuli on neural activity in the hippocampus and memory acquisition is unclear. The amygdala has a primary role in processing emotion and mediates affectively-influenced memory. The hippocampus is critically involved in memory. We evaluated the effect of pleasant and unpleasant stimuli on neuronal activity in these regions during picture-encoding using functional magnetic resonance imaging and memory acquisition. To test whether pleasant and unpleasant stimuli affect memory acquisition, a recall test was administered 20 minutes after encoding. Fifteen subjects (8 men; mean age 37.9±12.9 years [range 20-61]) participated. Pleasant and unpleasant stimuli increased blood oxygenation level-dependent (BOLD) signals in the amygdala. The pleasant stimulus enhanced neuronal activity in the hippocampus (i.e., increased BOLD signals) and increased memory acquisition. The unpleasant stimulus decreased both hippocampal neural activity and memory acquisition. Visual Analogue Scale scores for the pleasant and unpleasant stimuli were 8.7±0.5 and 9.0±0.6, respectively. Thus, pleasant and unpleasant stimuli might influence memory acquisition by increasing or reducing hippocampal activity during picture-encoding.
