Differential amygdala habituation to neutral faces in young and elderly adults.

Habituation is a highly adaptive property of the nervous system, which allows for the allocation of attention and other cognitive resources to more imperative environmental events. The amygdala is an important site of habituation in humans, but no studies to date have examined the effects of aging on amygdala habituation. Given the amygdala's role in evaluating the salience of a stimulus and initiating behavioral responses, the potential importance of amygdala habituation in aging may be far-reaching. Therefore, we assessed for differences in habituation in the amygdalae of healthy young and elderly adults during repeated presentations of neutral human faces using functional magnetic resonance imaging (fMRI). In addition, we evaluated the relationship between amygdala volume and habituation, to examine the effects of atrophy. Eighteen healthy young controls and 18 healthy elderly subjects were scanned with fMRI during viewing of repeatedly presented neutral human face stimuli. Significant fMRI signal decrement was observed across all subjects for early versus late face presentations. Analysis of group, condition, and hemisphere revealed a significant three-way interaction, with right greater than left habituation in the young, but left greater than right amygdala habituation in the elderly. Volumetric and correlational analyses demonstrated that amygdala volume is associated with habituation in the right, but not left, hemisphere. We conclude that, in healthy elderly adults, the amygdala retains its adaptive habituation response, but speculate that intrinsic changes in amygdala anatomy during aging may modulate its laterality.