A unique role for the human amygdala in novelty detection

Abstract

Previous research indicates that the amygdala and hippocampus are sensitive to novelty; however, two types of novelty can be distinguished - stimuli that are ordinary, but novel in the current context, and stimuli that are unusual. Using functional magnetic resonance imaging, we examined blood oxygen dependent level (BOLD) response of the human amygdala and hippocampus to novel, commonly seen objects versus novel unusual objects. When presented with the novel common stimuli, the BOLD signal increased significantly in both the amygdala and hippocampus. However, for the novel unusual stimuli, only the amygdala showed an increased response compared to the novel common stimuli. These findings suggest that the amygdala is distinctly responsive to novel unusual stimuli, making a unique contribution to the novelty detection circuit.

Figure 1. Amygdala and hippocampus show different patterns of response to two types of novel stimuli

(a) In the amygdala, BOLD response increases parametrically to familiar, novel common and novel uncommon images. Response is greater for novel common relative to familiar images for both the left amygdala, t(28) = 4.49, p = .0001, and right amygdala, t(28) = 4.04, p = .0004. Amygdala response is also increased for novel uncommon images compared to novel common for both left, t(28) = 2.14, p = .04, and right, t(28) = 2.49, p = .02, hemispheres. (b) Left and right hippocampi show increased responses to novel common relative to familiar pictures (t(28) = 6.21, p = .0004 and t(28) = 6.66, p = .0001,respectively), but do not respond differentially to the two types of novel pictures (both ps > .59; novel uncommon = novel common). Error bars indicate SEM.

Figure 2. Brain response to the three different picture types (versus baseline fixation cross)

Activation maps are superimposed on a coronal section of a single standard (MNI canonical T1 image) brain image. Spread and degree of BOLD response can be seen for the (a) amygdala (y = 0) and (b) hippocampus (y = -12). Activation maps are thresholded at t = 5 to illustrate differences in spread and degree across conditions. The color bar represents t-values (restricted to maximum t = 10, to emphasize color variations).