Hippocampal, but not amygdala, activity at encoding correlates with long-term, free recall of nonemotional information

Abstract

Participation of two medial temporal lobe structures, the hippocampal region and the amygdala, in long-term declarative memory encoding was examined by using positron emission tomography of regional cerebral glucose. Positron emission tomography scanning was performed in eight healthy subjects listening passively to a repeated sequence of unrelated words. Memory for the words was assessed 24 hr later with an incidental free recall test. The percentage of words freely recalled then was correlated with glucose activity during encoding. The results revealed a striking correlation (r = 0.91, P < 0.001) between activity of the left hippocampal region (centered on the dorsal parahippocampal gyrus) and word recall. No correlation was found between activity of either the left or right amygdala and recall. The findings provide evidence for hippocampal involvement in long-term declarative memory encoding and for the view that the amygdala is not involved with declarative memory formation for nonemotional material.

Figure 1

Brain regions in which rGMR while subjects listened to a word list correlated significantly with number of words freely recalled 24 hr later. The scale bar represents the P-values ranging from P < 0.05 to P ≤ 0.001. Only areas of statistically significant correlation (P < 0.05) are shown. Positive correlations are depicted in yellows/reds, negative correlations in blues. Axial brain sections (29) are taken from 61%, 47%, and 41% (Upper, left to right), and 34%, 28%, 21%, and 14% (Lower, left to right) of head height above the canthomeatal line. The front of the brain is toward the top of each image, and the left side of the brain is on the left of each image. Solid arrow (Lower, left) identifies the primary hippocampal region finding. The shaded arrow (Lower, second section from right) identifies the amygdala. [Reproduced with permission from ref. . Copyright 1996, JKB Sutherland.]

Figure 2

Scatterplot of rGMR versus recall for the left hippocampal region identified by the solid white arrow in Fig. 1.