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N-Acetylaspartate and creatine levels measured by ¹H MRS relate to recognition memory

From the Institute of Epileptology, King’s College Hospital (Drs. Ferrier, Alarcón, Koutroumanidis, Morris, Elwes, Cox, Binnie, and Polkey), and Institute of Psychiatry (Dr. Simmons and A. Glover), London, UK; Faculteit der Geneeskunde (Dr. Ferrier), Academic Medical Center, University of Amsterdam, the Netherlands; and Departamento de Fisiología (Dr. Alarcón), Facultad de Medicina, Universidad Complutense, Madrid, Spain.

Address correspondence and reprint requests to Dr. G. Alarcón, Department of Clinical Neurophysiology, King’s College Hospital, Denmark Hill, London SE5 9RS, UK.

OBJECTIVES: To investigate the relationship between recognition memory and metabolite levels in medial structures of the temporal lobes in the living human brain.

METHODS: Proton MRS (¹H MRS) and the intracarotid amobarbital test were performed in 16 epileptic patients found suitable for temporal lobectomy. All patients had mesial temporal sclerosis. Metabolite ratios between N-acetylaspartate (NAA), creatine and phosphocreatine (Cr + PCr), and choline-containing compounds (Cho) [NAA/(Cr + PCr), NAA/Cho, and NAA/(Cr + PCr + Cho)] were calculated for ¹H MRS voxels that included the amygdala, anterior half of the hippocampus, and underlying subiculum. Metabolite ratios were correlated with unilateral memory scores estimated by the intracarotid amobarbital test for words, objects, faces, and total score.

RESULTS: The total memory score, memory for objects and faces, and NAA/(Cr + PCr) were significantly lower for the hemisphere ipsilateral to the resection. The asymmetry indexes for NAA/(Cr + PCr) correlated with asymmetry indexes for words ( = 0.82, p = 0.0001) and total memory ( = 0.72, p = 0.002). Analysis of memory scores and metabolite ratios from all 32 hemispheres revealed a correlation between NAA/(Cr + PCr) and memory for words ( = 0.45, p = 0.009). A correlation between memory for words and NAA/(Cr + PCr) existed in the contralateral ( = 0.58, p = 0.019) and in the right ( = 0.51, p = 0.045) hemispheres, and a trend was found in the left hemispheres ( = 0.48, p = 0.06).

CONCLUSION: There is a correlation between memory for words and the NAA/(Cr + PCr) ratio from medial temporal structures in patients with mesial temporal sclerosis. The findings suggest that medial temporal structures and adjacent neocortex play a significant role in recognition memory in humans, particularly for words.

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