Nonlinear Trends in Hippocampal Metabolic Function and Verbal Memory: Evidence of Cognitive Reserve in Temporal Lobe Epilepsy?

The present study explored the possibility of nonlinear trends in the relationship between verbal memory and hippocampal function in a series of 33 patients with temporal lobe epilepsy (TLE). Right and left hippocampal metabolic function was quantified using levels of hippocampal creatine to N-acetylaspartate (Cr/NAA) derived from (1)H magnetic resonance spectroscopic imaging. An exploratory neural network analysis (multi-layer perceptron) suggested the possibility of either a quadratic or cubic trend in the relationship between left hippocampal Cr/NAA and verbal retention. Using regression-based curve estimation, the cubic function was found to optimally fit the data, explaining 41% of the variance in the relationship between verbal memory and hippocampal metabolic function. This was contrasted to the 28% variance explained by simple linear regression. These findings suggest that (1) the relationship between verbal retention and hippocampal function in patients with TLE is nonlinear, and (2) this could be explained in terms of a possible "cognitive reserve." Clinical implications are discussed.