Hippocampal adequacy versus functional reserve: Predicting memory functions following temporal lobectomy

Abstract

While decrements in memory following temporal lobectomy for intractable epilepsy remain a major neurocognitive complication of surgical intervention, it has been difficult to identify patients who are most at risk for developing decrements in their memory functions following surgery. Two different models of hippocampal functioning have been posited to account for postsurgical memory losses: hippocampal reserve versus functional adequacy. This article briefly examines the basis of the traditional model of hippocampal reserve, which suggests that it is the reserve or capacity of the contralateral hippocampus to support memory after surgery that determines whether changes in memory function will be observed. A more in-depth review of emerging data from studies of function (baseline neuropsychological testing and intracarotid amytal procedure) and structure (histologic cell densities and MRI volumetrics) supporting a functional adequacy model of hippocampal functioning is then presented. Data from these latter studies provide strong convergent support for the view that the risk for memory decrements following mesial temporal lobe epilepsy surgery is inversely related to the functional adequacy of the tissue to be resected. Data reviewed here have broad implications for the field of neuropsychology. Not only do the data reported in this article challenge some of the basic tenets of our understanding of the role of the hippocampus in memory functioning, but they illustrate the importance of ongoing research, critical reevaluation of basic concepts, and the importance of self-scrutiny in light of new technologies.