Is there evidence of a subordinate role of the hippocampal CA1 field for declarative memory formation?

Abstract

Sir, Neuropsychological investigations in patients with temporal lobe epilepsy (TLE) substantially contributed to our understanding of the neural underpinnings of human memory (Saling, 2009). Given the essential role of the hippocampus for episodic memory, epilepsy surgery for the treatment of pharmaco-resistant mesial TLE offers the unique possibility of relating the neuropathological status of the resected hippocampus to presurgical memory performance. With one exception (Zaidel et al. , 1998), almost all respective studies found significant correlations between segmental neuronal cell loss within the left hippocampus and preoperative verbal memory functions (Sass et al. , 1990, 1992, 1995; Baxendale et al. , 1998; Zentner et al. , 1999; Witt et al. , 2014). One of these studies also reported significant correlations between cell counts of the right hippocampus and non-verbal memory (Zentner et al. , 1999). Three additional studies found significant relationships between hippocampal cell loss and global memory performance of the isolated ipsilateral hemisphere during the Intracarotid Amobarbital Test (Sass et al. , 1991; O’Rourke et al. , 1993; Pauli et al. , 2006). However, no clear picture emerged regarding the eventual superior role of specific subfields of the cornu ammonis (CA) or the dentate gyrus for episodic memory processes. Six of 10 relevant studies (60%) found significant correlations between presurgical memory performance and cell densities within the subfields CA3 and CA4, followed by CA1 (50%), dentate gyrus (40%) and CA2 (30%). Sample characteristics (often small, heterogeneous, and in some cases including both mesial and lateral TLE) and the use of different memory tests may well have contributed to this heterogeneity. In a recent study of 104 patients with mesial TLE only, we analysed the role of segmental hippocampal cell loss and its underlying factor structure with regard to presurgical memory performance (Witt …