Mnemonic discrimination in patients with unilateral mesial temporal lobe epilepsy relates to similarity and number of events stored in memory

Mesial temporal lobe epilepsy (mTLE) is a neurological disorder associated with histopathological changes in different subfields of the hippocampus. These alterations have been associated with memory difficulties. In this study, we tested the hypothesis that these difficulties stem on mnemonic discrimination impairment due to a reduced ability to make similar representations more distinct, leading to an increased susceptibility to interference. With this aim, we used a visual mnemonic discrimination task and evaluated the ability of a group of patients with unilateral mTLE, relative to controls, to discriminate between a studied item and a new foil item, as a function of the similarity between them, and of the number of exemplars from a category stored in memory. We found that patients performed worse than controls when the studied item had to be discriminated from a physically similar new object from the same basic-level category. Crucially, reliable differences between groups were observable in the conditions in which more exemplars from a category were held in memory. In the conditions in which the studied item had to be discriminated from a foil from a different basic-level category, there were no differences between groups, with one exception. Neither a general cognitive impairment nor a general memory impairment could account for this pattern of results. Current findings indicate that patients found more difficulties in conditions with higher interference, which poses greater demands for pattern separation. A disruption of pattern separation processes resulting from hippocampal damage provides a reasonable interpretation for these results. Future studies should explore the causal relationship between hippocampal subfields integrity and mnemonic discrimination capacity in mTLE patients.