Mapping memories in the medial temporal lobe: contributions from single-neuron recordings in patients with epilepsy

Abstract

Abstract The medial temporal lobe (MTL) plays a central role in declarative memory. While the cytoarchitecture of the hippocampus, in particular, dominates neuroanatomical memory models, the manner by which neuronal activity in this area enables the permanent storage of declarative memories remains elusive. Patients with pharmacologically intractable epilepsy, who are candidates for resective surgery and whose seizure focus must be located via the implantation of intracranial electrodes, represent the only opportunity that scientists currently have to record the firing patterns of single neurons in the human hippocampus in vivo. Such direct recordings have shown that cells in the hippocampus appear to support declarative learning by distinguishing between novel and familiar stimuli via changes in firing patterns, using both excitation and inhibition to signal familiarity. Some cells with highly selective excitatory responses have also been described, and these responses seem to represent abstract concepts such as identity, rather than superficial perceptual features of items. New data show that selective and globally responsive cells behave differently depending on the conscious demands of the task. Cells in the MTL can be plastic or stable in terms of the information that they code: while some cells show highly-selective and reproducible excitatory responses when presented with a familiar object, other cells change their receptive fields in line with changes in experience and the cognitive environment. These findings present challenges for current network models of memory and inform our understanding of the neural basis of declarative memory.