Abstract
Besides decades of research showing the role of the medial temporal lobe (MTL) in memory and the encoding of associations, the neural substrates underlying these functions remain unknown. We identified single neurons in the human MTL that responded to multiple and, in most cases, associated stimuli. We observed that most of these neurons exhibit no differences in their spike and local field potential (LFP) activity associated with the individual response-eliciting stimuli. In addition, LFP responses in the theta band preceded single neuron responses by ~70 ms, with the single trial phase providing fine tuning of the spike response onset. We postulate that the finding of similar neuronal responses to associated items provides a simple and flexible way of encoding memories in the human MTL, increasing the effective capacity for memory storage and successful retrieval.
Highlights
Besides decades of research showing the role of the medial temporal lobe (MTL) in memory and the encoding of associations, the neural substrates underlying these functions remain unknown
We recorded single neuron and local field potential (LFP) activity during 21 sessions in 6 patients with pharmacologically intractable epilepsy, who were implanted with intracranial electrodes for clinical reasons
There was a significant correlation between the association score and the normalized difference in the response strength (Fig. 6b, r = −0.2, p = 1.9 × 10−2), spike (Fig. 6c, r = −0.28, p ~ 10−4), and LFP latency differences (Fig. 6d, r = −0.22, p = 1.4 × 10−2). We explored if these correlations were present for the response and the other did not (R–NR) pairs when looking at the difference in strength and LFP latency
Summary
Besides decades of research showing the role of the medial temporal lobe (MTL) in memory and the encoding of associations, the neural substrates underlying these functions remain unknown. Our results (from the follow-up sessions) show a strong relationship between the spike and LFP responses, with the latter consistently preceding the former, and that most MTL neurons responding to more than one stimulus exhibit “neural unitization”—i.e., they respond to the different stimuli eliciting significant responses, or in other words, if a neuron fires to more than one stimulus, the responses to these stimuli are indistinguishable from each other We postulate that such “unitized” coding is the basis for encoding long-term associations in the human MTL, and is crucial to understand the mechanisms that underlie memory coding and its capacity in the human brain
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