Disambiguation of Overlapping Experiences by Neurons in the Medial Entorhinal Cortex

Abstract

Hippocampal neuronal activity distinguishes separate events that share common elements. Here, we examined whether the capacity to disambiguate overlapping experiences is an exclusive feature of hippocampal processing or whether information processing one stage earlier in the hippocampal system also disambiguates common elements of distinct experiences. We compared the spatial firing patterns of neurons in the dorsocaudal medial entorhinal cortex (dcMEC) and hippocampal CA1 neurons in animals continuously alternating left-turn and right-turn routes through a T-maze. Neurons in the dcMEC more strongly distinguished left-turn from right-turn trials compared with CA1 neurons, whereas CA1 neurons more selectivity encoded places traversed within each route. These results indicate that dcMEC spatial firing patterns are experience dependent and reflect the mnemonic demands of a spatial memory task. Furthermore, the results suggest that neuronal populations in the dcMEC and CA1 differentially emphasize complementary aspects of spatial memory representations.

Towards a functional organization of the medial temporal lobe memory system: Role of the parahippocampal and medial entorhinal cortical areas

“…Is it possible that the MEC, like the parahippocampal cortex, also plays a broader role in temporal as well as spatial contextual processing? Results of a recent study by the present authors are consistent with this suggestion (Lipton et al, 2007). In this study, our aim was to determine whether MEC neurons, like hippocampal principal cells, distinguish routes traversed as temporally distinct episodes during performance of a T-maze alternation task.…”

“…It should be noted that in another study nearly all hippocampal neurons differentiated left-and right-turn trial types . However, in the current discussed situation in which the differentiation of trial types was weaker in hippocampal neurons (Lipton et al, 2007), MEC neurons showed stronger disambiguation. The combination of these and other analyses showed that both MEC and hippocampal neurons had significant spatial specificity and distinguished leftand right-turn trials.…”

“…Many of these MEC neurons also exhibited differential firing along the central stem of the maze during left-and right-turn trials, similar to hippocampal neurons (Lipton et al, 2007). The patterns of neuronal activity illustrated in Figure 3B represent typical trial-type specific activity exhibited by medial entorhinal neurons, as well as the range of spatial specificity exhibited by these neurons.…”

“…A significant main effect of trial type or a trial type by segment interaction qualified a cell as differentiating left-from rightturn trials . The results of this analysis indicated that 56% of medial entorhinal neurons (23/41 with place fields on the central stem) were significantly more active on either right-or left-turn trials, whereas 33% (16/48) of hippocampal neurons exhibited differential firing on the central stem (Lipton et al, 2007). Our log-likelihood ratio analysis quantified the degree to which firing patterns on left-turn and rightturn trials differed.…”

“…For each cell, log-likelihood ratios were summed over all central stem position bins for each trial. The magnitude of average absolute value of the summed log-likelihood ratio reflects how well the representations are distinguished by temporal context (for a more detailed description, see Lipton et al, 2007). This analysis revealed that MEC neurons more robustly distinguished left-from right-turn trials than did hippocampal neurons, such that the average log-likelihood ratio for medial entorhinal neurons was significantly greater than for hippocampal neurons (MEC, 2.82; Hippocampus, 1.7; Wilcoxon rank-sum test, P < 0.003).…”