Metric and non-metric inputs influence spatial and working memory processes of medial entorhinal neurons
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The medial entorhinal cortex (MEC) contains spatial cell types including grid and head direction (HD) cells. Grid cells fire action potentials when animals pass through environmental locations that form vertices of tessellating triangles. HD cells fire when animals face a preferred direction along their azimuth. These cells have been widely studied for their potential metric role in spatial navigation, but considerably less is known about their non-metric functions. This thesis examines non-metric influences on the MEC during working memory maintenance, 'look-ahead' activity, and 'more familiar' or 'less familiar' environmental rotations. The first experiment tests the hypothesis that persistent spiking of MEC neurons could represent a sensory cue during a working memory task. Animals ran a T-Maze where an auditory stimulus cued rats to move toward left- or right-reward arms. Instead of the hypothesized increase in spike rate during the delay period between cue and reward, MEC spike rates were suppressed. Additionally, MEC ensemble firing at the choice point suggests that these cells encoded reward locations. This indicates the MEC displays forward activation of possible future locations ('look-ahead' activity). To model this experiment's look-ahead results, a recent model of goal-directed navigation was adapted to the previous T-Maze task. This adaptation trains a virtual rat to associate cues to reward cells and corresponding place cells, a difference from previous models where goal locations were not cueassociated. The rat reliably learns goal locations, performs look-ahead scans at the choice point, and simulated MEC activity decodes to reward locations, successfully modeling look-ahead behavior. The final experiment examines effects of environmental recency on spatial tuning of MEC neurons. Rats performed spatial alternation on a T-Maze rotated into 'more familiar' or 'less familiar' configurations as MEC units were recorded. Spatial cells oriented their firing fields in register with the T-Maze more often during less familiar rotations. This implies a shift in animals' reference frame with learned experience suggesting the MEC is comparing contexts in the same environment. In summary, these results highlight previously uninvestigated, non-metric influences over MEC activity with strong implications for goal-directed behavior and spatial navigation.
Thesis (Ph.D.)--Boston University