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Functional neurons in the medial temporal lobe

functional-neurons-in-the-medial-temporal-lobe

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Most recordings in rats hippocampus, in humans _ hippocampus.

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Place cells

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Boundary cells

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Head direction cells

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O'keefe and Dostrovsky (1971)
Solstad et al (2008)
Lever et al (2009)
Taube et al (1990)
Hafting et al (2005)
Quiroga et al (2005)

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Abstract: study of the behavioral and spatial firing correlates of neurons in the rat postsubiculum. Recordings made from post-subicular neurons as rats moved freely throughout a cylindrical chamber, where the major cue for orientation was a white card taped to the inside wall. Approximately 26% of the cells were classified as head direction cells because they discharged as a function of the animal’s head direction in the horizontal plane, independent of the animal’s behavior, location, or trunk position. For each head-direction cell, vectors drawn in the direction of maximal firing were parallel throughout the recording chamber and did not converge toward a single point. Each firing-rate/head direction function was adequately described by a triangular function. Each cell’s maximum firing rate occurred at only one (the preferred) head direction; firing rates at head directions on either side of the preferred direction decreased linearly. Results from 24 head-direction cells in 7 animals showed an equal distribution of preferred firing directions over a 360 angle. Quantitative analysis showed the location of the animal within the cylinder had minimal effect on directional cell firing. For each head-direction cell, the preferred direction, peak firing rate, and directional firing range remained stable for days. These results identify a new cell type that signals the animals head direction in its environment.
•Intro: Navigation in mammals depends on a distributed and organised brain network. This network represents the position and direction of information indicated by the existence of ‘place cells’ and ‘head-direction cells’. dMEC (dorsocaudal medial entorhinal cortex) •Aim: dMEC had a map-like structural organisation by recording the spike activity in this area from rats in different enclosures. •Method: 14 rats, electrode implants. 6 rats tested on circle enclosures, 8 tested on squares. Recorded position and data. •Results: Found a geometric structure of firing fields in dMEC neurons. Grid cells show topographical organisation in dMEC. These cells share a number of properties including spacing, orientation and field size. It was found that at different locations from the postrhinal border, the fields changed in density and field size. •Discussion: The dorsocaudal medial entorhinal cortex (dMEC) contains a directionally oriented, topographically organized neural map of the spatial environment. Its basic unit is the ‘grid cell’, which is activated whenever the animal’s position coincides with any vertex of a regular grid of equilateral triangles spanning the surface of the environment.
•Intro: Computational models posit the existence of cortical “boundary vector cells,” whose activity patterns encode the animal’s distance from salient geometric borders. Aim: whether proximity to borders is represented by specific cell types in the entorhinal spatial representation circuit. •Method: A total of 624 principal cells were recorded from the dorsocaudal quarter of the medial entorhinal cortex (MEC) and adjacent parasubiculum in 13 rats. Animals foraged in various enclosures (square, circle, high walls) with movable walls and barriers. •Results: Cells recorded inc place and grid cells. Also unknown entorhinal cells that fired along one or several walls of the enclosure. Border cells context independent (no wall, extra wall, lower walls, longer walls). •Discussion: The persistence of border-related activity across environments, under conditions that often lead to realignment in grid cells and remapping in place cells, suggests that the firing of these cells is primarily defined by geometric borders and less by the content of the environment or the training history of the animal.
•Intro: Sec to rec place or objects. Unclear how. Previous research cells in MTL fire to particular stimuli. Can MTL neurons represent high‐level information in an abstract manner characterised by invariance to the metric characteristics of the images? •Method: 8 epileptic patients. Implanted with electrodes. 21 sessions. Images for 1 sec, 6 times. Randomized order. Images: famous and unknown individuals, animals and objects. In 18/21 sessions, letter strings with names of individuals/objects presented. Attention task. Analysed responses from hippocampus, amygdala, entorhinal cortex and parahippocampal gyrus. •Results: Responses showed to: particular individuals, landmark buildings, animals or objects (jennifer aniston, halle berry, sydney opera house, inc string). Responsive areas: hippocampus, parahippocampal gyrus, amygdala, and entorhinal cortex. •Discussion: These cells selectively respond to a diversity of distinct images of a single individual. High‐level visual responses in medial temporal lobe structures → involved in long‐ term memory and consolidation. Neurons in the MTL might have a fundamental role in learning associations between abstract representations.
•Method: We recorded from six rats with tetrodes located in the dorsal subiculum. Tested in different environments. •Results: The BVC (boundary vector cell) model. A BVC responds maximally when a boundary is perceived at a preferred distance and allocentric direction from the animal, regardless of the animal’s heading direction. Thirty-six dorsal subicular pyramidal cells had spatial firing characteristics consistent with those predicted for BVCs. Twentyfour of these cells were recorded in several environments differing in boundary geometry and other non-spatial aspects of the recording context. •Discussion: The main determinant of BVC firing is the vector from the rat to the boundary regardless of the color, material, or shape of the boundary. BVC fire in wall-less enviro, similar to wall enviro. Boundaries may be defined by sensory cues and limitations in movement. the difference between environment a vs environment b produced strong remapping in CA1 PCs without disrupting BVC firing. Environmental boundaries are also important determinants of firing in entorhinal cortex, and cells there are also less likely to remap than hippocampal PCs.
Measured rats behaviour. Electrodes fixed to the freely moving rat. Rats with hippocampal damage hyperactive in novel enviro, resistant to change or learnt bhvr, poor at spatial tasks. Findings suggest that the hippocampus provides the rest of the brain with a spatial reference map. The activity of cells in such a map would specify the direction in which the rat was pointing relative to environmental landmarks and the occurrence of particular tactile, visual etc stimuli whilst facing in that orientation. Deprived of this map, the rat would not immediately notice environmental changes, be they incidental ones such as floor texture or more important ones such as reinforcement contingencies, furthermore, it could not learnt to go from where it happened to be in the environment to a particular place independently of any particular route, but would be forced to rely on other inherently less flexible strategies to find its way. We suggest that it is the loss this spatial reference map which results in all or most of the behavioural deficits reported for hippocampectomized rats.

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