Exploring the brain's representation of three-dimensional
Doctoral thesis, UCL (University College London).
Previous research has shown that in two‐dimensional environments, ‘place cells’ and ‘grid cells’ in the hippocampal formation comprise two units of O’Keefe and Nadel’s (1978) ‘cognitive map’, coding for the rat’s present location and for metric distance information, respectively. Since the world is three‐dimensional, an important question is whether the cognitive map is also volumetric. To explore this issue, place and grid cells were recorded from rats as they shuttled up and down between two ends of a spiral staircase (the ‘helical track’), allowing the same horizontal locations to be sampled at different vertical levels. Using this novel paradigm, it was possible to investigate whether place and grid cell receptive fields are globular or planar. The first experiment demonstrated that place fields extended in the vertical, as well as horizontal, dimension, suggesting that they were globular. However, the vertical extent was larger than the horizontal extent, suggesting either a coarser representation of height, or contextual modulation of fields in the vertical dimension, in the absence of metric vertical distance. Both possibilities imply that the cognitive map is anisotropic (not uniform in all dimensions). The second set of experiments involved probe trials that showed that both distal and local cues influence place fields in the vertical dimension. The third experiment demonstrated that grid cells produced several subfields on the helical track that, similar to the place fields, were vertically elongated. However, they were more elongated than place fields, and showed no vertical periodicity, suggesting the lack of metric vertical information. Overall, these observations suggest that three‐dimensional space is anisotropically represented in the rat brain as a contextually modulated flat map, in which only the current navigation plane is metrically represented. Due to the complexity of a truly three‐dimensional representation, an anisotropic representation is likely in all surface‐navigating animals including humans.
|Title:||Exploring the brain's representation of three-dimensional space|
|Additional information:||Permission for digitisation not received|
|UCL classification:||UCL > School of Life and Medical Sciences > Faculty of Brain Sciences > Psychology and Language Sciences (Division of) > Experimental Psychology|
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