116 — The cognitive map in humans: spatial navigation and beyond

Epstein et al (10.1038/nn.4656)

Read on 14 December 2017
#neuroscience  #spatial-reasoning  #cognitive-map  #grid-cells  #fMRI  #mapping  #navigation  #hippocampus  #entorhinal-cortex  #group:review 

The “cognitive map” model of the brain hypothesizes that the brain draws conclusions about its location and orientation in space based upon a hierarchy of cells in the region of the hippocampus — namely, place cells, grid cells, and several other cell types that encode head location, proprioception, and orientation. Rather than simply moving in direct response to the location of stimuli, 1940s research showed that rodents appeared to make educated guesses about their environment based on internal, mental maps. This hypothesis is supported by years of electrophysiological studies, and this review explores recent functional research that even further supports the cognitive map hypothesis.

Through electrophysiology recordings, researchers discovered that neurons in the medial entorhinal cortex (dubbed grid cells) fire in response to where you are along an imaginary hexagonal grid across the (real-world) floor; other cells (named head direction cells) changed their firing patterns as a function of head orientation over the navigational plane (i.e when turning your head left and right). These cells, along with a multitude of others, virtualize a location in space and provide tremendous context for navigation.

In recent studies, fMRI revealed periodic activity in the regions of (and surrounding) human hippocampus when the subject navigated a 3D game space. The period of these responses aligned to 60º of virtual rotation (a function of the response to 0º looked similar to that at 60, 120…and 300º), which supports the hexagonal-representation claim made by ephys recordings.

Further studies, including intracranial recording in epilepsy patients (who often participate in these studies because an invasive surgery is already scheduled), showed evidence of grid cells when subjects played a driving game in which they navigated a closed circuit track.

These studies are interesting to me because they show the extent to which brain areas (such as hippocampus) which are so often associated with memory also play such an important, tangible role in spatial reasoning and navigation. These systems are not unique to humans but exist in mammals and (possibly??) some non-mammals as well. Do spatially “embedded” memories exist in invertebrates as well? Do Drosophila or octopus navigation systems work the same way?

I also think it’s very interesting that these circuits all appear to be hexagonal. There must be something fundamentally efficient about hexagons in the brain.