Churchland lab

Being able to navigate in the world requires a stable representation of space. A key part of the neural substrate supporting this ability is the entorhinal cortex, where individual cells’ responses constitute a grid that tiles the space being explored.

The existence of such cells has been known for a while, and certainly they seem reasonable for the task at hand, but it has been an ongoing challenge to understand what kind of neural machinery would give rise to them. Ila Fiete, from UT Austin, has been tackling this problem from a theoretical point of view. I heard her give a talk at a recent meeting organized by the McKnight Foundation, which funds systems neuroscientists working at the molecular, cellular, systems and theoretical level.

Ila’s idea is that the grid cells reflect a stable 2-dimensional manifold driven by continuous attractors (see left panel of figure, below). The gist is that short-range excitatory and long-range inhibitory connections give rise to stable “bumps” of activity. This kind of mechanism has been put forth previously in the visual and oculomotor systems. Here, Ila proposes that the same continuous attractors might be used by entorhinal cortex, there to drive the individual nodes of activity of the grid cells. This model predicts some specific features of the resulting population: for example, the model predicts that even though the absolute phase of individual neurons might change a bit over time the relative phase of the neurons to each other should be fixed. This prediction is born out by real measurements of grid cells: their phase can change over time, but their relative phase is extremely stable. A second prediction is that tuning curves of all the neurons will be stereotyped, a prediction that is again born out by the data.

A continuous attractor, some grid cells, and some measurements about them

This work presents a challenge to alternative explanations for grid cells, such as that they are driven by oscillations in the cortex. To my mind, a key next step will be to manipulate the circuit, perhaps by suppressing the activity of interneurons which play a key role, and examining the effects on the phase of grid cells.