This chapter focusing, on head direction cells, began the discussion on how the mammalian processes space and position in it.
Head directions (HD) cells fire most when the animal’s head is pointing in a particular direction with respect to the environment and are modulated by both internal and external input. Internal input, such as vestibular and proprioceptive input, seem to influence how the cell will fire. For instance, once type of HD cell will fire when the head is pointing in the cell’s preferred direction when the head is moving to the left, but not when it is moving to the right.
Further, external stimuli, such as visual cues can influence the signaling behavior of HD cells. One interesting study showed that when a cue external to an eight-arm radial arm maze was rotated along with the location of a reward, HD cells in some animals seem to rotate with the cue which led to proper determination of reward location. In others, however, failure in the HD cells to rotate resulted in an error in pursuing the reward.
HD cells are often coupled in activity with place cells, though the firing of the two cell types can be independently modulated. HD signals seem to couple with theta oscillations to produce the grid cell firing characteristics in the entorhinal cortex, which apparently computes distance of distal landmarks.
It’s interesting just how many nuclei seem to contain HD cells. From the info provided in this chapter it seems these may be the first step the brain takes in understanding where it is, how it got there and how to get to where it wants to go.
Though they did discuss what causes the peak firing rate (preferred direction of the head with respect to cues), they did not seem to mention the dynamics firing over time. The reason I bring this up is because the studies which involved rotating cues, and thus altering spatial orientation made me think about how HD cells may be involved in establishing orientation. Once you drop the animal in an altered environment, what happens in the seconds before the animal knows where it is? Is spatial information needed for spatial activity? For instance, can place cell activity disinhibit HD cells when a cognitive map has been established and allow vestibular input to control firing rate?
Seems I have a lot to learn about navigation and spatial cognition.