Head direction cells

Many mammals possess head direction (HD) cells: Neurons that are active only when the animal's head points in a specific direction within an environment. These neurons fire at a steady rate (i.e. they do not show adaptation), but show a decrease in firing rate down to a low baseline rate as the animal's head turns away from the preferred direction (usually returning to baseline when facing about 45° away from this direction).

These cells are found in many brain areas, including the post-subiculum, retrosplenial cortex, the thalamus (the anterior and the lateral dorsal thalamic nuclei), lateral mammillary nucleus, dorsal tegmental nucleus, and striatum.

The system is related to the place cell system, which is mostly orientation-invariant and location-specific, while HD cells are mostly orientation-specific and location-invariant. However, HD cells do not require a functional hippocampus, where strong place cells are found, to show their head direction specificity. Head direction cells are not sensitive to geomagnetic fields (i.e. they are not "magnetic compass" cells), and are neither purely driven by nor are independent of sensory input. They strongly depend on the vestibular system, and the firing is independent of the position of the animal's body relative to its head.

Some HD cells exhibit anticipatory behaviour: the best match between HD activity and the animal's actual head direction has been found to be up to 95 ms in future. That is, activity of head direction cells predicts, 95 ms in advance, what the animal's head direction will be.

In familiar environments, HD cells show consistent preferred directions across time as long as there is a polarizing cue of some sort that allows directions to be identified (in a cylinder with unmarked walls and no cues in the distance, preferred directions may drift over time).

For a review on the HD system and place field system, see Muller (1996): “A quarter of a Century of Place Cells”, Sharp et al. (2001): “The anatomical and computational basis of rat HD signal”