HD-Coding
Knowing one's facing direction in the environment is fundamental for accurate navigation. In the rodent, a network of head direction (HD) cells code for this information by showing increased firing rate when the animal faces the cell's preferred orientation. This firing, however, rapidly diminishes as the animal turns away from the cell's preferred direction. With a population of cells coding for different orientations, the net result is akin to a neural compass that provides the animal with a constant directional signal in the environment.
Figure 1. Head direction cells in the rodent. Left: The schematic of a circular environment presented from the top-down, containing a mouse facing in four different directions. Right: The corresponding firing rate of the cell. Here we can see that as the animal is facing 0 degrees, the blue cell is firing maximally, but as the animal turns away from 0 towards 90 degrees, the blue cell ceases firing and the purple cell starts to fire maximally. A network of these cells provides a signal akin to a neural compass indicating the animal’s direction in the environment.
To study this crucial spatial signal in humans, the Wolbers’ lab has used immersive virtual reality (VR) in combination with functional magnetic resonance imaging (fMRI). Shine, Valdés-Herrera, Hegarty and Wolbers (2016) used repetition suppression to identify brain regions sensitive to repeats of the same HD after participants had learned a virtual environment with access to both body-based and visual cues (Figure 2). Consistent with the rodent literature, Shine et al. (2016) found significantly reduced activity in the retrosplenial cortex (RSC), thalamus and precuneus (Figure 3). These data suggest that, by providing participants with full access to both body-based and visual cues via immersive VR and using repetition suppression, fMRI is a viable tool to study the neural correlates of HD coding in humans.
Figure 2. Outside of the scanner participants wore a head mounted display as they learned a virtual environment comprising four galleries (left column), surrounded by four global landmarks (middle). Each gallery contained four unique images (right column) that the participant had to learn in association with a global head direction.
Figure 3. Using repetition suppression, Shine et al. (2016) found regions in the human brain sensitive to head direction coding.
Since August 2019, Postdoctoral researcher Dr Jonathan Shine and PhD student Matthieu Bernard have been working on a grant provided by the European Fund for Regional Development (EFRE), to uncover the basic mechanisms of the HD signal in humans and how they change across the lifespan, using combination of cutting-edge immersive VR and high-resolution functional imaging.
