Vestibular Influences
The HD compass is inertial: it continues to operate even in the absence of light. Experiments have shown that the inertial properties are dependent on the vestibular system, especially the semicircular canals of the inner ear, which respond to rotations of the head. The HD system integrates the vestibular output to maintain a signal of cumulative rotation. The integration is less than perfect, though, especially for slow head rotations. If an animal is placed on an isolated platform and slowly rotated in the dark, the alignment of the HD system usually shifts a little bit for each rotation. If an animal explores a dark environment with no directional cues, the HD alignment tends to drift slowly and randomly over time.
Read more about this topic: Head Direction Cells
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