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Head direction (HD) cells are neurons present in the brains of many mammals, which increase their firing rates above baseline levels only when the animal's head points in a specific direction. When stimulated, these neurons fire at a steady rate (i.e.—they do not show adaptation), but decrease back to their baseline rates as the animal's head turns away from the preferred direction (usually 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, striatum and entorhinal cortex (Sargolini ''et al.'', Science, 2006). 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. HD cells continue to fire in an organized manner during sleep, exactly as if animals were awake. However, instead of always pointing toward the same direction - the animals are asleep and thus immobile - the neuronal "needle" moves constantly. In particular, during Rapid Eye Movement Sleep, a brain state rich in dreaming activity in humans and whose electrical activity is virtually indistinguishable from the waking brain, this needle moves exactly as if the animal was awake. HD neurons are sequentially activated and the individual neurons representing a common direction during wake are still active, or silent, at the same time. == 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「head direction cells」の詳細全文を読む スポンサード リンク
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