Abstract and Keywords
In laboratory conditions, with the head restrained and held upright, eye-in-head orientation vectors are constrained to a tilted two-dimensional (2D) range called Listing’s plane. However, in most real-world conditions gaze control utilizes a three-dimensional (3D) range. For example, when the head is allowed to move naturally, the accompanying saccades and vestibulo-ocular reflex movements include coordinated torsional components, out of, and then back into Listing’s plane. The head itself rotates more like a set of Fick gimbals, resulting in a non-planar range of orientation vectors. To control this complex behaviour, the brainstem reticular formation appears to have struck upon an elegant solution: it encodes the 3D components of posture and movement in coordinates that align with the Listing and Fick behavioural constraints, such that its control signals collapse to 2D (zero torsion) when these constraints are upheld, but it retains the capacity for torsional control whenever required. In contrast, the superior colliculus and cortex appear to only encode 2D gaze direction. Surprisingly, after many years of research on this topic, we still know very little—other than a few clues—about the neural mechanisms that transform high-level 2D gaze direction commands into the 3D control signals for eye and head orientation.
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