Visual Processing: Subcortical Mechanisms for Gaze Control

Saccadic Gaze Shifts

Gaze can be redirected by moving the eyes alone at very high speeds (up to 900° per second [°/s] in humans). These movements are called saccades, and they can be made either involuntarily (e.g., to a sudden movement or noise) or voluntarily (e.g., when visually scanning the environment, locating the source of a sound, or reading). Even when we think we are staring directly at an object, the eyes constantly make extremely tiny saccades (as if the eye is jiggling) to prevent the image from fading and to improve visual acuity. Many scientists think that saccades are guided by feedback mechanisms that compare current eye position (i.e., where am I looking now?) and desired eye position (i.e., where do I want to look?). For gaze shifts larger than 150°/s (e.g., while watching a tennis match), gaze is shifted through a coordinated combination of eye and head movements. The eyes, which are lighter and have less inertia, begin to move toward the new visual target first, followed by the heavier head. The head's relative contribution to the overall gaze shift depends on various parameters, including the oculomotor range of the animal (e.g., cats have a small oculomotor range of ±250, and thus move their heads much more than humans, who have an oculomotor range of ±500), and the position of the eyes in the orbits at the start of the gaze shift (i.e., if the eyes are already pointing in the direction of the target, then the head will contribute more). Once the eyes reach the target, the vestibulo-ocular reflex (VOR) [Page 1105]is engaged. The VOR causes the eyes to rotate by the same amount as the head, but in the opposite direction, and thus serves to keep the eyes on target while the head catches up. Eye plus head gaze shifts typically end with the eyes nearly centered in the orbits.

Thus, a combined eye plus head gaze shift can be broken down into three components: (1) eye-in-head: the eye movement relative to the head, (2) head-in-space: the head movement relative to space, and (3) eye-in-space: the eye movement relative to space (i.e., gaze, which is the result of 1 + 2). These components are present for all three directions of motion that the eye and head are capable of moving in: (a) horizontal movements cause gaze to be shifted right and left, (b) vertical movements cause gaze to be shifted up and down, and (c) torsional movements cause gaze to be shifted clockwise and counterclockwise around the line of sight (e.g., bringing the right ear down toward the right shoulder involves a clockwise rotation of the head). Horizontal and vertical movements are important for determining gaze direction, whereas torsion is important for eye and head coordination and binocular perception, and is usually reset to zero at the end of each gaze shift (Listing's law). The brain stem mechanisms associated with gaze shifts carefully monitor and control all three directions of motion (a-c) for all three components of gaze (1–3).

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