Quick Summary

• Head and eye movements are not time-locked to each other. The head can start anytime between slightly before the eye starts moving to 200ms after.
• The head is noted to continue moving even when eye → target error reduces close to 0, a delay upwards to 400ms; the SD of this is 80ms.
• Head movements follow an approximate bell-shaped velocity curve, while the eye velocity profile (relative to the head space) is “more ragged” and features greater variation.

Experimental Setup

Subjects performed two different tasks. 1) Randomly sequenced gaze shifts were performed between five targets: one central and four at $70$ or $90\degree$ eccentricity arranged in a square; i.e., in the $\pm45$ and $\pm135\degree$ directions: up and left, up and right, down and right, down and left. Subjects viewed the targets (red dots, 8mm across, at a distance of ~1m) binocularly, so that the vergence angle was nearly constant at ~$3.5\degree$, and made movements between them in response to randomly sequenced verbal commands (up right, down left, center, etc.). 2) Large spontaneous gaze shifts with the head free in which subjects looked randomly between targets in the room. Throughout all experiments, subjects sat in a chair with a firm back support and kept their bodies still.