For the past decade, there has been an ongoing debate as to whether 3-D eye movements are implemented neurally by brain stem circuits,
23 or mechanically by the positioning of orbital pulleys.
31 32 33 34 For example, during saccades and smooth pursuit, when eye movements are made from an eccentric initial eye position, the eye velocity vectors do not remain confined to Listing’s plane, but deviate in the same direction as gaze by approximately half as much (Listing’s half-angle rule).
2 23 By what mechanism are the eye velocity vectors confined to these different planes? Do the burst neurons send different phasic commands depending on the initial eye position, as suggested by Tweed and Vilis,
23 or do burst neurons send the
same command regardless of initial eye position, and the eye rotates about different axes because of the mechanical properties of the eye muscles and other orbital tissues? This latter theory, called the
linear plant model, was first proposed by Tweed et al.
35 Soon after, a more specific mechanism was proposed
34 36 : mobile, connective-tissue sleeves in the orbit, called “pulleys,” influence the pulling directions of the extraocular muscles so that their pulling axes change when the eye moves. Refined over the years, this idea became known as the
active pulley hypothesis,
34 which states that contraction of muscle fibers in the global layer of extraocular muscles rotates the eyeball, whereas contraction of fibers in the orbital layer moves the pulleys.