We presented you with an intro in Part 1, and this classic textbook ranks right up there with Kandel’s Principles of Neural Science, Campos & von Noorden’s Binocular Vision, Griffin/Grisham/Borsting’s Binocular Anomalies, Scheiman & Wick’s Binocular Vision, and Adler’s Physiology of the Eye that have multiple editions that just keep getting better. The 5th edition of Leigh & Zee is in excess of a thousand pages, and Chapter 3 on the Vestibular-Optokinetic System is a doozy at 90 pages in length and over 1200 references! So let’s delve into it a bit further, shall we?
Here’s the primer on the vestibular system from Kandel’s Principles of Neural Science. Leigh & Zee provide a nice complement to that in Chapter 3 in noting that there are three basic types of VOR:
- r-VOR: rotational, in response to the angular component of head motion.
- t-VOR: translational, in response to the linear component of head motion.
- s-VOR: static ocular counterroll or torsion in the direction opposite to a sustained lateral head tilt (“Doll’s Eye”).
On p. 75, Leigh & Zee diagram otolith pathways for head translation and static head tilt, with the medial portion of the utricle activated by head tilt and neurologically linked to the vertical torsional eye muscles (vertical recti and obliques), while the lateral portion of the utricle is activated by head translation and is neurologically linked to the horizontal recti. The latter is believed to occur via polysynaptic connections through the cerebellum to the lateral and medial recti.
From a clinical standpoint, response of the VOR as well as the COR (cervico-ocular reflex) plays an increasingly important part in head movements coupled with eye movements, as occurs more frequently in patients with ocular motor imbalances head involving even subtle head and neck tilts and postural skews, as we have discussed related to paretic strabismus and torticolis. Leigh & Zee point out (p. 95) that the COR can be actively trained in labryrinthine-deficient patients using plus and minus lenses in a similar way as adaptation of the gain of the r-VOR. Adaptive changes in the COR can be induced in normal subjects, with the mental set and perception of the subject and the context in which the neck afferents are being stimulated playing a role. Furthermore (p. 59), the latency of the t-VOR may be influenced by the perceived distance and location of the target of interest.
If the VOR is impacted by disease or trauma, then vision during motion and especially walking is impaired. This has been reported with long-term use of antibiotics, as occurs with the ototoxic effects of streptomycin. Leigh & Zee (p.60) cite the case of a physician suffering ototoxicity and labyrinthine deficiency who, when walking in the street could not recognize faces or read signs unless he stood still. Labyrinthitis may contribute to the signals between the ears, eyes, muscles and joints are no longer being in sync.
In other instances the opposite problem occurs, in which the VOR is inadequately inhibited. As Leight & Zee note (p. 61) VOR suppression, or cancellation of the VOR refers to modulation of VOR responses during combined eye-head tracking, when the object of interest is not stationary but moves in the direction of the head. In contrast to problems with reading signs while walking, this would impair the ability to following the movement of other pedestrians or cars while moving, making spatial judgments while bicycling, or navigating busy places like malls or supermarkets.
With this neurology primer in mind, I’ll suggest that you re-visit the eight part series we did in 2014 on “Eco-Optics”. Please play particular attention to the applications for vision therapy, and to the comments at the end of each blog:
Eco-Optics Part 1: Introducing the Gibsons
Eco-Optics Part 2: Ecological Perception
Eco-Optics Part 3: Optic Flow
Eco-Optics Part 4: The t-VOR
Eco-Optics Part 5: The OFR
Eco-Optics Part 6: Slotnick Dynamic Therapy
Eco-Optics Part 7: Sunbeck’s Infinity Walk
Eco-Optics Part 8: Looking and Seeing