Naso-temporal Asymmetry of OKN (Motion Perception) / Reversing Grating Test

Ai-Hou Wang, M.D., Ph.D.

Monocular optokinetic nystagmus (OKN) in infants and young children is nasal-temporally asymmetric. Images moving from temporal to nasal side induce a temporal beating nystagmus; however, images moving from nasal to temporal side do not induce a nasal beating nystagmus. In other words, the right eye prefers leftward-moving images, producing a rightward-beating nystagmus, but not a leftward-beating nystagmus for rightward-moving images. The left eye, in contrast, prefers rightward-moving images, producing a leftward-beating nystagmus, but not a rightward-beating nystagmus for leftward-moving images.

The right eye prefers leftward-moving images.

The left eye prefers scenes moving to the right

In individuals without strabismus, this nasal-temporal asymmetry gradually develops into nasal-temporal symmetric OKN between three and seven or eight months of age. In patients with congenital esotropia, this nasal-temporal asymmetry persists and does not develop into nasal-temporal symmetry.

This preference for the temporal beating nystagmus is identical with the direction of latent nystagmus (LN), a characteristic of early-onset strabismus. Furthermore, the timeline for the OKN development from nasal-temporal asymmetry into symmetry coincides with the development of stereopsis. Furthermore, OKN is also nasal-temporal asymmetric in animals with bilaterally positioned eyes that lack binocular vision. All of this suggests a close correlation between nasal-temporal symmetry in OKN and binocular vision. Tychsen L of the University of Washington has provided a detailed explanation in the binocular vision chapter of previous editions of Adler’s physiology of the eye.

Tychsen L. Binocular vision. In: Hart WM, editor. Adler's Physiology of the Eye: Clinical Applications, 9th ed. St. Louis: Mosby; 1992: 773–853.

Nasal-temporal asymmetry of OKN is a characteristic sign of early-onset strabismus. In patients with anisometropic amblyopia, even if one eye has severe amblyopia and lacks stereopsis, OKN is still nasal-temporal symmetric. Nasal-temporal symmetry of OKN can therefore help differentiate between strabismic amblyopia and anisometropic amblyopia.

We developed a Reversing Grating Test on personal computer screen to test the nasal-temporal symmetry of OKN.

On a computer, the moving stripe pattern used in OKN testing involves looping four stripe patterns with 90° phase shifts (see image). If played from top to bottom, the stripes move to the left; if played from bottom to top, they move to the right.

Reversing grating is a dynamic grating, played back and forth repeatedly between two grating patterns of opposite black and white colors. Unlike OKN, where four grating patterns 90° out of phase are played consecutively, reversing gratings can be thought of as two grating patterns 180° out of phase played back and forth repeatedly (see image).

Each reversal can be viewed as a stripe-width moving either to the right or to the left. The dynamic image of reversing grating is physically left and right symmetrical, or in other words, the weight of leftward and rightward movements is equal. When the reversing grating is played to a normal person with symmetrical nasal/temporal OKN, it appears as flickering stripes. However, when viewed by individuals of asymmetric nasal-temporal OKN, it appears similar to OKN stripes moving to the left when viewed with right eye, while appears as OKN stripes moving to the right when viewed with left eye. And at the same time OKN will be evoked.

It is the same image of a reversing grating, when viewed with the right eye, it produces a rightward beating nystagmus; when viewed with the left eye, produces a leftward beating nystagmus – identical with the direction of latent nystagmus!

The moving stripes of OKN can be physically described by a wave equation of brightness, while the reversing grating can be described as a standing wave. The reversing grating can be thought of as the sum of two wave equations of identical wavelength, one moving to the left and one moving to the right, with the same speed (see figure).

Reversing Grating OKN to L OKN to R

If a normal person with symmetrical nasal-temporal OKN looks at a reversal grating without moving his/her eyes, they see flickering stripes. However, if he/she gazes following a pen tip that moves left and right across the screen, he/she will see leftward-moving stripes when gazing leftward, and see rightward-moving stripes when gazing rightward.

Since a reversing grating can be viewed as the sum of equal weighted rightward moving OKN stripe and leftward moving OKN stripe, a leftward shifting gaze increases the weight of the leftward-moving stripe in the sum, resulting in the perception of a leftward-moving OKN stripe. Conversely, a rightward shifting gaze increases the weight of the rightward-moving stripe in the sum, resulting in the perception of a rightward-moving OKN stripe.

Look at the moving red dot over the reversing grating.

When the dot goes leftward the stripes look like leftward moving OKN stripes;

when the dot goes rightward the stripes look like rightward moving OKN stripes.

Individuals with nasal-temporal asymmetry of OKN have a physiological preference for leftward-moving stripes in the right eye and rightward-moving stripes in the left eye. Without external induction of leftward or rightward gaze shifts, the right eye naturally perceives a reversing grating as a leftward-moving stripe, which induces leftward pursuit. The leftward gaze shift mentioned above then increases the weight of the leftward-moving stripe within the total reversing grating, further strengthening the tendency to perceive the reversing grating as leftward-moving. This creates a positive feedback loop, amplifying the latent nystagmus that typically occurs only when one eye is covered, in early-onset strabismus. Viewing the same reversing grating, right eye viewing manifests rightward beating nystagmus, while left eye viewing manifests leftward beating nystagmus. Clinically, reversing grating test can be used to diagnose the early-onset nature of strabismus.

Interestingly, the reversing grating test, or the test for nasal-temporal symmetry in OKN, is primarily a monocular test, but its interpretation is often associated with binocular vision. A person with esotropia cannot fuse and does not pass stereopsis tests even after surgical correction to beautiful alignment, reversing grating test can be performed on either single eye. If the OKN is symmetrical nasal-temporally, it suggests late-onset strabismus, and stereopsis may be possible to recover later-on. If the OKN is asymmetrical nasal-temporally, it suggests early-onset strabismus, and stereopsis is unlikely to be restored.

For infantile esotropia (congenital esotropia), the general consensus is to correct the eye alignment surgically before the age of two years. More proactive physicians may even perform surgery as early as four months of age. The goal of early surgery is to remove binocular suppression and restore binocular vision. But what aspects of binocular vision are being restored?

(1) Random-dot stereopsis seems impossible to recover;

(2) Peripheral fusion in monofixation syndrome may be restored;

(3) Jampolsky A, Norcia AM, and Tychsen L tested the symmetry of nasal-temporalward motion perception with visual evoked potential (VEP) and found that it can be restored through early surgery.

Norcia AM, Hamer RD, Jampolsky A, Orel-Bixler D. Plasticity of human motion processing mechanisms following surgery for infantile esotropia. Vision Res. 1995 Dec;35(23-24):3279-96.

Tychsen L, Wong AMF, Foeller P, Bradley D. Early Versus Delayed Repair of Infantile Strabismus in Macaque Monkeys: II. Effects on Motion Visually Evoked Responses. Invest Ophthal & Vis Sci March 2004, Vol.45, 821-827.