Optokinetic nystagmus

1. Optokinetic nystagmus aka “OKN”

2. OKN • another field-holding reflex • supplements the VOR during sustained rotation so that compensatory eye movements can be maintained • smooth pursuit may also contribute to compensation

3. Smooth pursuit vs. OKN • Foveal vision is essential for good smooth pursuit but not for OKN • OKN is dominated by inputs from peripheral retina and can be elicited even in a patient with macular disease if he/she has good peripheral vision

4. Circular vection • A common experience • If we are stationary and a large object moves in our peripheral vision, we feel that we are moving • Due to optokinetic responses in our brains • Visual input to vestibular pathways

5. More prolonged nystagmus in light than in darkness

6. Optokinetic after nystagmus • The optokinetic system exhibits memory or “storage” of recent activity • eye movements continue after visual stimulation stops • There is optokinetic after nystagmus (OKAN) in darkness after a period of optokinetic stimulation

7. OKAN: practical importance • Diseases of the peripheral vestibular system are often characterized by abnormalities of OKAN • Diseases of central vestibular connections may abolish OKAN.

8. Neurons within the vestibular nuclei respond to sustained rotation with an initial increase in discharge rate • declines over ~ 15 sec. • Decline is partially due to the return of the cupula to its resting position • Cupula returns to resting position in ~ 5 sec • central nervous system seems to “remember” and extend the time constant of decay of the reflex

9. VOR during constant velocity turning of patient

10. Sinusoidal rotation is better

11. Removing the fast phases

12. Visual responses in vestibular system • Cells in the vestibular nuclei respond to full field motion • What is typically called “optokinetic stimulation” • Rotation in the light stimulates vestibular neurons via two inputs • vestibular • optokinetic

13. Vestibular neurons • in dark : brief discharge at beginning of movement, decaying over 5-15 sec. • eye movements are briefly compensatory, then fade away • in light : steady discharge during movement • eye movements are maintained through the period of rotation

14. Visual input • Supplements vestibular input by maintaining compensatory eye movements during prolonged rotation in the light • To test vestibular responses in isolation, must test in darkness

15. OKAN • When the lights are turned off after a period of optokinetic stimulation, vestibular neurons continue to discharge for some time • This produces OKAN • In the light, OKAN acts to counter post-rotatory nystagmus of vestibular origin

16. Vestibular input alone (in dark)

17. When do we experience movement? • Whenever the hair cells send new input to the brain • When we start moving • When we stop moving • When inputs from the left and right canals differ • If we have damage to the hair cells on one side • If debris in the canals stimulates the hair cells

18. Visual and vestibular (in light)

19. Visual only (OKN)

20. Binocular vs. monocular input • Binocular OKN should always be symmetrical • Drum movement to left or right should elicit equally vigorous OKN • Monocular OKN may be asymmetrical • In young infants, nasalward movement of stimuli produces better OKN than does temporalward movement of stimuli

21. Nasal bias • OKN to nasally moving stimuli is largely mediated by a direct pathway from the retina to the nucleus of the optic tract (NOT) in the pretectum • OKN to temporally moving stimuli requires participation of cortical circuits as well • Cortical circuits for OKN develop more slowly than do subcortical circuits.