Slow phases are in the direction of visual motion (in the direction of the moving object) and can be horizontal, vertical, or torsional depending on the stimulus1.
The rVOR (rotational or angular vestibulo-ocular reflex) cannot provide continuous stable gaze during persistent rotation, since the semicircular canals detect accleration, not constant velocity. For example, on a spinning amusement park ride, the initial acceleration of the ride is easily detected, but if the ride reaches a constant speed and occupant has their eyes are closed, the sense of motion gradually becomes less and less, and then is lost. Once constant velocity is reached, the endolymph fluid ceases to displace the hair bundles and a motion signal is no longer sent to the brain.
Consquently, in conditions such as running around in a circle or constant rotation in light, the requirement for image stabilization is provided by the visual system and not the vestibular system, by a process which generates optokinetic nystagmus (OKN). With the optokinetic reflex, image velocity signals from retinal ganglion cells are fed back to the eye muscles in order to reduced retinal slip.
The OKN builds up at approximately the same rate as the rVOR decays. The OKN also maintains a stable image of the world on the retina, but rather than using the vestibular system as an input, it uses the motion of the visual image to initiate the eye movements. For example, the OKN is activated when a stationary observer watches a merry-go-round3. OKN function is thus to hold gaze steady on a stationary background, and prevent the wide-field relative motion which would result from the pursuit of a foreground object4.
(vv)ForestOKN.mp4(tt)
The brain normally interprets movement of the entire visual field as evidence for self-motion, as is shown by the sometimes overwhelming motion illusions experienced when watching IMAX movies, or when the train adjacent to an observer starts to move out of the station4 (for a version of this, see Video 3). Note that determing direction of motion is not necessarily straightforward: vection (the sensation of movement produced by visual input) is dictated by the background, as is shown nicely in this little movie:
(vv)Chasing Dots.mp4(tt)
The optokinetic reflex is classically considered to be the reflexive response of the eyes to the motion of a large visual field, which stabilizes the eyes during tracking of a large moving visual scene and matches slow-phase eye velocity to the velocity of the visual surround. The OKN reflex therefore is complementary to the rVOR, but acts at low-frequencies. As a consequence, despite the fact that, during constant-velocity rotation eye velocity will decay to zero, the same rotation in the presence of a surrounding visual field results in a steady-state nystagmus pattern that is sustained indefinitely, for however long it is necessary to compensate for the applied stimulus.
- Circularvection refers to the subject’s compelling sensation of self-rotation induced by a sustained rotating visual stimulus: the smooth pursuit system contributes to the initial ocular following response at the onset of optokinetic nystagmus, but the optokinetic system takes over with a sustained rotating stimulus1; this sense of self-rotation is in the opposite direction to the direction of motion of the large visual scene or of the drum.
- Translational vection refers to the subject’s sensation of linear self motion induced by a translational optic flow stimulus (eg, watching a passing train or handheld optokinetic tape or drum); these eye movements are due to activation of the pursuit system alone.
(vv)Traingag.mp4(tt)
From: Top Secret. Paramount Pictures 1984.
In the laboratory, the OKN system is usually stimulated by rotating a large patterned drum around the stationary subject. Under these circumstances, the nystagmus pattern typically consists of an increase in eye velocity to a steady-state level that is proportional to the rotational velocity of the drum; the eyes will track the full-field rotation of the patterned stripes on the drum with a nystagmus pattern of slow phases in the direction of drum rotation and quick phases in the opposite direction4.
From: Brandt T, Strupp M. General vestibular testing. Clin Neurophysiol. 2005;116(2):406-426. doi:10.1016/j.clinph.2004.08.009
(vv)Optokinetic To The Right For Projection 2.mp4(tt)
From: Stanton J. Optokinetic to the right for projection. https://www.youtube.com/watch?v=CG5n516PCXM
Slowly decaying optokinetic after-nystagmus (OKAN) in the dark occurs after visual stimulation has stopped, and shares the same velocity-storage mechanism with the VOR4. When head rotation (or drum rotation) is suddenly halted, OKAN cancels out the postrotatory vestibular nystagmus pattern, and eye velocity drops quickly to zero.
The slow component of OKN–OKAN appears designed to perfectly complement the rVOR. Despite the fact that during constant-velocity rotation eye velocity decays to zero, the same rotation in the presence of a visual surround elicits a steady-state nystagmus pattern that is sustained indefinitely, as necessary to compensate for the applied stimulus. This happens because the OKN builds up at approximately the same rate as the rVOR decays.
The smooth pursuit system also participates at the onset of the response to an optokinetic (full-field) stimulus bringing the eyes quickly to the maximum velocity. Accordingly, the cerebellar regions involved in generating both pursuit and the VOR contribute to the tracking response to an optokinetic stimulus.
These visual–vestibular interactions are impaired after lesions that also impair velocity storage5: floccular Lesions affecting pursuit and OKN
Examination of eye movements with the optokinetic drum allows combined testing of smooth pursuit movements and saccades in horizontal and vertical directions. It is especially helpful with uncooperative or drowsy patients and with children.
It should be noted that although clinicians sometimes may try to elicit OKN using simpler devices such as the drum below, however this largely tests the pursuit and saccadic systems, since it can only stimulate a portion of the visual field. It can be useful in evoking convergence retraction nystagmus in persons with dorsal midbrain lesions, as well as asymmetrical tracking in persons with latent nystagmus" 6. Note that smooth pursuit is typically elicited voluntarity by the images of small objects being maintained on the fovea, all features which make the standard clinical OKN drum a useful method of testing pursuit4.
(vv)Optokinetic Reflex.mp4(tt)
(vv)Okn.mp4(tt)
From: Embalance Project. University College London.Optokinetic Nystagmus. Retrieved from: https://www.youtube.com/watch?v=LInm9cZcHyk
Use of the OKN drum primarily tests smooth-pursuit movements in the direction of drum rotation, and saccadic eye movements opposite to the direction of drum rotation: eg, drum rotation toward the patient’s right side generates a rightward ocular pursuit movement followed by a leftward saccade.
Asymmetry of the OKN drum–induced responses can be caused by a unilateral lesion of the cerebral pathways which control ipsilateral smooth pursuit, and which descend from the ipsilateral parietal, middle temporal, or medial superior temporal areas to the brainstem ocular motor centers.
One should look for asymmetries in the following clinical situations:
- Between right and left (indicates a unilateral cortical or pontine lesion)
- Vertical worse than horizontal (indicative of a vertical supranuclear gaze palsy due to a mesencephalic lesion)
- Dissociations of the two eyes (a sign of diminished adduction in INO)
- Reversal of pursuit (indicates congenital nystagmus)6
The best known example is that of parietal lobe lesions: with an infarction of right posterior cerebral cortex which affects secondary visual areas concerned with motion processing, the OKN response will be reduced as the stripes move to the patient’s right (impaired/broken-up ipsilateral smooth pursuit) and fewer corrective quick phases will be triggered. That is, there will be less nystagmus as the stripes move to the patient’s right compared with moving to the left7. This is particularly useful with attempting to establish whether a homonymous hemianopia has arise from occipital or parietal lobe lesions. In the case of an occipital lobe lesion, pursuit and testing with the OKN drum will be normal, whereas lesions of the parietal lobe will cause OKN asymmetry.