The unbalance of signals transmitted through the VOR pathways from both sides of the irritated or nonfunctional labyrinth, such as in Meniere's disease or due to head trauma or labyrinth disease, can result in vestibular nystagmus. For instance, irritation of the left ear labyrinth can increase the discharge of afferent fibers supplying the left horizontal semicircular duct, causing a left nystagmus with a slow phase to the right and a fast phase to the left. The destruction of the right ear labyrinth produces similar effects as irritation of the left labyrinth, and interestingly, the nystagmus is temporary, demonstrating the adaptability of these circuits over time.

The optokinetic reflex (OKR) is initiated by visual stimuli (retinal slip), and photoreceptors are the starting point of the reflex arc. The nucleus of the optic tract (NOT) and the accessory optic nuclei (AON) in the rostral midbrain's tegmentum and pretectal region play crucial roles in this reflex. These nuclei receive visual information from direction-selective, motion-sensitive retinal ganglion cells. Additionally, input from primary and higher order visual cortical areas in the occipital and temporal lobes is significant in primates and humans. The NOT and AON cells have large receptive fields and selectively respond to the direction and speed of visual scene movement. Notably, the preferred movement directions of the NOT/AON cells closely align with motion caused by rotation around axes perpendicular to the semicircular canals, facilitating coordination between the VOR and OKR to maintain stable retinal images.

The efferent connections of these nuclei are complex and not completely understood. There are polysynaptic pathways to the oculomotor and abducens nuclei and monosynaptic input to the vestibular nuclei, allowing interaction with the VOR. Various precerebellar nuclei, including the inferior olivary nucleus and basilar pontine nuclei, receive projections. These pathways loop through the flocculus and back to the vestibular nuclei. Ultimately, activity reaches the oculomotor nuclei via multiple parallel pathways, activating motor neurons and resulting in proper counterrotation of the eyes.

Clinical testing of labyrinthine function involves rotating the patient in a Bárány chair to stimulate the labyrinths in both ears or introducing cold or warm water into the external auditory canal of one ear (caloric test). During rotation in the Bárány chair, nystagmus develops, with the fast phase of the nystagmus aligning with the rotation direction. When the chair rotation stops, nystagmus develops in the opposite direction (postrotatory nystagmus), as stopping rotation has a similar effect to accelerating in the opposite direction.
Whenalabyrinthisirritatedinoneear,asinMeniere’s disease, orwhenalabyrinthisrenderednonfunctional,asmayhappenasaresultofheadtraumaordiseaseofthelabyrinth,thesignalstransmittedthroughtheVORpathwaysfromthetwosidesbecomeunbalanced.Vestibularnystagmuscanthenresult.Forexample,irritationofthelabyrinthoftheleftearcanincreasethedischargeofafferentfibersthatsupplythelefthorizontalsemicircularduct.Thesignalproducedresemblesthatnormallygeneratedwhentheheadisrotatedtotheleft.Becausethestimulusisongoing,aleftnystagmusresults,withaslowphasetotheright(causedbytheVORpathway)andafastphasetotheleft.Destructionofthelabyrinthintherightearproduceseffectssimilartothoseinducedbyirritationoftheleftlabyrinth.Interestingly,thenystagmusistemporary,whichshowstheabilityofthesecircuitstoadaptovertime.
Circuits Underlying the Optokinetic Reflex
The stimulus eliciting the OKR is visual (retinal slip), and so photoreceptors are the start of the reflex arc. Key brainstem centers for this reflex lie in the tegmentum and pretectal region of the rostral midbrain. They are the nucleus of the optic tract (NOT) and a group of nuclei collectively known as the accessory optic nuclei (AON). Direction-selective, motion-sensitive retinal ganglion cells are a major afferent source carrying visual information to these nuclei. In addition, input comes from primary and higher order visual cortical areas in the occipital and temporal lobes. These latter afferent sources are particularly important in primates and humans. Cells of the NOT and AON have large receptive fields, and their responses are selective for the direction and speed of movement of the visual scene. Of interest is that the preferred directions of movement of the NOT/AON cells correspond closely to motion caused by rotation about axes perpendicular to the semicircular canals, thereby facilitating coordination of the VOR and OKR to produce stable retinal images.
The efferent connections of these nuclei are numerous and complex and not fully understood. There are polysynaptic pathways to the oculomotor and abducens nuclei and monosynaptic input to the vestibular nuclei, which allow interaction with the VOR. There are projections to various precerebellar nuclei, including the inferior olivary nucleus and basilar pontine nuclei. These pathways then loop through the flocculus and back to the vestibular nuclei. In sum, via several pathways operating in parallel, activity ultimately arrives at the various oculomotor nuclei whose motor neurons are activated, and proper counterrotation of the eyes results.
Saccades are generated in response to activity in the superior colliculus or the cerebral cortex (frontal eye fields and
ClinicaltestingoflabyrinthinefunctionisdoneeitherbyrotatingthepatientinaBáránychairtoactivatethelabyrinthsinbothearsorbyintroducingcoldorwarmwaterintotheexternalauditorycanalofoneear(caloric test). WhenapersonisrotatedinaBáránychair,nystagmusdevelopsduringtherotation.Thedirectionofthefastphaseofthenystagmusisinthesamedirectionastherotation.Whentherotationofthechairishalted,nystagmusintheoppositedirectiondevelops(postrotatorynystagmus)becausestoppingarotationhasthesameeffectasacceleratingintheoppositedirection.