Velocity Storage with Vestibular and Optokinetic Nystagmus Explanation
Rotating in a chair for 1 minute at 0.2Hz and 0.5Hz and recording the eye movements with Infrared goggles is very useful for adding to the ‘Vestibular’ suite of tests.
The results are compared to normal values and this helps us decide on which therapies are best suited to the particular patient.
If the rapid eye movements (nystagmus) that occur after abruptly stopping the rotation, take less than 8 seconds, or greater than 20 seconds to stop, we interpret this to mean that the Velosity Storage system is dysfunctional.
Velocity storage estimation gained from assessing vestibular and optokinetic nystagmus is a concept related to the interaction of the vestibular and visual systems, respectively, playing crucial roles in how we perceive motion and maintain balance and visual stability.
Velocity Storage - Neural Integration
Velocity storage is the mechanism within the vestibular system that prolongs the response of the brain to rotational movements. This mechanism is part of the body's system for maintaining balance and spatial orientation.
Function: When you rotate your head or body, the vestibular organs in your inner ear detect this motion. The velocity storage mechanism extends the time over which the brain perceives this rotation, even after the actual movement has stopped. This extension helps to stabilise gaze during and after head movements.
Purpose: It plays a crucial role in maintaining a stable perception of the world despite head movements. By prolonging the sensation of rotation, it allows for more effective integration of vestibular information with visual and proprioceptive (body position) information, aiding in balance and spatial orientation.
Clinical Relevance: Abnormalities in the velocity storage mechanism can contribute to symptoms like dizziness, vertigo, and balance disorders. It is a focus in the study of various vestibular disorders.
Vestibular Nystagmus: With the eyes closed, perceived movement is largely from the otoliths and semi-circular canals of the vestibular apparatus. Movement of these structures signal to the brain what movement is occuring. With an abrupt cessation of movement the fluid within the vestibular apparatus takes a little while to stabilise followed by the velocity storage mechanism being activated. Measuring the time it takes for the nystagmus to subside gives an indication of the vestibular component of the velocity storage mechanism without the visual contribution from having the eyes open.
Optokinetic Nystagmus (OKN): Optokinetic nystagmus is an involuntary eye movement induced by the motion of the visual field. It is a reflex that helps maintain a stable image on the retina during sustained head rotation or while observing a moving visual scene.
Function: OKN involves a combination of slow-phase and fast-phase eye movements. When you watch objects move across your field of vision (like looking out the window of a moving train), your eyes will follow the moving objects smoothly (slow phase) and then quickly snap back (fast phase) to refocus on another moving object.
Purpose: This reflex helps to stabilise the visual image on the retina, allowing for clear vision despite movement in the environment or of the head.
Clinical Relevance: OKN can be used in clinical settings to assess the function of the visual and vestibular systems. Abnormal OKN responses can indicate issues with the brain, inner ear, or eye function. We use the abnormal persistence of OKN as biomarker of dysfunction of the brain stem nuclei that generate the activity or the mechanism that drives those nuclei. In migraine and concussion the velocity storage time constant can be shortened allowing insight into cerebellar, cortical or brain stem dysfunction.
In summary, velocity storage is a vestibular mechanism that extends the brain's perception of rotational movement, aiding in balance and spatial orientation, while optokinetic nystagmus is a reflex in the visual system that stabilises the visual field during head or environmental movement. Both are critical for maintaining balance and visual stability in a dynamic environment.