Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_4382_Библиотеки_им_академика_М_И_Перельмана

or VVOR for visual-vestibular ocular reflex) investigates how the visual and vestibular system interact with one another in order to augment, or improve, the neurophysiological VOR response. We have previously discussed the relatively weak nature of the VOR in the absence of any visual cues (i.e., lack of VOR gain unity during SHA testing in darkness). Specifically, we have seen how the sen­sitivity (gain) of the vestibular system, particularly for frequencies below 1 Hz, is inherently weak, with gain values often [well] below 1.0 (see Fig­ure 6–18). Unfortunately, if such a weak VOR gain were to persist in the presence of visual cues (i.e., anything less than 1.0) the functional outcomes would be catastrophic during head movement, as even minor retinal slips of the visual target off the fovea can cause significant blurring in visual acuity, which would subsequently be reported as “dizziness” in most patients. We are reminded that the fovea represents a viewing area of only 1° (approximately 1 mm) in a field of vision as large as 200°, and any deviation of the target from the fovea severely compromises visual clarity. In fact, the required precision of the visual system is so demanding that, even if the visual-vestibular sys­tem produced a VOR gain of 0.90 with a peak head velocity (movement) of 90°/sec (such as that expe­rienced during running), an unassisted vestibular response would cause a retinal image velocity of 8°/sec. This would produce more than enough retinal slip to reduce visual acuity by a factor of 2 (Demer, 1996). If such retinal slip occurred dur­ing every active head movement, the resultant effects during daily activities would not only be devastating, they would be life limiting. In fact, if the VOR were solely responsible for providing compensatory eye responses to every head move­ment, the vestibular system’s inadequacies would be highly detrimental to the maintenance of visual acuity during nearly every natural head move­ment. Fortunately, the inadequate VOR is effec­tively modified and enhanced by the visual system to near perfect sensitivity (gain), so such visual blurring is rarely a problem during routine daily activities. This visual-vestibular relationship, or enhancement, is heavily mediated through the central nervous system.

Prior to discussing the rotational protocol for visual-vestibular enhancement, let’s first consider the various neurophysiological substrates asso-

FIGURE 8–2. Abnormally high visual-vestibular enhancement in a patient with an

FIGURE 8–4. Diagram illustrating the differences in applied GIA force across the utricular epithelium (blue

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. As a result of such slow acceleration, the degree of cupular displace­ment is significantly less (in accordance with the cupular pendular model) than during VST. Con­sequently, the VOR response is subsequently less and habituates more quickly once the target velocity stimulus is achieved. Thus, the contribu­tions from the horizontal semicircular canals are considerably less and extinguish rapidly follow­ing the acceleration period. The exact methods for eccentric testing will be discussed shortly. First, let us consider the anatomical substrate and normal physiological response that occurs during eccen­tric rotational testing.

Anatomical and Physiological Response During Eccentric Rotation

Centrifugal Force Versus Centripetal Force: Which Is Correct?

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