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The vestibular labyrinth consists of ? ve compartments: the lateral, anterior, and posterior semicircular canals, the utricule, and the saccule. At the beginning of the twentieth century, Robert Bárány proposed the caloric test as a clinical test of the lateral semicircular canal. This test enabled clinicians to assess the individual lateral semicircular canal function easily by using the simple method of irrigating the external ear canal with cold or warm water and observing the induced nyst- mus. We believe that the caloric test was a breakthrough in the ? eld of vestibular research. However, as far as the other compartments were concerned, there was no simple clinical test equal to the caloric test for the lateral semicircular canal function. At the end of the twentieth century, the vestibular evoked myogenic potential (VEMP) test was proposed as a new method for assessing the individual saccule function. This test has some unique attributes. First, it uses sound stimulation, even though it is a test of the vestibular system. This appears to be a contradiction n- rophysiologically. As a result, in its early stages there were controversies conce- ing the origin of the responses. However, such controversies have been overcome by basic neurophysiological studies and clinical studies. Above all, sound sensit- ity of the saccular afferents shown in cats and guinea pigs with single-unit recording methods became the main supporting evidence. Nowadays, VEMP is one of the routine clinical tests for balance disorder.
(cont.) In the second part of the thesis, we develop a computational model of VEMP physiology, and use the model to separate external sources of variability from internal noise. The model outputs are also used to define a statistical measure, the inverse coefficient of variation (iCOV), which correlates with altered vestibular sensitivity, but is relatively robust to other changes. When applied to the experimental data, the iCOV is found to yield estimates of vestibular sensitivity that are more stable than the conventional VEMP amplitude. This measure also reveals a diversity of response threshold and growth characteristics within the clinically normal population. These findings suggest that the proposed approach could lead to the development of an improved clinical tool, but could also yield new insights into the physiological mechanisms of vestibular pathology.
"It is uncertain whether clinically useful information of otolith function can be well characterized by vestibular-evoked-myogenic-potentials (VEMPs), i.e., electromyogenic activity recorded from sternomastoid muscles in response to brief loud auditory clicks. We aimed to assess the utility of VEMP testing in the differential diagnosis of dizziness (81 dizzy patients, 12 normals). We found that: (1) VEMP was reliably elicited from all controls and from 96% of patients without loss of vestibular function; (2) in patients with documented unilateral peripheral vestibular disorders, VEMP could reveal loss of otolith function that usefully facilitated diagnoses; and (3) such otolith function loss was independent of canal function loss revealed by caloric testing. We conclude that the VEMP-test is clinically useful to assess vestibular (otolith and/or inferior vestibular nerve) function, providing information complementary to that obtained from caloric testing (horizontal semicircular canal and/or superior vestibular nerve function)." --