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The video head impulse test (vHIT) has been proposed as an objective approach to detect peripheral vestibular disorder in a clinical setting. Data from several studies indicate that the vHIT is a useful addition to the vestibular test battery and can give complementary information to caloric testing. This study explores the relationship between lateral canal vestibular occular reflex gain measured using the vHIT system and canal paresis indicated using air calorics in a sample of patients attending a clinic for balance disorder. Sensitivity and specificity of the vHIT test relative to calorics was studied for a clinical sample of 51 patients (20 male, 31 female) who attended a private clinic for balance disorders. vHIT gains were compared to the manufacturer's normative range and to data from a normative study using 30 young volunteers. Of 14 patients in the clinical sample that had significant canal paresis indicated by air calorics, only 4 showed a significant abnormality in either canal using a measurement of vHIT gain. vHIT gain does not correlate with canal paresis as indicated by air caloric testing. vHIT gain appears relatively insensitive to peripheral vestibular disorder as indicated by air caloric testing, although patients that had no caloric response on one side showed abnormal vHIT gain. vHIT testing may be a useful addition to the existing vestibular test battery, but it does not appear to be an alternative to it.

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Large circular displays rotating around the line of sight produce an illusion in which the seen orientation of the true vertical is shifted in a direction opposite to the display's motion. Two experiments were performed to determine whether the magnitude of this illusory tilt is a function of the area of display elements, of their boundary length, or of their spatial frequency. In Experiment 1, 12 subjects viewed each of nine displays across which the number and area of the circular elements were independently varied. Three of the displays were equated for the area of their elements. The results suggested that tilt magnitude and onset latency could be explained by a boundary length effect. A second experiment tested eight subjects on two displays, equated for element boundary length but differing in the spatial frequency of the elements. The displays produced closely similar illusory tilts corroborating the view that, within broad limits, element boundary length-and not spatial frequency or area-determines the size and onset latency of illusory tilt. A third experiment confirmed previous research in finding greater tilt and more rapid onset with more peripherally projected displays. This paper is concerned with the structural characteristics of large circular displays rotating around the line of sight and their effects upon the magnitude and onset latency of the illusory tilt produced in the seen orientation of the true vertical. Using such a stimulus, Dichgans, Held, Young, and Brandt (1972) demonstrated that the illusion increases as a negatively accelerated function of display velocity, reaching a maximum around 30 deg/sec, while Held, Dichgans, and Bauer (1975) observed that it grows as a roughly linear function of the logarithm of the total display area. It has also been found that elements within the display falling on the periphery of the retina yield substantially greater illusory effects than those projecting onto more central areas (Brandt, Dichgans, and Koenig, 1973;Brandt, Wist, and Dichgans, 1975;Held et al., 1975).The first experiment reported here deals with the interaction between the number and size of the circular elements within the display. It has been shown that tilt magnitude increases as a logarithmic function of the number of small circular elements present within the rotating display (Brandt et aI., 1975). Since element size was held constant in this study, it was possible for these investigators to equate their number with the total moving area of the stimulus and to suggest that the observed relationship between tilt magnitude and element number was due either to an area effect (in which the presence of more elements stimulated a larger number of motion-receptive units) or to a spatial frequency effect (in which larger tilts arose as the result of the increased density of the moving elements within the visual field).There is, however, a third and as yet unexplored possibility-namely, that the motion-receptive units were responding to an increase in the total boundary length of the movi...

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