Figure 1.

The anatomical and physiological basis for enhanced oVEMP responses after a semicircular canal dehiscence. (A) Time series records of the oVEMP response of a patient with an SCD showing a greatly enhanced oVEMP n10 component beneath the eye contralateral to the SCD in response to 500 Hz brief tone bursts. The typical oVEMP n10 response of a healthy subject to the same stimulus shows a much smaller n10 component [reproduced with permission from (4)]. (B) A view from a medial view point of the bony wall of the superior semicircular canal in a guinea pig showing the small (0.1 mm diameter) dehiscence of the bony wall of the canal made during the experiment by shaving away the thin bone using a fine scalpel blade, while continuing to record from the same single neuron. In guinea pigs the canal is clearly visible after removal of the overlying cerebellum [reproduced with permission from (5)]. (C) After such an SCD, anterior canal neurons with irregular resting activity are activated and phase-lock to ACS and BCV at stimulus levels used for human clinical testing, whereas they do not respond to the same stimuli before SCD (6). In these experiments the same neuron was tested before and after the SCD. The response of one superior canal neuron to high-frequency air-conducted sound, before and after a small dehiscence in the bony wall of the superior canal. (a) The response of the neuron to pitch angular acceleration in the plane of the superior canal identifies the neuron as being a superior canal afferent. (b) Before SCD an 8 s burst of 1,483 Hz ACS has no effect on the neural response—there are very few action potentials during the tone burst. (c) After the SCD a 10 s burst of an air-conducted sound of 1,479 Hz causes strong activation of this same neuron. Reproduced with the permission of John Wiley and Sons Inc., from (3). (D) The projections of otolithic (D1) and canal (D2) neurons to IO and SCM [redrawn from (7)]. These are schematic diagrams of a view of the brainstem to show the otolithic projections to IO and SCM on the left (1) and the superior canal projections to IO and SCM on the right (2). These projections were derived from experiments using electrical stimulation to identify the projections. Stimulation in animals with intact labyrinths causes the otolithic neural connections shown in 1 to be activated, so ACS and BCV generate the oVEMP and cVEMP responses without any input from semicircular canal neurons, since canal afferents are not activated by ACS and BCV at frequencies above 200 Hz (8). However, after an SCD, the otoliths are activated even more strongly but in addition superior semicircular canal neurons are also activated by ACS and BCV as well as the otolithic neurons. Some canal afferents can be activated by frequencies of 3,000 Hz and above (3) (9). The superior canal neurons project to III nucleus by the crossed ventral tegmental tract (dashed lines) and the MLF. This combination of otolithic and canal afferent activation will result in a larger oVEMP 10 (as shown in A above). Reproduced with the permission of John Wiley and Sons Inc., from (3).