![]() Eye and head angular positions were represented by rotation vectors with torsional, vertical, and horizontal coordinates ( Migliaccio & Todd., 1999). ![]() Eye and head angular position signals were low-pass filtered with a single-pole analog filter that had a 3dB bandwidth of 100Hz, sampled at 5000Hz at 16-bit resolution, then digitally filtered with a 50-tap zero-phase low pass FIR filter with bandwidth 200Hz. A search coil embedded in a bite block was used to measure head rotation. The identical technique has been described in detail elsewhere ( Migliaccio et al 2006). Binocular eye movements were recorded in three-dimensions using magnetic search coils embedded in a silicone annulus placed around the cornea (Skalar, Delft, The Netherlands). The sound-evoked VOR was measured in all subjects while the stimuli were delivered. ![]() Concordant changes in the two measures would indicate that the “OVEMP” is indeed an EMG correlate of the sound-evoked VOR which could be widely used for non-invasive assessment of vestibulo-ocular reflex pathways. ![]() We examined modulation of both responses by gaze direction, stimulus type and frequency. Since both OVEMP and VOR are amplified and thus better defined in SCD we used subjects with dehiscence to determine the temporal relationship between the eye movement and the muscle potential. Does it also represent in part, the muscle contraction that leads to eye movement? Can it function as a surrogate measure of the VOR? To answer these questions, we measured the VOR and OVEMP simultaneously for the first time. The OVEMP, an excitatory potential is thus likely to represent the contraction of extra ocular muscles in response to vestibular stimulation. The OVEMP is distinct from the R1 component of the blink reflex: being abolished with vestibulopathy when the R1 is preserved and preserved in facial palsy although the R1 is abolished (Smulders et al., 2008). When recorded infraorbitally, it amplifies on upgaze, but not on contraction of the orbicularis oculi muscles and is therefore thought to be an extraocular muscle response, possibly arising from the inferior oblique muscle ( Rosengren et al., 2005). The OVEMP is larger in the eye contralateral to the stimulus ( Rosengren et al., 2007). The typical OVEMP response to an 8ms pure tone burst is an excitatory potential which begins at about 8ms has a first negative (excitatory) peak at 11-12 ms, followed by a later positivity at about 18 ms ( Welgampola et al., 2008). Known as “OVEMP”, (ocular vestibular-evoked myogenic potential), it is optimally recorded with active electrode directly beneath the eye and a reference 1cm below this ( Todd et al., 2007). This EMG response is abolished in vestibulopathy and preserved despite profound hearing loss, thus vestibular in origin. A smaller, identical potential is seen in normal subjects ( Rosengren et al 2005, Todd et al 2007). In SCDS, the same stimulus also evokes a large surface electromyographic (EMG) potential recordable from peri-ocular sites ( Halmagyi et al 2003). This response is essentially a “sound-evoked VOR”. the upper pole of the eye torting away from the stimulus) torsional eye movement in the plane of the affected canal ( Halmagyi et al., 2003 Aw et al 2006). In these subjects, a brief intense 110 dB nHL (normalized hearing level) click, delivered in the affected ear, evokes a vertical upward and contraversive (ie. In the superior canal dehiscence syndrome (SCDS), absence of the bony roof overlying the superior semicircular canal gives rise to sound- and pressure-evoked vertigo or oscillopsia ( Minor, 2005). They provide an indirect means of studying the VOR. Non-physiological vestibular stimuli like intense sound and vibration can evoke reflex ocular movements even in the absence of head displacement ( Zhou et al., 2004 Karlberg et al 2003). In health, vestibulo-ocular reflexes (VOR) stabilize the visible world upon the retina during head and body movement.
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