Eye position dependency of nystagmus during constant vestibular stimulation

Abstract

Alexander's law, the eye position dependency of nystagmus due to peripheral vestibular lesions, has been hypothesized to occur due to adaptive changes in the brainstem velocity-to-position neural integrator in response to non-reciprocal vestibular stimulation. We investigated whether it develops during passive head rotations that produce constant nystagmus for >35 s. The yaw rotation stimulus consisted of a 1-s acceleration (100°/s(2)), followed by a lower acceleration ramp (starting at 7.3°/s(2) and increasing at 0.04°/s(2)/s) until 400°/s was reached after 38 s. This stimulus was designed to offset the ~15 s vestibular ocular reflex time constant (and the 150 s adaptation time constant) and produce constant velocity slow phases. In contrast to peripheral lesions, this vestibular stimulation is the result of real head turns and has the push-pull characteristics of natural movements. The procedure was successful, as the average velocity of 31°/s was unchanged over the final 35 s of the acceleration period. In all 10 healthy human subjects, we found a large and stable Alexander's law, with an average velocity-versus-position slope of -0.366 in the first half that was not significantly different in the second half, -0.347. These slopes correspond to integrator time constants of <3 s, are much less than normal time constants (~25 s), and are similar to those observed in patients with peripheral vestibular lesions. Alexander's law also developed, on average, in 10 s. We conclude that Alexander's law is not simply a consequence of non-reciprocal vestibular stimulation.

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“…The trajectory with the negative slope represents the slow phases of nystagmus. We found that the SPV in the rst 3 s increased gradually, and became stable in the last 4 s. This phenomenon was consistent with the report of Bockisch et al 18 . Then, we discarded the volunteers' SPV in the rst 3 s and calculated the average of the SPV in the last 4 s (see Table 1).…”

“…We discarded the rst and last data of nystagmus slow phases, and removed the phases shorter than 50 ms to reduce the statistical error 22 . Besides, the rst 3 s of the nystagmus was discarded because the SPV generally increases within 3 s after the head senses the change in acceleration 18 . We also removed the nystagmus data when the volunteers blinked.…”

Alexander's Law During High-Speed, Yaw-Axis Rotation: Adaptation or Saturation?

“…For all subjects at a mean value of 12.1°/s (range 6.4–18.1°/s) the inferred calculation of the TcNI increased relatively abruptly and soon reached a plateau at its previous normal high value. This finding is in accord with the result shown in Figure 4C of Bockisch et al ( 14 ) in which the AL effect was greater for a higher SPV. This can be interpreted as the higher the SPV, the more likely the circuits that implement the VOR will show the non-linear effects of saturation, in which (1) neurons lose their ability to fire at higher rates in their excitatory direction and, (2) because of the effects of inhibitory cutoff at higher speeds of rotation, neurons can no longer decrease their discharge in a reciprocal fashion.…”