Figure 1.
EVORs to human bipolar EVS in healthy subjects, bilateral and unilateral vestibular deafferented patients.
(A) Normal eVOR from healthy subjects (N = 17) (group means ± SEM) comprised conjugate torsional and horizontal eye rotations, binocularly equal in amplitude, rotated away from cathode towards anode, and vertical divergence with the intorting eye upwards (cathode side) and extorting eye downwards (anode side). (B) EVOR was absent from BVD patients (N = 5) with torsional, vertical and horizontal eVOR positions ≤0.01°. (C) Bidirectional eVOR from grouped as right UVD patients with left functioning ear (N = 12). The excitatory eVOR to left-cathode/right-anode (lc/ra) cathodal EVS comprised conjugate torsional and horizontal eye rotations away from cathode and a vertical divergence with the eye on the anodal side moving downwards. The inhibitory eVOR to right-cathode/left-anode (rc/la) anodal EVS was in the opposite direction and at about half the amplitude of the excitatory eVOR with the eye on the anodal side moving upwards. (The schemes illustrate EVS polarities and eye rotation directions).
Figure 2.
Spatio-temporal characteristics of the eVOR to human bipolar EVS.
(A, B) Mean eVOR velocity and acceleration time-series to EVS intensities of [0.9, 2.5, 5.0, 7.5, 10.0]mA from grouped as right UVDs with functioning left ears (N = 12). Tonic and phasic eVORs graded to all current intensities for lc/ra cathodal EVS excitation or rc/la anodal EVS inhibition exhibiting excitatory-inhibitory asymmetries. (C) Relationship of eVOR with EVS duration and frequency. Normal eVOR was linearly correlated with EVS duration when EVS duration was incremented from 10–100 ms in 10 ms-step. (D) Comparison of excitatory and inhibitory eVOR positions to a 1 ms EVS step at [1, 50, 100, 200]Hz and to a 100 ms EVS step at 1 Hz. We found that the eVOR correlated well to EVS duration but not to frequency of stimulation.
Figure 3.
Latency of the eVOR after UVD.
(A) Excitatory and inhibitory eVOR latencies of binocular torsional, vertical and horizontal eVOR to 5.0 mA lc/ra cathodal EVS and rc/la anodal EVS from grouped as right UVDs with functioning left ears (N = 12, group means ± SEM) showing their mean latencies. (B) Schematic depicting the excitatory (solid red line) and inhibitory (dashed red line) horizontal semicircular canal pathways showing the 3-neuron reflex arc comprising 1: vestibular nerve; 2: vestibulo-ocular secondary neuron; 3: abducens motorneuron. LR: lateral rectus muscle; MR: medial rectus muscle; ON: oculomotor nucleus; AN: abducens nucleus; VN: vestibular nucleus; MLF: medial longitudinal fasciculus. Equivalent 3-neuron reflex arcs also exist for the vertical semicircular canal pathways.
Figure 4.
Effect of current spread in human unipolar EVS of UVD patients.
The eVOR to unipolar 5.0(A) The eVORs from left-cathode/right-anode (lc/ra) and to right-cathode/left-anode (rc/la) bipolar EVS were largest. (B) When left-cathode/C7-anode (lc/C7a) and C7-cathode/left-anode (C7c/la) unipolar EVS stimulated the functioning left ear, the eVOR was about half the bipolar eVOR. (C) However when right-cathode/C7-anode (rc/C7a) and C7-cathode/right-anode (C7c/ra) unipolar EVS stimulated the right UVD ear, instead of the expected absent response, it was one-third the bipolar eVOR. The polarity of the eVOR was consistent with the C7- electrode polarity suggesting that current may have spread from the C7-electrode closer to functioning left ear. (D) When the eVOR from all three configurations were grouped according to direction of their responses (i.e. CW or CCW) and then normalized, they share similar spatio-temporal characteristics with the mean excitatory eVOR latency of 8.9 ms and inhibitory latency of 11.4 ms suggesting that the eVORs were generated from the functioning left ear. (Schemes show electrode locations on the patient).
Figure 5.
Comparison of the mean eVOR in UVS to normal and UVD subjects.
(A) The mean eVOR in UVS (N = 4) was similar to UVD grouped as right lesions, but different from normal. Right UVS showed bidirectional, excitatory eVOR to 5 mA cathodal EVS and inhibitory eVOR to anodal EVS of the left ear at half the amplitude and opposite in direction, with excitatory-inhibitory asymmetrical response pattern similar to the right UVD. Mean latencies from 4 individual UVS patients (P1, P2, P3, P4) were compared to normal and right UVD subjects. (C) MRI from UVS patients (P1, P2)