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Notizen: Summary The otolith contribution and otolith-visual interaction in eye and head stabilization were investigated in alert cats submitted to sinusoidal linear accelerations in three defined directions of space: up-down (Z motion), left-right (Y motion), and forward-back (X motion). Otolith stimulation alone was performed in total darkness with stimulus frequency varying from 0.05 to 1.39 Hz at a constant half peak-to-peak amplitude of 0.145 m (corresponding acceleration range 0.0014–1.13 g) Optokinetic stimuli were provided by sinusoidally moving a pseudorandom visual pattern in the Z and Y directions, using a similar half peak-to-peak amplitude (0.145 m, i.e., 16.1°) in the 0.025–1.39 Hz frequency domain (corresponding velocity range 2.5°–141°/s). Congruent otolith-visual interaction (costimulation, CS) was produced by moving the cat in front of the earth-stationary visual pattern, while conflicting interaction was obtained by suppressing all visual motion cues during linear motion (visual stabilization method, VS, with cat and visual pattern moving together, in phase). Electromyographic (EMG) activity of antagonist neck extensor (splenius capitis) and flexor (longus capitis) muscles as well as horizontal and vertical eye movements (electrooculography, EOG) were recorded in these different experimental conditions. Results showed that otolith-neck (ONR) and otolith-ocular (OOR) responses were produced during pure otolith stimulation with relatively weak stimuli (0.036 g) in all directions tested. Both EMG and EOG response gain slightly increased, while response phase lead decreased (with respect to stimulus velocity) as stimulus frequency increased in the range 0.25–1.39 Hz. Otolith contribution to compensatory eye and neck responses increased with stimulus frequency, leading to EMG and EOG responses, which oppose the imposed displacement more and more. But the otolith system alone remained unable to produce perfect compensatory responses, even at the highest frequency tested. In contrast, optokinetic stimuli in the Z and Y directions evoked consistent and compensatory eye movement responses (OKR) in a lower frequency range (0.025–0.25 Hz). Increasing stimulus frequency induced strong gain reduction and phase lag. Oculo-neck coupling or eye-head synergy was found during optokinetic stimulation in the Z and Y directions. It was characterized by bilateral activation of neck extensors and flexors during upward and downward eye movements, respectively, and by ipsilateral activation of neck muscles during horizontal eye movements. These visually-induced neck responses seemed related to eye velocity signals. Dynamic properties of neck and eye responses were significantly improved when both inputs were combined (CS). Near perfect compensatory eye movement and neck muscle responses closely related to stimulus velocity were observed over all frequencies tested, in the three directions defined. The present study indicates that eye-head coordination processes during linear motion are mainly dependent on the visual system at low frequencies (below 0.25 Hz), with close functional coupling of OKR and eye-head synergy. The otolith system basically works at higher stimulus frequencies and triggers Synergist OOR and ONR. However, both sensorimotor subsystems combine their dynamic properties to provide better eyehead coordination in an extended frequency range and, as evidenced under VS condition, visual and otolith inputs also contribute to eye and neck responses at high and low frequency, respectively. These general laws on functional coupling of the eye and head stabilizing reflexes during linear motion are valid in the three directions tested, even though the relative weight of visual and otolith inputs may vary according to motion direction and/or kinematics.

Notizen: Abstract  The purpose of this study was to investigate changes in neck muscle and eye movement responses during the early stages of vestibular compensation (first 3 weeks after unilateral vestibular neurectomy, UVN). Electromyographic (EMG) activity from antagonist neck extensor (splenius capitis) and flexor (longus capitis) muscles and eye movements were recorded during sinusoidal visual and/or otolith vertical linear stimulations in the 0.05–1 Hz frequency range (corresponding acceleration range 0.003–1.16 g) in the head-fixed alert cat. Preoperative EMG activity from the splenius and longus capitis muscles showed a pattern of alternate activation of the antagonist neck muscles in all the cats. After UVN, two motor strategies were observed. For three of the seven cats, the temporal activation of the individual neck muscles was the same as that recorded before UVN. For the other four cats, UVN resulted in a pattern of coactivation of the flexor and extensor neck muscles because of a phase change of the splenius capitis. In both subgroups, the response patterns of the antagonist neck muscles were consistent for each cat independently of the experimental conditions, throughout the 3 weeks of testing. Cats displaying alternate activation of antagonist neck muscles showed an enhanced gain of the visually induced neck responses, particularly in the high range of stimulus frequency, and a gain decrease in the otolith-induced neck responses at the lowest frequency (0.25 Hz) only. By contrast, for cats with neck muscle coactivation, the gain of the visually induced neck responses was basically unaffected relative to preoperative values, whereas otolith-induced neck responses were considerably decreased in the whole range of stimulation. As concerns oculomotor responses, results in the two subgroups of cats were similar. The optokinetic responses were not affected by the vestibular lesion. On the contrary, otolith-induced eye responses showed a gain reduction and a phase lead. Deficits and short-term changes after UVN of otolith- and semicircular canal-evoked collic and ocular responses are compared.