Canal-otolith convergence on cat ocular motoneurons

Do motoneurons encode the noncommutativity of ocular rotations?

As we look around, the orientation of our eyes depends on the order of the rotations that are carried out, a mathematical feature of rotatory motions known as noncommutativity. Theorists and experimentalists continue to debate how biological systems deal with this property when generating kinematically appropriate movements. Some believe that this is always done by neural commands to a simplified eye plant. Others have postulated that noncommutativity is implemented solely by the mechanical properties of the eyeball. Here we directly examined what the brain tells the muscles, by recording motoneuron activities as monkeys made eye movements. We found that vertical recti and superior/inferior oblique motoneurons, which drive sensory-generated torsional eye movements, do not modulate their firing rates according to the noncommutative-driven torsion during pursuit. We conclude that part of the solution for kinematically appropriate eye movements is found in the mechanical properties of the eyeball, although neural computations remain necessary and become increasingly important during head movements.

A review of otolith pathways to brainstem and cerebellum

Our knowledge of otolith pathways is developing rapidly, but is still far from complete. Primary afferents from the sacculus and utricle terminate mainly in the lateral, inferior and caudal superior vestibular nuclei, and the ventral cerebellum, in particular the nodulus. Otolith signals descend via reticulo- and vestibulospinal pathways in the spinal cord to influence neck motoneurons and ascending proprioceptive afferents. Utricular information can reach the extraocular eye muscles via mono-, di-, and multisynaptic pathways, but saccular afferents probably only by multisynaptic pathways. The otolith signals are relayed from the vestibular nuclei, medullary reticular formation, inferior olive, and lateral reticular nucleus to sagittal zones in the caudal cerebellar vermis (nodulus and uvula), and influence the deep cerebellar nuclei. The graviceptive information could be channeled by the cerebellar efferents back to the vestibular and inferior olive complex, or fed into ascending pathways that would innervate the mescencephalon, the thalamus, and cerebral cortex.

Skew deviation with ocular torsion: a vestibular brainstem sign of topographic diagnostic value

Fifty-six patients with unilateral brainstem infarctions presenting with skew deviation of the eyes were analyzed for static vestibular function in the roll plane. Ischemic lesions were allocated to the level and side of the brainstem by the clinical syndrome and neuroimaging. Two findings of clinical relevance were obtained: (1) All skew deviations were ipsiversive (ipsilateral eye was undermost) with caudal pontomedullary lesions and contraversive (contralateral eye was lowermost) with rostral pontomesencephalic lesions. (2) All skew deviations were associated with concomitant ocular torsion and tilts of subjective visual vertical toward the undermost eye. Thus, skew deviation or more correctly, ocular skew torsion is a sensitive brainstem sign of localizing and lateralizing value. Evidence is presented that the ocular skew torsion sign indicates a vestibular tone imbalance in the roll plane secondary to graviceptive pathway lesions.

The behavior and vestibular nuclear morphology of otoconia-deficient pallid mutant mice

Pallid mutant mice with no otoconia in their gravity receptors were behaviorally tested and their vestibular nuclei measured to determine if this developmental sensory deprivation had any detrimental effect on central vestibular structure and function. This absence of otoconia was correlated with disorientation in water and absence of air righting reflexes. The inferior and superior vestibular nuclei were smaller than normal, but neurons within regions that receive direct gravity receptor input were smaller only unilaterally in medial vestibular nucleus. It was tentatively concluded that the sensory deprivation resulting from reduced gravity receptor input was responsible for the behavioral and central morphological abnormalities.

Comparison of human subjective and oculomotor responses to sinusoidal vertical linear acceleration

Human subjects were vertically oscillated in the dark over a frequency range of 1-0.02 HZ at peak-to-peak acceleration amplitudes of 0.2-0.6 g, using both a vertical movement simulator and real flight. Subjective tracking of the movement was unreliable and showed no systematic dependence of phase upon frequency. In contrast, reflex oculomotor response, although weak, clearly demonstrated progressive and substantial phase lag with increasing frequency. The similarity of this characteristic to that previously obtained from brainstem neural responses in cat suggests the oculomotor response was primarily of a vestibular origin. The in-flight studies demonstrated that head movement in a changing linear accelerative field, especially at 0.1 Ha, is highly provocative of motion sickness despite subthreshold angular movement of the aircraft.