| Animals were rotated about earth-horizontal axes to study central mechanisms of integration of information from semicircular canal and otolith organ sensors.; Electromyographic wire electrodes were implanted in dorsal neck muscles of alert, untrained squirrel monkeys to measure vestibulocervical reflexes. Monkeys were rotated in the dark in several different vertical planes to determine the direction of rotation that produced maximal activation of individual muscles. Rotations occurred while the monkey was either upright or upside down. The phase of the electromyographic response often changed when the monkey was turned upside down. The results suggest that reflex contraction of neck muscles in response to passive head rotation includes an interplay of compensatory and righting responses that varies from animal to animal.; Small coils were implanted or acutely attached to the eyes of normal and mutant mice to measure the vestibuloocular reflex (VOR). As in other animals, rotations about earth-horizontal axes produced more accurate low-frequency VOR than rotations about earth-vertical axes, presumably due to a central neuronal computation of head angular velocity based on otolith afferent information related to changes in the direction of the head's linear acceleration. This same central otolith-based computation has been assumed to account for the continuous compensatory nystagmus generated by horizontal axis rotations of constant velocity, observed long after the signals from canal afferents have returned to normal. Lesions of the medial nodulus in mice significantly reduced this response, as had been reported previously in primates. However mutant pcd/pcd mice, which lack any nodular output, were shown to have this compensatory nystagmus, suggesting that other circuits, such as those in brain stem, can produce this neuronal computation. Three-dimensional eye movement recordings in normal mice showed torsional VOR performance relative to vertical VOR was better than that reported in humans, about equivalent to that in rabbits, and poorer than in pigeons. |
