Characteristics of saccades induced by neck torsions: a re-examination in the normal guinea pig.

Torsion of the neck relative to the fixed head results in several reflexes involving the eyes, the neck and the body. One of these reflexes, the cervico-ocular reflex, has been described as having a small gain in the normal animal. However, when the body of a guinea pig is moved relative to the fixed head with a ramp-like velocity profile, saccades are systematically elicited in the direction of body movement. We re-examined the characteristics of this reflex in the normal guinea pig and demonstrated that: (1) it occurs mainly in the range of high velocity body movements; (2) the latency of the saccades is shorter than previously suspected; (3) the saccades are triggered at specific positions relative to the starting and ending positions of rotation, revealing some degree of flexibility in the reflex. We hypothesize that these saccades of nuchal origin are under the control of the same neuronal circuit as visually triggered saccades and quick phases of vestibular nystagmus. Thus, this nuchal reflex may fundamentally subserve orienting behaviour in normal animal.