Spasm of fixation: a quantitative study.

Spasm of fixation, consisting of impaired initiation of saccades in the presence of fixation target, but normal initiation in the absence of a fixation target, was measured in a patient with cerebral hemispheric damage. When a central target was constantly present, the patient made horizontal saccades to the sudden appearance of a second target at very prolonged latencies (mean 369 ms). In the absence of a central fixation target, saccadic latency decreased to normal (197 ms). Extinction of a target for a gap interval elicited very short latency movements (122 ms), termed express saccades. The intervals between self-paced horizontal refixation saccades with the head immobile were prolonged, whereas voluntary refixation saccades with the head free to move occurred at shorter intervals. We postulate that cerebral hemispheric damage may cause spasm of visual fixation by disinhibiting the substantia nigra pars reticulata, thereby inhibiting the superior colliculus.