By antidromic stimulation in the superior colliculus of neurons in the arcuate frontal eye fields, it was determined that cells bearing eye movement or fixation signals but not those with peripheral visual information project to the superior colliculus from the frontal cortex. This implies that the cerebral cortex sends only a motor message to the brainstem saccadic eye movement system. Monkeys trained on a saccadic adaptation paradigm learn quickly to change the amplitude of their saccades in response to intrasaccadic stimulus steps. Stimulation of the superior colliculus in the adapted case yields the same saccades as the unadapted case. The activity of single neurons also shows no evidence of adaptation: the visual and movement signals emanating from the colliculus are the veridical signals for stimulus and movement, rather than movement signals in a visual frame which are modified by the brainstem. Thus the change in saccade amplitude which occurs as a result of adaptation is compensated for at early sites in the neural chain from stimulus to response. A small percentage of humans with reading disorders have fixational instability manifest by saccadic intrusions or square-wave jerks while they attempt to fixate a spot of light. A single dose of methylphenidate decreases or eliminates the saccadic intrusions and results in temporary improves in both the ocular mechanics of oral reading and the reading performance itself.
