CEREBRAL CORTICAL MECHANISMS FOR EYE MOVEMENTS AND VISUAL ATTENTION

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The activity of single neurons in the prefrontal cortex that projects to the frontal eye fields has been studied in a number of visual and oculomotor tasks. Neurons in this region are visually responsive and show two sorts of activity before the beginning of a saccade: presaccadic enhancement and presaccadic reactivation. These results suggest that the prefrontal cortex participates in the planning of visually guided saccades. Monkeys trained on a short saccadic adaptation paradigm learn quickly to change the amplitude of their saccades. Stimulation of the superior colliculus in the short term adapted case yields the same saccades as the unadapted case. The activity of some single neurons in the superior colliculus shows evidence of this adaptation: When a monkey makes a saccade of adjusted amplitude in response to a visual stimulus, some collicular neurons discharge before saccades ordinarily not associated with activity from those neurons. These observations were extended to a longer-term adaptation paradigm. One or more extraocular muscles were weakened by injection of botulinum toxin, and the non-paretic eye masked. The monkeys adapted the gain of their eye movements so that the seeing, weak eye made close to normal saccades and the non-seeing, normal eye made saccades of much larger amplitude than normal. Stimulation of the superior colliculus resulted in the production here too of saccades that did not reflect the adaptation process. A class of patients with formal reading scores within the normal range who nonetheless consider themselves to be poor readers were found to have a higher than normal amount of unwanted rapid eye movements (square wave jerks) during visual fixation, and a higher than normal amount of backward eye movements during reading. Fixational instability did not correlate with performance on a quantitative assessment of skeletal motor functions which correlates with attentional deficit disorder in children.