CEREBRAL CORTICAL MECHANISMS FOR EYE MOVEMENTS AND VISUAL ATTENTION

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The frontal eye fields of the cerebral cortex contain three different populations of neurons which are active before visually guided saccades: visual neurons responsive only to light, movement neurons active before purposive saccades, and visuomovement neurons which have both visual and movement activity. Since the frontal eye fields are known to project to the superior colliculus by anatomical methods, antidromic electrical stimulation from the superior colliculus was performed on behaviorally and physiologically characterized frontal neurons to see if any cell types projected preferentially. Of the antidromically activated neurons, two thirds were movement or visuomovement with very strong movement activities. Fewer than 2% were purely visual. One third of the neurons discharged during active fixation or were excited by the signal to make a saccade, but had no specific movement field. The frontal eye fields occupy parts of two classically defined cytoarchitectonic areas, 8a and 45. Physiological identification of the borders of the frontal eye fields and quantitative measurement of cell size revealed a new cytoarchitectonic region whose hallmark is large neurons in Layer V. This region is coterminous with the physiologically identified frontal eye fields and bridges the border between areas 8a and 45. Monkeys can be trained to adjust the gain of saccades within a few hundred trials. When a monkey has been trained to make an adapted saccade of a certain dimension, stimulation of the superior colliculus results in the generation of an unadapted rather than an adapted succade.