A natural scene may contain multiple objects which can compete for a response. Selection of a response is handled reflexively by animals more primitive than humans. A frog will flick its tongue toward a small object that moves in its world. Alternatively, large looming objects cause the frog to take evasive action. A reflexive response is not always adaptive, since the small object could be a fishhook, and the large object a tree blowing in the wind. During evolution, the primate brain developed a neocortex that provides for more adaptive behavior, partially by affording better discrimination between objects. Better discrimination allows a more selective response, as well as suppression of responses that are inappropriate.

Our studies of the supplementary eye fields (SEF) in the primate frontal cortex focus on how decisions are made to respond or not respond to a moving object. The SEF is part of a complex of frontal brain regions associated with the readiness, or bereitschafts potential, a scalp-recorded electrical signal that occurs before movement (1; 2). Interestingly, this potential occurs also when a planned movement is cancelled, indicating that it is more associated with preparing than executing the response. Work in our laboratory targets more specifically the role of the SEF in movement preparation. We have shown that this area is involved in generating anticipatory smooth pursuit (3), eye movements that precedes target motion, reducing response time. Our new studies focus on activity accompanying the decision of whether or not to act upon a moving object. To this end, we have devised a paradigm adopted from the sport of American baseball.

In traditional baseball, a player swings a bat at a moving ball. A contingency on swinging the bat is that the ball cross a "strike zone," a small volume of space in front of the player. In our baseball paradigm, the player uses an eye movement to follow a small spot of light moving on a video screen. The strike zone is a rectangular image centered around an illuminated fixation spot. The player in "eye movement" baseball fixates this spot while another spot begins to move at the edge of the screen. If the moving spot enters the strike zone ("strike"), the player must pursue it. If the spot crosses the front edge of the strike zone without entering it ("ball"), the player must maintain gaze on the fixation point, and suppress the eye movement. Discrete populations of neurons in the SEF respond for strikes or balls. "Strike neurons" have early activity during movement preparation and activity during movement execution. The activity of "ball neurons" however, accompanies suppression of an eye movement to a moving visual target, both when the target starts to move and when it crosses the front edge of the strike zone.