Discharge of pursuit-related neurons in the caudal part of the frontal eye fields in juvenile monkeys with up-down pursuit asymmetry.

The smooth-pursuit system uses retinal image-slip-velocity information of target motion to match eye velocity to actual target velocity. The caudal part of the frontal eye fields (FEF) contains neurons whose activity is related to direction and velocity of smooth-pursuit eye movements (pursuit neurons), and these neurons are thought to issue a pursuit command. During normal pursuit in well-trained adult monkeys, a pursuit command is usually not differentiable from the actual eye velocity. We examined whether FEF pursuit neurons signaled the actual eye velocity during pursuit in juvenile monkeys that exhibited intrinsic differences between upward and downward pursuit capabilities. Two, head-stabilized Japanese monkeys of 4 years of age were tested for sinusoidal vertical pursuit of target motion at 0.2-1.2 Hz (+/-10 degrees, peak target velocity 12.5-75.0 degrees/s). Gains of downward pursuit were 0.8-0.9 at 0.2-1.0 Hz, and peak downward eye velocity increased up to approximately 60 degrees/s linearly with target velocity, whereas peak upward eye velocity saturated at 15-20 degrees/s. The majority of downward FEF pursuit neurons increased the amplitude of their discharge modulation almost linearly up to 1.2 Hz. The majority of upward FEF pursuit neurons also increased amplitude of modulation nearly linearly as target frequency increased, and the regression slope was similar to that of downward pursuit neurons despite the fact that upward peak eye velocity saturated at approximately 0.5 Hz. These results indicate that the responses of the majority of upward FEF pursuit neurons did not signal the actual eye velocity during pursuit. We suggest that their activity reflected primarily the required eye velocity.