Figure 1.

A, Modified double-step task. Arrows indicate the direction of the saccade. Trials began with the presentation of a central fixation spot. After the fixation spot disappeared, a target flashed at a noncentral location. Subjects were instructed to saccade to this location as quickly as possible. On double-step trials, a second target flashed at some variable delay following onset of the first target (target step delay). A target step was the cue for the subject to withhold a saccade to the first target and instead redirect gaze toward the second target. On most double-step error trials, a corrective saccade was made to the second target location. On single-step trials, a second target did not appear. Trial types were randomly interleaved. B, CD Schematic. Left, Schematic of not compensating for the change in eye position brought about by the first saccade. The gray arrow is the saccade vector from fixation to T2. The dotted arrow represents the saccade that would be made from T1 if the subject had not adjusted for change in position brought about by the first saccade (CD loss-saccade occurrence). Angle θ₁ represents the bias in the second saccade vector if the subject had not compensated at all for the first eye movement. Any saccade that fell in the direction between the ideal saccade and saccade indicating no CD was assigned a positive value. Right, Schematic of not compensating for variability in amplitude of the first saccade. The gray arrow is the saccade vector from T1 to T2. The dotted arrow represents the saccade that would be made from the endpoint of the first saccade if the subject had not adjusted for error in the endpoint of the first saccade (CD loss-saccade amplitude). Angle θ₂ represents the bias in the second saccade vector if the subject had not compensated at all for spatial error in the first saccade endpoint. Any saccade that fell in the direction between the ideal saccade and saccade indicating no CD was assigned a positive value.