The Neural Control of Visually Guided Eye Movements
D. Temporal Factors in Visually Guided Saccadic Eye Movements

Figure 29 delineates the conventions we shall use in presenting data about express saccades in this section. We use a coordinate system in which each position on the screen is defined by two numbers: the radial angle in degrees (0° is horiontal left, 90° is up, 180° is left and 270° is down) and the linar distance from the center of the screen in degrees of visual angle. Thus 0/0 designates the center of the screen where the fixation spot (white disk) is located in the upper display (A). The red target (a) is positioned at 225/5. The vector of the saccade made to it is 225/5 (direction/amplitude). In the lower display (B) the white fixation spot is placed at 180/7. Two targets are shown, a and b. Target a is in the same position as in the upper display (225/5). An accurate saccade made to this target now, starting from the new fixation spot has a vector of 315/5. The second target, b, is located at 200/11. The saccadic vector to this target b is 225/5. Thus while the fixation spot and target b in the lower display are in different positions from those in the upper display, the saccadic vector to target b is the same as that made to target a in the upper display.

We shall examine the express saccade phenomenon by posing a series of questions about the conditions under which they do and do not arise and will then proceed to make inferences about the brain structures involved. All the data shown were obtained in monkeys.