Co-ordination of head and eyes in the gaze changing behaviour of cats

1. Saccadic changes of gaze have been studied in conscious cats implanted with electro-oculographic electrodes to register eye position and held in a device for measuring head position.2. When the head is free to move, almost all saccades larger than 4 deg in amplitude are accompanied by head rotation. This head movement starts at a variable delay after the beginning of the eye movement. Most frequently, the latency is 30 msec.3. After finishing its saccade, the eye rotates retrogressively in the orbit so as to maintain a constant direction of gaze during the remaining portion of the head movement. This compensatory eye movement has an average gain of 0.96 in the absence of visual feed-back.4. The duration and peak velocity characteristics of head-fixed saccades were measured. A typical horizontal saccade of 20 deg in amplitude attains a peak velocity of 250 deg/sec and lasts 110 msec. The duration-amplitude relationship increases by 2.1 msec/deg.5. Saccades aimed obliquely within the orbit possess the same duration/peak-velocity/amplitude relationships as purely horizontal saccades. Consequently, the horizontal component of an oblique saccade lasts longer, and has a lower peak velocity, than a purely horizontal saccade of the same amplitude as this component. This finding is discussed in relation to brain-stem control of oblique saccades.6. A saccade is initially programmed to achieve the desired gaze change without aid from the head. Consequently, the saccade is slowed down so as to prevent gaze overshoot when the head moves too. Since this saccadic attenuation occurs even during passive head movement, it must be due to feed-back from peripheral sensors of head rotation, and probably represents the initial stage of the compensatory eye movement.7. Since the axes of horizontal head and eye rotation are horizontally separated, it may be predicted that the gain of compensatory eye movements must rise as the viewing distance decreases if they are to maintain fixation during head movements. This prediction was confirmed.