Eye Tracking of Observer-Generated Target Movements

When an observer moves his arm he shows more precise visual tracking of a target mounted on his fingertip-the eye lags behind the target less and makes fewer corrective saccades-than when he relaxes his arm and the experimenter moves it in a similar manner. Apparently the control system for eye movements can use outflow (efferent) signals in order to anticipate motion of the self-moved target. When an observer attempts to keep his line of sight fixed on a moving target, his eyes perform tracking movements. The eye can remain on target with little or no lag when the target moves periodically at a low frequency (1, 2). If motion of the target is made aperiodic, its "predictability" decreases and tracking performance deteriorates, as indicated by increasing lag between orientation of the eye and direction of target. These results have been found with targets that move independently of control by the observer. The tracking of targets that are moved by the observer himself has not hitherto been examined, although one might expect that any additional information about target motion would aid tracking. We now report results indicating that the observer can more accurately track a target that he moves himself than a target that is moved without his control. In order to compare the accuracy of tracking self-produced target motion with that of independent motion, the observer tracked a spot on his own finger under two conditions: (i) when he himself moved his arm in an irregular manner (hereafter called the active condition), and (ii) when his relaxed arm was moved in a similar manner by an external force (passive condition). The movements of both eye and target were recorded during the two conditions of target motion. In both cases, the displacements and rates of movement of the arm were comparable, but only in the case of observerproduced movement was there additional information available and potentially useful to the oculomotor control system for tracking, namely the efferent or outflow signals to the musculature present with all voluntary movements (3). The target was a 2-mm square of black tape fastened on the fingernail of the third finger of the right hand of the observer. His right arm rested on a flat lever, pivoted near the elbow, restricting movement of the target to an arc ,in afrontal plane 30.5 cm from * Movement '0 degrees (in visual angle) to the right and left of the observer's midsagittal plane. Registration of arm (and therefore target) position was provided by a potentiometer geared to the pivot of the lever. Horizontal eye movements were recorded using a photoelectric technique yielding measurements accurate, in this instance, to one-half degree of horizontal displacement of the eye (2). The observer's head was held in position by having him bite a bar holding a wax dental impression. Although he viewed his right hand binocularly, the horizontal movements of only the right eye were recorded. Four male and two female students and employees at Massachusetts Institute of Technology were used as observers in the study. Their ages ranged from 19 to 33 years. The visual acuity of all observers was sufficient to perform the task without corrective lenses. In the active condition, the observer was instructed to look at the 'black square of tape on his fingertip and to follow it as best he could while rapidly moving his hand back and forth at an irregular rate of approximately three to four reversals of direction per second within the 20-degree range of travel of the lever. In the passive condition, the observer relaxed his arm on the lever, allowing the experimenter to move the arm through a pattern of displacements similar to the one the observer used in the active condition. The observer was again instructed to look at the target at all times. Arm (target) position and eye position were recorded simultaneously on two channels of a Sanborn recorder, operating at a paper speed of 100 mm/sec. Only the data from observers whose movements in the active condition were very similar to their movements in the passive condition (determined by counting the number of direction reversals and checking average amplitude during the first 15 seconds of recording) were kept for analysis. Figure 1 shows a typical record of hand and eye movements under active and passive conditions. For each condition the target (hand) record is on top, and maximum peak-to-trough