A comparison of oculomotor and motion parallax cues of egocentric distance.

Using the head motion procedure, the apparent distance of a point of light in an otherwise dark visual field was measured under conditions in which oculomotor cues (accommodation, convergence) and absolute motion parallax were varied together and separately. It was concluded that absolute motion parallax is almost as effective a cue to distance as are oculomotor cues from monocular observation, but is not as effective as oculomotor cues from binocular observation. Evidence was also presented that the null adjustment method, used in conjunction with the head motion procedure, provides an unbiased measure of apparent distance. In a series of articles (Gogel, 1976, 1977; Gogel and Newton, 1976; Gogel and Tietz, 1973, 1974, 1977) a new method of measuring apparent distance called the head motion procedure has been described and applied to the measurement of perceived distance from several distance cues. With the head motion procedure, the head is moved in a frontoparallel plane and judgments are obtained of the apparent motion of the stimulus object concomitant with the motion of the head. From these judgments, as will be explained, the experimenter can compute the distance of the object as perceived by the observer. The head motion procedure has several important advantages over other, more direct, methods for measuring apparent distance such as that of obtaining verbal reports of apparent distance. Unlike direct methods, with the head motion procedure the observer is unaware of the relation between his judgment and apparent distance and, therefore, cannot modify the response in an attempt to be veridical. From the above studies, it is clear that the head motion procedure provides a useful and sensitive measure of apparent distance. In the present study several variations of this procedure are applied to the evaluation of the relative effectiveness of oculomotor and absolute motion parallax cues of egocentric distance. Also, evidence is presented supporting the assumption that the measures obtained from the head motion procedure are not only a monotonic increasing function of apparent distance but are indeed equal to apparent distance. Figure 1 is useful in discussing a number of aspects of the head motion procedure and its assumptions. The prime notation indicates perceived (apparent) characteristics and the notation without primes indicates physical characteristics in this and the following figures. Figure 1A illustrates the situation in which, as the head is moved repetitively left and right through a distance K between Positions 1 and 2, a point of light physically at a distance D from the observer also is moved laterally through a physical horizontal distance h (between s1 and s2) concomitant with, but in a direction opposite to, the motion of the head. The visual direction between the observer and the point of light pivots around a hypothetical point at a distance D, from the observer. As can be seen from Fig. IA, D, can be changed by changing h. The distance D, is called the pivot distance and in this example D, < D. Suppose that for some reason the perceived distance D’, of the point of light is less than D, In this case, as shown in Fig. lA, the point of light will appear to move concomitantly with the head motion through an apparent distance h’ (between n’, and n’*) in the same direction as the motion of the head. On the other hand, if the perceived distance of the point D’, is greater than D, the apparent concomitant motion h’ will be opposite to the head motion (between f,’ and f2’). It follows from Fig. 1 that: D’ = D,(K h’)/K. (I) In equation I it is assumed that the observer correctly senses the distance K through which the head is moved and the change in direction to the point of light & = 4, + & resulting from the head motion. The value of h’ in equation I is taken as positive when the apparent concomitant motion is in the direction of the head motion and as negative when it is opposite to that of the head motion. Figure 1B is similar to Fig. 1A except that a physical vertical component of concomitant motion, [ is added to the physical horizontal component of con‘comitant motion, h, of the point of light. With the phase of the components of physical motion h and V illustrated in Fig. lB, the point moves physically at an angle a from the horizontal, between lower left and upper right, as the head moves between Positions 1 and 2. Also as shown, if the perceived distance of the point of light is less than D,, (e.g. at D’n), the point will appear to move at an angle a’ concomitantly with the head, between lower right and upper left (a’ > 9P). If the perceived distance of the point of light is greater than D, (e.g. at D’,) the point will appear to move concomitantly with the head betweeen lower left and upper right (a’ < 90’). It follows from Fig. 1B that