Change in straight ahead during adaptation to prism

Abstract

The course of adaptation to prism was examined by recording S's localization of (a) an object which was objectively straight ahead and (b) straight ahead. It was found that, as the object of regard changed in apparent position and came to be localized correctly, straight ahead changed concomitantly with it. Mean data for 20 Ss with a 25 diopter prism showed that, of the total change in localization during adaptation to the prism, 71% resulted from a concomitant change in the test stimulus and straight ahead, while only 29% could be attributed to a reduction in the disparity between the test stimulus and straight ahead. Introduction If an individual has one eye occluded and a prism in front of his other eye, the object of regard will be optically displaced. This can be demonstrated by having him reach for or point toward the object under conditions such that he cannot see his hand. He will be inaccurate in reaching or pointing, the direction and magnitude of his error being determined (within a margin of error) by the orientation and deviation-angle of the prism. Over one hundred years ago, Czermak (1863) showed that the error in visual localization produced by a prism could be eliminated by having S touch the object of regard repeatedly while seeing both his hand and the object. In the present experiment, this phenomenon-prism adaptation-was investigated by following the course of adaptation in terms of S's localization of (a) an object which is straight ahead, and (b) straight ahead. Apparatus The visual display consisted of a vertical luminous white line 15 cm high and 0.2 cm wide, seen against a uniform black background at a viewing distance of 33 cm and equidistant from S's two eyes. The bottom of the line was 6 cm belowS'shorizontalline of sight. The S, using a chin rest, had his left eye occluded and a 25 diopter prism base in (i. e., base to the left) before his right eye. (A 25 diopter prism produces a deviation of 8.33 cm on a tangent scale 33 cm distant.) The apparatus was in a darkened room. Two lamps, shielded from S's direct view, illuminated the visual display. These lamps could be turned on and off by the experimenter. With the lamps on, S could see his hand and the pointer as well as the line. With the lamps off, he could see only the line. The S's task was to move a pOinter along a horizontal track perpendicular to his line of sight. The pointer was tapered upward, and its tip was 0.1 cm below the G······ !!] Stf~lgJlt ahtad d,uU 0" ••-0 On U te Ime dark) On tre line h&nts on) Fig. 1. Results for one S. Each point represents one setting of the pOinter. The zero point on the ordinate represents the actual position of the stimulus line. Points above zero represent settings to S's left; points below zero represent settings to S's right. The thickened portion of the ordinate represents the direction and magnitude of the deviation produced by the prism. The solid line connects consecutive pointer settings. The accuracy of "line" settings with the light on is attributable to the fact the S, being able to see both pOinter and line, could align them precisely. vertical white line. The experimenter was able to read the pOSition of the pointer to the nearest 0.1 cm to right or left of the center of the line. Procedure Prior to each experiment, S's inter-pupillary distance was measured by a standard clinical technique (Kramer, 1953), and the horizontal distance between the prism and the chin rest was adjusted. The height of the chin rest was adjusted, and S was shown how to move the pointer. During each experiment, S made two types of settings: "on the line," in which the aim was to place the pointer directly beneath the line; and "straight ahead," in which the aim was to place the pointer exactly straight ahead. Two settings ("line" and "straight ahead") in the dark were followed by one setting ("line") with the lights on. (As in Czermak's original observation, the settings in which S was unable to see his hand constituted measurements of localization; and the settings with hand visible were adaptation trials.) This sequence was repeated, without interruption, until S's "line" setting in the dark was within 0.5 cm of the center of the line-i. e., until the error in localization had been reduced to 6% of the prism deviation. If S had not achieved this criterion after 20 adaptation trials, the experiment was terminated. 107