Ashton Graybiel

THE DELAY IN VISUAL REORIENTATION FOLLOWING EXPOSURE TO A CHANGE IN DIRECTION OF RESULTANT FORCE ON A HUMAN CENTRIFUGE.

Abstract : Three subjects were exposed on a human centrifuge to a change in direction of resultant G relative to the body axis. Under the conditions of the experiment the illusion was created wherein a horizontal target line appeared to rotate through an angle corresponding to the change in direction of the resultant force. The subjects task was to keep the line horizontal at all times. Thus, if the line appeared to rotate clockwise, the subject actually rotated the line counterclockwise in order to maintain the line horizontal for himself. A marked discrepancy was noted between the time required to impress the physical force on the subject and the period during which he found it necessary to make adjustments in keeping the line horizontal. This discrepancy was regarded as a measure of the delay in the subjects visual reorientation to a change in direction of resultant G. It is thought that a possible cause of the delay may lie in the characteristic behavior of the otolith organ. This lag phenomenon is of importance to aviation inasmuch as the full disorientating effects of a change in direction of resultant G relative to the body axis will not become manifest if the exposure time is short. (Author)

THE ROLE OF VESTIBULAR NYSTAGMUS IN THE VISUAL PERCEPTION OF A MOVING TARGET IN THE DARK

Abstract : Six well-trained subjects reported their visual perceptions both during and following rotation while observing a moving target in the dark and in a lighted room. The subjects were rotated at speeds varying from 2 to 15 rpm both to the right and left, while the target described a circular path 2 meters distant and 15 degrees above the level of the subjects eyes. The subjects were asked to give a qualitative description of their own sensations of rotation and of the apparent movement of the movement of the model plane during and following rotation. When a subject was accelerated to 15 rpm in the dark, there was a rapid displacement of the target in the opposite direction, although, at the same time, as a result of nystagmus the target appeared motionless. This paradox made it necessary for the subjects to distinguish between rate of motion and amount of displacement. As the target went by on successive rotations, it appeared to pass more rapidly until, after approximately 30 seconds, the subject felt himself to be stationary while the target rotated rapidly around him. Similar results occurred at slower speeds or rotation. The post-rotation phenomena were stronger, since the deceleration was more rapid. These phenomena which did not occur in a lighted room can be considered as a summation of the effects of real motion of the target, vestibular nystagmus, and the subjects sensations of their own motion.

Apparent Rotation of a Fixed Target Associated by Linear Acceleration in Flight

Abstract : The purpose of this study was to determine the effects of linear acceleration and deceleration on the visual perception of a target in the dark. Three subjects observed a collimated star in the dark while the pilot of an SNJ-6 aircraft executed various degrees of linear acceleration. When the subjects faced to the left in the aircraft the radiating lines of the star appeared to rotate about the central point to a new position. This rotation was clockwise during deceleration and counter clockwise during acceleration. As the force became smaller, the star appeared to rotate back to its normal position. This illusory rotation occurred at all stimulus levels. The thresholds of linear deceleration for motion and displacement were 0.067 G and 0.078 G respectively. All of the linear accelerations used were above threshold. The mean maximum rotation during deceleration increased as the decelerative force increased, the maximum estimated rotation reported being 15 degrees. There was also a positive relation between the maximum force and the duration of the illusory effect. The accelerative forces did not show a consistent variation with the maximum change in the accelerative force. The results are similar to those observed on a human centrifuge and show that linear acceleration and deceleration during flight have a marked influence upon visual perception in the dark. (Author)

Egocentric localization of the visual horizontal in normal and labyrinthine-defective observers as a function of head and body tilt

Perception of the visual horizontal by observers in five different combinations of head and body position was studied to determine the effect of 20-degree body tilts. Both normal and labyrinthine-defective observers made five settings to the visual horizontal for each condition using, a goggle device which presented a collimated line of light to the right eye while the other eye was covered. The results showed no significant constant errors in the settings by either group, and it is suggested that the absence of the E-phenomenon was due primarily to adequate contact cues and kinesthetic cues. The data also make it clear that vestibular information is not required for veridical perception of the visual horizontal under these experimental conditions.