Publications from 1995-1996

Titles:

Effects of Body Orientation and Retinal Image Pitch on the Perception of Gravity-Referenced Eye-Level (GREL)

The Effects of Environmental Pitch on Perceived Optic Slant and Eye Level: Lines vs Dots

Accuracy and Adaptation of Reaching and Pointing in Pitched Visual Environments

The effects of Pictorial Realism, Delay of Visual Feedback, and Observer Interactivity on the Subject Sense of Presence

Is there Dissociation of Perceptual and Motor Responses to Figural Illusions?

Elevator Illusion and Gaze Direction in Hypergravity

Reduction of the Elevator Illusion from Continued Hypergravity Exposure and Visual Error-corrective Feedback

Vulnerability to Height Fears

Effects of Body Orientation and Retinal Image Pitch on the Perception of Gravity-Referenced Eye-Level (GREL)

M. M. Cohen, L. T. Guzy

INTRODUCTION: It has been asserted that the pitch orientation of a visual array and of an observer's body jointly determine the perception of GREL. The current study formally tests this assertion over an extended range with multiple combinations of visual and body pitch orientations. METHODS: Ten subjects were individually secured in a CircOlectric bed surrounded by a room (pitch room) with walls that could be pitched at various angles with respect to gravity. The bed and the walls of the room were independently adjusted to each of five positions relative to gravitational vertical: -15, -7.5, 0, +7.5, and +15 degrees, yielding 25 combinations of body x room pitch angles, and retinal image pitch (RIP) conditions ranging from -30 to +30 degrees. Each subject set a target to apparent GREL while viewing it against a background of two electroluminescent strips on the outer edges of the far wall of the room. RESULTS: As determined by ANOVA, the orientation of the room, and its interaction with that of the observer, significantly altered GREL (p<0.01). CONCLUSION: Regression analysis showed that GREL was best described as a linear summation of the weighted independent contributions from a body-referenced mechanism (B) and a visual mechanism given by the orientation of the background array on the retina (RIP). The equation for this relationship is: GREL = .74 (B) + .64 (RIP) - 1.42; r-squared = .994.

Abstract from Aviation, Space, and Environmental Medicine, 1995, 66:505.

The Effects of Environmental Pitch on Perceived Optic Slant and Eye Level: Lines vs Dots

A. E. Stoper, M. M. Cohen , J. Randle

Visually perceived eye level (VPEL) has been shown to be strongly affected by the pitch of the visible environment (Stoper and Cohen, 1989, Perception and Psychophysics, 46:469-475 ), even if this environment consists only of two luminous lines pitched from the vertical ( Matin and Li, 1992, JEP:HP&P, 18:257-289). Here, two luminous vertical lines or 32 randomly distributed luminous dots were mounted on a plane that was viewed monocularly and was pitched (slanted in the pitch dimension) 30 deg. forward or backward from the vertical. In addition to measuring VPEL, we measured the perceived optic slant (rather than the perceived geographic slant) of this plane by requiring each of our 10 subjects to set a target to the visually perceived near point (VPNP) of the plane. We found that, for the lines, VPNP shifted 50% and VPEL shifted 26% of the physical pitch of the plane. For the dots, VPNP shifted 28%, but VPEL shifted only 8%. The effect of the dots on VPEL was weaker than might have been predicted by their effect on VPNP, which was used to indicate perceived optic slant. The weakness of this effect with the dots implies that changes in VPEL result from a direct effect of stimuli, rather than one mediated by the perceived pitch of the plane. The non-zero effect of the dots shows that pitched from vertical line segments are not necessary to shift VPEL.

Paper presented to the 19th European Conference on Visual Perception, Strasbourg, France, September 7, 1996.

Accuracy and Adaptation of Reaching and Pointing in Pitched Visual Environments

Robert B. Welch, Robert B. Post

Visually perceived eye level (VPEL) and the ability of subjects to reach with an unseen limb to targets placed at VPEL were measured in a statically pitched visual surround (pitchroom). VPEL was shifted upward and downward by upward and downward room pitch, respectively. Accuracy in reaching to VPEL represented a compromise between VPEL and actual eye level. This indicates that VPEL shifts reflect in part a change in perceived location of objects. When subjects were provided with terminal visual feedback about their reaching, accuracy improved rapidly. Subsequent reaching, with the room vertical, revealed a negative aftereffect (i.e., reaching errors that were opposite those made initially in the pitched room). In a second study, pointing accuracy was assessed for targets located both at VPEL and at other positions. Errors were similar for targets whether located at VPEL or elsewhere. Additionally, pointing responses were restricted to a narrower range than that of the actual target locations. The small size of reaching and pointing errors in both studies suggests that factors other than a change in perceived location are also involved in VPEL shifts.

Perception & Psychophysics 1996, 58 (3), 383-389

The effects of Pictorial Realism, Delay of Visual Feedback, and Observer Interactivity on the Subject Sense of Presence

Robert B. Welch, Theodore T. Blackmon, Andrew Liu2, Barbra A. Mellers, and Lawrence Stark

Two experiments examined the effects of pictorial realism, observer interactivity, and delay of visual feedback on the sense of "presence." Subjects were presented pairs of virtual environments (a simulated driving task) that differed in one or more ways from each other. After subjects had completed the second member of each pair they re ported which of the two had produced the greater amount of presence and indicated the size of this difference by means of a l - l OO scale. As predicted, realism and interactivity increased presence while delay of visual feedback diminished it. According to subjects' verbal responses to a postexperiment interview, pictorial realism was the least influential of the three variables examined. Further, although some subjects reported an increase in the sense of presence over the course of the experiment, most said that it had remained unchanged or become weaker.

Presence, Vol. 5, No. 3, Summer 1996, 263-273 © 1996 by the Massachusetts Institute of Technology

Is there Dissociation of Perceptual and Motor Responses to Figural Illusions?

Robert B Post, Robert B Welch

Open-loop reaching for locations within figural illusions was measured in three experiments. The experiments differed with respect to whether subjects were provided a visible target toward which to direct their reaching or were required to form a mental representation of the intended target. In the first experiment, subjects' reaching errors for vertices of a Muller-Lyer figure were similar to those for a nonillusory control stimulus. In experiment 2, subjects' errors while reaching to the imaginary bisector of the Judd illusion were consistent with the presence of an illusion of bisector location. However, when a bisector line was added to the Judd figure, reaching errors were similar to those obtained with a control figure. In experiment 3, subjects open-loop reaching at the perceived midpoint of a triangle was biased toward its illusory perceptual midpoint. When a mark was placed at the midpoint between a vertex and the opposite side, reaching errors were similar to those obtained with a control figure. The results of the experiments indicate that hand-eye coordination is biased in the direction of illusions of bisector location only when no target is present at the intended goal of the reaching response and subjects are required instead to form a mental image of the target. Under these conditions, reaching responses appear to utilize the spatial map of the visual system, and are influenced by figural illusions of bisector location. The present data can be understood without invoking the notion of visual-motor dissociation.

Perception, 1996, volume 25, pages 569-581

Elevator Illusion and Gaze Direction in Hypergravity

M. M. Cohen

INTRODUCTION: A luminous visual target in a dark hypergravity (Gz>1) environment appears to be elevated above its true physical position. This "elevator illusion" has been attributed to changes in oculomotor control caused by increased stimulation of the otolith organs. Data relating the magnitude of the illusion to the magnitude of the changes in oculomotor control have been lacking. The present study provides such data. METHOD: Sixteen paid subjects were exposed to 1.0, 1.5, and 2.0 Gz on the human-rated centrifuge at NASA-Ames Research Center. The subjects viewed a small illuminated target in an otherwise darkened gondola, and adjusted the elevation of the target so that it appeared to be at their horizon; they also attempted to direct their gaze to the horizon, both in the dark and with the target illuminated. An ISCAN infra-red video camera system was used to the record the actual elevation of the subjects' eyes. RESULTS: Analyses of variance revealed that settings of the target (F=31.43, df 2/30, p<0.001), of the eyes when viewing the target (F=31.64, df 2/30, p<0.001), and of the eyes in total darkness when no target was viewed (F=29.87, df 2/30, p<0.001) all changed with Gz according to the same linear relationship of the form: Elevation = m (Gz) + b, where m ranged between -5.80 and -6.40 degrees per Gz, and b ranged between 3.61 and 4.84 degrees. The equation relating eye elevation and target elevation was simply: eye elevation = target elevation (r-square = .95). CONCLUSIONS: These data strongly suggest that the elevator illusion is a direct result of changes in eye elevation, and that subjects misjudge the elevation of the target simply because they misjudge the elevation of their eyes in their heads.

Abstract from Aviation, Space & Environmental Medicine, 1996, 67:676.

Reduction of the Elevator Illusion from Continued Hypergravity Exposure and Visual Error-corrective Feedback

Robert B. Welch, Malcolm M. Cohen, and Charles W. DeRoshia

Ten subjects served as their own controls in two conditions of continuous, centrifugally produced hypergravity (+2 Gz) and a 1-G control condition. Before and after exposure, open-loop measures were obtained of (1) motor control, (2) visual localization, and (3) hand-eye coordination. During exposure in the visual feedback/hypergravity condition, subjects received terminal visual error-corrective feed back from their target pointing, and in the no-visual feedback/hypergravity condition they pointed open loop. As expected, the motor control measures for both experimental conditions revealed very short lived underreaching (the muscle-loading effect) at the outset of hypergravity and an equally transient negative aftereffect on returning to 1 G. The substantial (approximately 17° initial elevator illusion experienced in both hypergravity conditions declined over the course of the exposure period, whether or not visual feedback was provided. This effect was tentatively attributed to habituation of the otoliths. Visual feedback produced a smaller addition al decrement and a postexposure negative aftereffect, possible evidence for visual recalibration. Surprisingly, the target-pointing error made during hypergravity in the no-visual-feedback condition was substantially less than that predicted by subjects' elevator illusion. This finding calls into question the neural outflow model as a complete explanation of this illusion.

Perception & Psychophysics 1996, Jan;58(1):22-30.

Vulnerability to Height Fears

L. Shevalier, P. A. DI Nardo, L. T. Guzy & S. J. Gilbert

Contemporary models of specific phobia emphasize the interaction of vulnerability factors in the development of phobias. Using questionnaire measures of space and motion discomfort (SMD), anxiety sensitivity (ASI), and acrophobia in a group of college females, a multiple regression analysis showed that the combined SMD and ASI factor accounted for a greater proportion of variance in acrophobia scores that either SMD or ASI separately. Implications for understanding etiological models of specific phobia are discussed.

Paper presented to the Eastern Psychological Association, Philadelphia, PA, March 1996.