Malcolm M. Cohen

Effect of structured visual environments on apparent eye level

Each of 12 subjects set a binocularly viewed target to apparent eye level; the target was projected on the rear wall of an open box, the floor of which was horizontal or pitched up and down at angles of 7.5° and 15°. Settings of the target were systematically biased by 60% of the pitch angle when the interior of the box was illuminated, but by only 5% when the interior of the box was darkened. Within-subjects variability of the settings was less under illuminated viewing conditions than in the dark, but was independent of box pitch angle. In a second experiment, 11 subjects were tested with an illuminated pitched box, yielding biases of 53% and 49% for binocular and monocular viewing conditions, respectively. The results are discussed in terms of individual and interactive effects of optical, gravitational, and extraretinal eye-position information in determining judgments of eye level.

Elevator illusion: Influences of otolith organ activity and neck proprioception

Each of nine Ss experienced gravitational-inertial forces (GIFs) of 1.00, 1.25, 1.50, and 1.75 Gz with his head erect and pitched forward at angles of 15 and 30 deg. The magnitude of the elevator illusion was shown to depend on both the intensity of the GIFs and the orientation of the head. Contributions of otolith organ activity and of neck proprioception to the illusion were examined, and theoretical baseg for their influences were discussed.

Judgments of eye level in light and in darkness

Subjects judged eye level in the light and in the dark by raising and lowering themselves in a dental chair until a stationary target appeared to be at the level of their eyes. This method reduced the possibility of subjects’ using visible landmarks as reference points for setting eye level during lighted trials, which may have contributed to artificially low estimates of the variability of this judgment in previous studies. Chair settings were 2.5° higher in the dark than in the light, and variability was approximately 66% greater in the dark than in the light. These results are discussed in terms of possible interactions of two separate systems, one sensitive to the orientations of visible surfaces and the other sensitive to bodily and gravitational information.

Visual and somesthetic influences on postural orientation in the median plane.

We investigated optic and somesthetic contributions to perceived body orientation in the pitch dimension. In a within-subject factorial design, each of 12 subjects attempted to set his/her body erect or 45° back from erect while restrained in a movable bed surrounded by an adjustable box The box provided a visual environment consisting of either a grid pattern, two luminous lines, or complete darkness. Both the grid pattern and the luminous lines were effective at-biasing settings of body position when the box was pitched; the pitched grid was more effective than the pitched lines. Although the pitch of the box influenced orientation to both goals, the effect was greater for the diagonal goal than for the erect goal. We present a model of postural orientation in the median plane that involves vestibular, somatosensory, and visual inputs.

Reduction of the elevator illusion from continued hypergravity exposure and visual error-corrective feedback

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 feedback 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 additional 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.

Effects of optical pitch on oculomotor control and the perception of target elevation

In two experiments, we used an ISCAN infrared video system to examine the influence of a pitched visual array on gaze elevation and on judgments of visually perceived eye level. In Experiment 1, subjects attempted to direct their gaze to arelaxed or to ahorizontal orientation while they were seated in a room whose walls were pitched at various angles with respect to gravity. Gaze elevation was biased in the direction in which the room was pitched. In Experiment 2, subjects looked into a small box that was pitched at various angles while they attempted simply to direct their gaze alone, or to direct their gaze and place a visual target at their apparent horizon. Both gaze elevation and target settings varied systematically with the pitch orientation of the box. Our results suggest that under these conditions, an optostatic response, of which the subject is unaware, is responsible for the changes in both gaze elevation and judgments of target elevation.

Human spatial orientation in the pitch dimension

Two experiments were conducted. In Experiment I, each of eight Ss attempted to place himself at 13 different goal orientations between prone and supine. Deviations of achieved body pitch angles from goal orientations were determined. In Experiment II, each of eight Ss attempted to align a visual target with his morphological horizon while he was placed at each of the 13 goal orientations. Changes in settings of the target were examined. Results indicate that Ss underestimate body pitch when they are tilted less than 60 deg backward or forward from the vertical, overestimate body pitch when they are nearly prone, and accurately estimate body pitch when they are nearly supine. In contrast, Ss set the visual target maximally above the morphological horizon when they are tilted 30 deg forward from the vertical. The findings are discussed in terms of common and different physiological mechanisms that may underlie judgments of these types.

Perception and action in altered gravity

Human perception and action are dramatically influenced by the gravitationalinertial forces (GIFs) in the surrounding environment. Forces due to acceleration and those due to gravity interact with one another, and have virtually identical physical effects. These forces cause novel mechanical loadings of the entire organism, and provide systematically altered stimulation to the otolith organs and the somesthetic receptors. Over the past several years, studies of human spatial orientation, visual localization, force estimation, and hand-eye coordination under a variety of conditions that involve exposure to altered GIFs have revealed how these various functions are influenced by the force environment. This presentation reviews some of our earlier studies, and presents a general framework for examining the effects of altered GIFs on both perception and action. The critical role of motor-sensory feedback in adaptation to altered GIFs is highlighted from the results of these studies.

Effects of gravitational and optical stimulation on the perception of target elevation

To examine the combined effects of gravitational and optical stimulation on perceived target elevation, we independently altered gravitational—inertial force and both the orientation and the structure of a background visual array. While being exposed to 1.0, 1.5, or 2.0 Gz in the human centrifuge at NASA Ames Research Center, observers attempted to set a target to the apparent horizon. The target was viewed against the far wall of a box that was pitched at various angles. The box was brightly illuminated, had only its interior edges dimly illuminated, or was kept dark. Observers lowered their target settings as Gz was increased; this effect was weakened when the box was illuminated. Also, when the box was visible, settings were displaced in the same direction as that in which the box was pitched. We attribute our results to the combined influence of otolith—oculomotor mechanisms that underlie the elevator illusion and visual—oculomotor mechanisms (optostatic responses) that underlie the perceptual effects of viewing pitched visual arrays.

Chapter 7 Visual-Motor Control in Altered Gravity

Publisher Summary Human perception and performance are affected by changes in the magnitude of the gravitational-inertial environment. The extra weight of the limbs during exposure to hypergravity (muscle loading) causes initial (ballistic) reaching responses to be too low, whereas the reduced weight of the limbs during exposure to hypogravity (muscle unloading) causes them to be too high. If a visual target is viewed in an otherwise dark setting under altered gravitational-inertial conditions, the elevator illusion will also occur and will induce target-pointing errors opposite in direction from those caused by muscle loading/unloading. This chapter examines the effects of alternating hypogravity and hypergravity, as produced by parabolic flight, on open-loop target-pointing. The results exhibit that adaptation is specific to those conditions under which motor-sensory feedback is provided; the subjects who pointed at the target during the hypogravic phase, but were passive during the hypergravic phase, adapted only to the hypogravic condition; likewise, the subjects who pointed at the target only during the hypergravic phase adapted to that condition. In both cases, the adaptation is based on an elimination of the effects of muscle loading/unloading.