The effect of long-term exposure to microgravity on the perception of upright.

Going into space is a disorienting experience. Many studies have looked at sensory functioning in space but the multisensory basis of orientation has not been systematically investigated. Here, we assess how prolonged exposure to microgravity affects the relative weighting of visual, gravity, and idiotropic cues to perceived orientation. We separated visual, body, and gravity (when present) cues to perceived orientation before, during, and after long-term exposure to microgravity during the missions of seven astronauts on the International Space Station (mean duration 168 days) and measuring perceived vertical using the subjective visual vertical and the perceptual upright. The relative influence of each cue and the variance of their judgments were measured. Fourteen ground-based control participants performed comparable measurements over a similar period. The variance of astronauts' subjective visual vertical judgments in the absence of visual cues was significantly larger immediately upon return to earth than before flight. Astronauts' perceptual upright demonstrated a reduced reliance on visual cues upon arrival on orbit that re-appeared long after returning to earth. For earth-bound controls, the contributions of body, gravity, and vision remained constant throughout the year-long testing period. This is the first multisensory study of orientation behavior in space and the first demonstration of long-term perceptual changes that persist after returning to earth. Astronauts showed a plasticity in the weighting of perceptual cues to orientation that could form the basis for future countermeasures.

Asymmetrical representation of body orientation.

The perceived orientation of objects, gravity, and the body are biased to the left. Whether this leftward bias is attributable to biases in sensing or processing vestibular, visual, and body sense cues has never been assessed directly. The orientation in which characters are most easily recognized--the perceived upright (PU)--can be well predicted from a weighted vector sum of these sensory cues. A simple form of this model assumes that the directions of the contributing inputs are coded accurately and as a consequence participants tilted left- or right-side-down relative to gravity should exhibit mirror symmetric patterns of responses. If a left/right asymmetry were present then varying these sensory cues could be used to assess in which sensory modality or modalities a PU bias may have arisen. Participants completed the Oriented Character Recognition Test (OCHART) while manipulating body posture and visual orientation cues relative to gravity. The response patterns showed systematic differences depending on which side they were tilted. An asymmetry of the PU was found to be best modeled by adding a leftward bias of 5.6° to the perceived orientation of the body relative to its actual orientation relative to the head. The asymmetry in the effect of body orientation is reminiscent of the body-defined left-leaning asymmetry in the perceived direction of light coming from above and reports that people tend to adopt a right-leaning posture.

Enhancing visual cues to orientation: suggestions for space travelers and the elderly.

Establishing our orientation in the world is necessary for almost all aspects of perception and behavior. Gravity usually defines the critical reference direction. The direction of gravity is sensed by somatosensory detectors indicating pressure points and specialized organs in the vestibular system and viscera that indicate gravity's physical pull. However, gravity's direction can also be sensed visually since we see the effects of gravity on static and moving objects and also deduce its direction from the global structure of a scene indicated by features such as the sky and ground. When cues from either visual or physical sources are compromised or ambiguous, perceptual disorientation may result, often with a tendency to replace gravity with the body's long axis as a reference. Orientation cues are compromised while floating in the weightlessness of space (which neutralizes vestibular and somatosensory cues) or while suspended at neutral buoyancy in the ocean (which neutralizes somatosensory cues) and the ability to sense orientation cues may also be compromised in the elderly or in clinical populations. In these situations, enhancing the visual cues to orientation may be beneficial. In this chapter, we review research using specially constructed virtual and real environments to quantify the contribution of various visual orientation cues. We demonstrate how visual cues can counteract disorientation by providing effective orientation information.

Perceptual upright: the relative effectiveness of dynamic and static images under different gravity States.

The perceived direction of up depends on both gravity and visual cues to orientation. Static visual cues to orientation have been shown to be less effective in influencing the perception of upright (PU) under microgravity conditions than they are on earth (Dyde et al., 2009). Here we introduce dynamic orientation cues into the visual background to ascertain whether they might increase the effectiveness of visual cues in defining the PU under different gravity conditions. Brief periods of microgravity and hypergravity were created using parabolic flight. Observers viewed a polarized, natural scene presented at various orientations on a laptop viewed through a hood which occluded all other visual cues. The visual background was either an animated video clip in which actors moved along the visual ground plane or an individual static frame taken from the same clip. We measured the perceptual upright using the oriented character recognition test (OCHART). Dynamic visual cues significantly enhance the effectiveness of vision in determining the perceptual upright under normal gravity conditions. Strong trends were found for dynamic visual cues to produce an increase in the visual effect under both microgravity and hypergravity conditions.

The effect of altered gravity states on the perception of orientation.

We measured the effect of the orientation of the visual background on the perceptual upright (PU) under different levels of gravity. Brief periods of micro- and hypergravity conditions were created using two series of parabolic flights. Control measures were taken in the laboratory under normal gravity with subjects upright, right side down and supine. Participants viewed a polarized, natural scene presented at various orientations on a laptop viewed through a hood which occluded all other visual cues. Superimposed on the screen was a character the identity of which depended on its orientation. The orientations at which the character was maximally ambiguous were measured and the perceptual upright was defined as half way between these orientations. The visual background affected the orientation of the PU less when in microgravity than when upright in normal gravity and more when supine than when upright in normal gravity. A weighted vector sum model was used to quantify the relative influence of the orientations of gravity, vision and the body in determining the perceptual upright.

The influence of retinal and extra-retinal motion cues on perceived object motion during self-motion.

Eye, head, and body movement are intimately linked. During self-motion, the eyes track objects by a combination of vestibular reflexes and smooth pursuit eye movements but although the world appears stable during saccadic gaze changes, it does not appear stable during physical self-motion. We determined the amount by which a fixated object needed to be moved in space in order to appear earth stationary to a linearly moving observer. Observers were oscillated sinusoidally either passively or under their own control, under lit and fully darkened conditions. The visual targets always needed to move (in space) in the same direction as the observer to be judged as earth stationary. Targets needed to be moved more in order to be judged as earth stationary when movement was in the dark, rather than in the light, and also when movement was passive rather than when it was active. Efference copy motor signals, visual movement, and non-visual cues all contribute significantly and approximately additively to the estimate of self-motion. Errors in perceived self-motion can produce subsequent illusory visual motion.

The subjective visual vertical and the perceptual upright.

The direction of 'up' has traditionally been measured by setting a line (luminous if necessary) to the apparent vertical, a direction known as the 'subjective visual vertical' (SVV); however for optimum performance in visual skills including reading and facial recognition, an object must to be seen the 'right way up'--a separate direction which we have called the 'perceptual upright' (PU). In order to measure the PU, we exploited the fact that some symbols rely upon their orientation for recognition. Observers indicated whether the symbol 'horizontal P' presented in various orientations was identified as either the letter 'p' or the letter 'd'. The average of the transitions between 'p-to-d' and 'd-to-p' interpretations was taken as the PU. We have labelled this new experimental technique the Oriented CHAracter Recognition Test (OCHART). The SVV was measured by estimating whether a line was rotated clockwise or counter-clockwise relative to gravity. We measured the PU and SVV while manipulating the orientation of the visual background in different observer postures: upright, right side down and (for the PU) supine. When the body, gravity and the visual background were aligned, the SVV and the PU were similar, but as the background orientation and observer posture orientations diverged, the two measures varied markedly. The SVV was closely aligned with the direction of gravity whereas the PU was closely aligned with the body axis. Both probes showed influences of all three cues (body orientation, vision and gravity) and these influences could be predicted from a weighted vectorial sum of the directions indicated by these cues. For the SVV, the ratio was 0.2:0.1:1.0 for the body, visual and gravity cues, respectively. For the PU, the ratio was 2.6:1.2:1.0. In the case of the PU, these same weighting values were also predicted by a measure of the reliability of each cue; however, reliability did not predict the weightings for the SVV. This is the first time that maximum likelihood estimation has been demonstrated in combining information between different reference frames. The OCHART technique provides a new, simple and readily applicable method for investigating the PU which complements the SVV. Our findings suggest that OCHART is particularly suitable for investigating the functioning of visual and non-visual systems and their contributions to the perceived upright of novel environments such as high- and low-g environments, and in patient and ageing populations, as well as for normal observers.

Is an internal model of head orientation necessary for oculomotor control?

In order to test whether the control of eye movement in response to head movement requires an internal model of head orientation or instead can rely on directly sensing information about head orientation and movement, perceived gravity was separated from physical gravity to see which dominated the eye-movement response. Internal model theory suggests that the oculomotor response should be driven by perceived, internalized gravity, whereas the direct sensing theory predicts it should always be driven by vestibularly sensed gravity. Subjects lay on an airbed either supine or on their side and were sinusoidally translated along their dorsoventral body axis. The direction of perceived gravity was separated from physical gravity by performing the experiments in a room built on its side with the direction of its "floor" orthogonal to both physical gravity and the subject's translation. The swinging sum of the imposed sinusoidal acceleration with physical gravity was thus in a plane orthogonal to its sum with perceived gravity. Oculomotor responses to these swinging vectors were looked for and responses were found only to the sum of the acceleration with physical gravity, not perceived gravity. It was concluded that an internal model is not used to drive these compensatory eye movements.

Shape-from-shading depends on visual, gravitational, and body-orientation cues.

The perception of shading-defined form results from an interaction between shading cues and the frames of reference within which those cues are interpreted. In the absence of a clear source of illumination, the definition of 'up' becomes critical to deducing the perceived shape from a particular pattern of shading. In our experiments, twelve subjects adjusted the orientation of a planar disc painted with a linear luminance gradient from one side to the other, until the disc appeared maximally convex-that is, until the luminance gradient induced the maximum perception of a three-dimensional shape. The vision, gravity, and body-orientation cues were altered relative to each other. Visual cues were manipulated by the York Tilted Room facility, and body cues were altered by simply lying on one side. The orientation of the disc that appeared maximally convex varied in a systematic fashion with these manipulations. We present a model in which the direction of perceptual 'up' is determined from the sum of three weighted vectors corresponding to the vision, gravity, and body-orientation cues. The model predicts the perceived direction of 'up', contributes to our understanding of how shape-from-shading is deduced, and also predicts the confidence with which the 'up' direction is perceived.

Two illusions of perceived orientation: one fools all of the people some of the time; the other fools all of the people all of the time.

In a series of three separate experiments, we studied two different orientation illusions, in both of which vertical lines appear tilted as a result of being set against a tilted background pattern. The 'simultaneous tilt illusion' (STI), in which a target grating is viewed within an abutting tilted grating surround, is thought to originate early in the cortical processing of visual contours. In contrast, the 'rod-and-frame' illusion (RFI), which is induced by a distant tilted frame, is thought to originate much later in the perceptual processing system. In the first two experiments we found that orientation-guided action was virtually impervious to the RFI, whereas both perceptual experience and action were equally influenced by the STI. In the third experiment, in which the two illusions were pitted one against the other, an opposite pattern of effects emerged, such that visuomotor performance was now deceived by the illusion much more than was perceptual experience. This pattern of association and dissociation in the effects of visual illusions on perception versus action can best be explained within a two-visual-systems model of cortical processing.