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.

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.

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.

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.

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.