Richard T. Dyde

The subjective visual vertical and the perceptual upright

AbstractThe 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 ‘ ’ 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.

Why do some perceptual illusions affect visually guided action, when others don't?

In 1995, Aglioti and his colleagues [1] reported that the powerful Ebbinghaus–Titchener size-contrast illusion had no effect on visually guided grasping. Pairs of discs were presented within annular arrays of (respectively) smaller or larger circles, generating a strong perceptualsize illusion; yet the illusion did not affect the extent of hand opening during reaches made to pick up one or other of the discs. Milner and Goodale [2] interpreted these data within their proposed association of the cortical ventral and dorsal visual streams with ‘perceptual’ and ‘visuomotor’ processing, respectively. Ventral-stream processing would be contextually relative, they argued, in order to provide suitably coded visual information for purposes of recognition and storage. In contrast, size and location would need to be coded in absolute metrics in the dorsal stream, in order to be readily translated into motor coordinates. Since 1995, several studies have produced similar dissociations to those of Aglioti et al., but others have found a significant effect, albeit generally a weak one, of perceptual illusions on action [3–5]. Most, if not all, of these results can be encompassed within the two-visual-streams framework. For example, the effect sometimes found of the Ebbinghaus illusion on grasp aperture might be an artefact of the different spaces ‘available’ around the target discs, if the surrounds are treated by the visuomotor system as ‘obstacles’. The preparatory hand posture appears to be highly sensitive to this factor of ‘grasp space’ [6]. When grasp space is equalized between the two targets, the effect of the illusion on grasp disappears [7]. A quite different reason why a perceptual illusion might influence a visually guided action relates to where in the brain the illusion originates. It is likely that ‘contextual’ illusions like the Ebbinghaus have their effects chiefly within the depths of the ventral stream. But other illusions are likely to originate in primary visual cortex (V1), or in one of the other retinotopic areas, which feed not only into the ventral stream but also into the dorsal stream. There are two ways of deceiving the visual system about the orientation of a central stimulus. The rod-and-frame illusion (RFI, Fig. 1a) appears to be due to a ‘contextual’ effect, in which the whole visual frame of reference becomes rotated. The surrounding features of the scene induce a relative percept of the target object, which dominates our conscious judgements [8]. By contrast, the simultaneous-tilt illusion (STI, Fig. 1b) depends on local interactions within the visual field, most probably mediated by short-range inhibitory connections between cortical columns in V1 that respond to different orientations. These interactions would predict a shift in the distribution of neurons responding to a target grating pattern when surrounded by a grating set at an orientation a few degrees away [9]. The two-streams theory [2] must predict a dissociation between perception and action in the RFI, because the frame is most unlikely to influence the target through local interactions in retinotopic visual areas. The STI, however, should not only affect activity in the perceptual system, but

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.

Multisensory determinants of orientation perception in Parkinson's disease.

Perception of the relative orientation of the self and objects in the environment requires integration of visual and vestibular sensory information, and an internal representation of the bodys orientation. Parkinsons disease (PD) patients are more visually dependent than controls, implicating the basal ganglia in using visual orientation cues. We examined the relative roles of visual and non-visual cues to orientation in PD using two different measures: the subjective visual vertical (SVV) and the perceptual upright (PU). We tested twelve PD patients (nine both on- and off-medication), and thirteen age-matched controls. Visual, vestibular and body cues were manipulated using a polarized visual room presented in various orientations while observers were upright or lying right-side-down. Relative to age-matched controls, patients with PD showed more influence of visual cues for the SVV but were more influenced by the direction of gravity for the PU. Increased SVV visual dependence corresponded with equal decreases of the contributions of body sense and gravity. Increased PU gravitational dependence corresponded mainly with a decreased contribution of body sense. Curiously however, both of these effects were significant only when patients were medicated. Increased SVV visual dependence was highest for PD patients with left-side initial motor symptoms. PD patients when on and off medication were more variable than controls when making judgments. Our results suggest that (i) PD patients are not more visually dependent in general, rather increased visual dependence is task specific and varies with initial onset side, (ii) PD patients may rely more on vestibular information for some perceptual tasks which is reflected in relying less on the internal representation of the body, and (iii) these effects are only present when PD patients are taking dopaminergic medication.

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.

Multisensory determinants of orientation perception: task specific sex differences

Females have been reported to be more ‘visually dependent’ than males. When aligning a rod in a tilted frame to vertical, females are more influenced by the frame than are males, who align the rod closer to gravity. Do females rely more on visual information at the cost of other sensory information? We compared the subjective visual vertical and the perceptual upright in 29 females and 24 males. The orientation of visual cues presented on a shrouded laptop screen and of the observer’s posture were varied. When upright, females’ subjective visual vertical was more influenced by visual cues and their responses were more variable than were males’. However, there were no differences between the sexes in the perceptual upright task. Individual variance in subjective visual vertical judgments and in the perceptual upright predicted the level of visual dependence across both sexes. When lying right‐side down, there were no reliable differences between the sexes in either measure. We conclude that heightened ‘visual dependence’ in females does not generalize to all aspects of spatial processing but is probably attributable to task‐specific differences in the mechanisms of sensory processing in the brains of females and males. The higher variability and lower accuracy in females for some spatial tasks is not due to their having qualitatively worse access to information concerning either the gravity axis or corporeal representation: it is only when gravity and the long body axis align that females have a performance disadvantage.

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.

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 gravitys physical pull. However, gravitys 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 bodys 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.

Is an internal model of head orientation necessary for oculomotor control

Abstract: 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 subjects 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.