Centre for Vision Research

Directional data arising from psychophysical studies requires careful processing due to its cyclical nature. Unlike linear variables such as response time and intensity, directions can be represented as angles or vectors on a circle, may have no natural zero-point or magnitude, and are defined on a periodic circle rather than an infinite line. Because of these unique features, directional data necessitates the use of circular statistical methods; however, unfamiliarity and limited availability of software that support circular data analysis have largely led to its under-use and misapplication. Here the implications of using linear statistics for circular data were explored by submitting data from a behavioural study to both circular and linear statistical analysis.

In this study we examined the effects of simulated horizontal and vertical viewpoint jitter on the vection and postural sway induced by radial patterns of optic flow. During each trial, observers were exposed sequentially to 20 s periods of radially expanding flow, radially contracting flow, and static visual scenes. For half the trials, simulated viewpoint jitter was added to the radially expanding/contracting optic flow patterns. In experiment 1, we found that, while this jitter increased the backward postural sway induced by radial expansion, it actually decreased forward postural sway induced by radial contraction. However, in experiment 2 we found that jitter increased both the forward and backward vection induced by radially expanding and contracting flow patterns. We conclude that the processes involved in postural control are more sensitive to the sensory conflicts generated by viewpoint jitter than those involved in the perception of self-motion, and that the observed asymmetries in forward and backward sway are ecological in origin.

We examined the vection induced by consistent and conflicting multisensory information about self-motion. Observers viewed displays simulating constant-velocity self-motion in depth while physically oscillating their heads left - right or back - forth in time with a metronome. Their
tracked head movements were either ignored or incorporated directly into the self-motion display (as an added simulated self-acceleration). When this head oscillation was updated into displays, sensory conflict was generated by simulating oscillation along: (i) an orthogonal axis to the head movement; or (ii) the same axis, but in a non-ecological direction. Simulated head oscillation always produced stronger vection than 'no display oscillation' even when the axis/direction of this display motion was inconsistent with the physical head motion. When head-and-display oscillation occurred along the same axis: (i) consistent (in-phase) horizontal display oscillation produced stronger vection than conflicting (out-of-phase) horizontal display oscillation; however, (ii) consistent and conflicting depth oscillation conditions did not induce significantly different vection. Overall, orthogonal-axis oscillation was found to produce very similar vection to same-axis oscillation. Thus, we conclude that while vection appears to be very robust to sensory conflict, there are situations where sensory consistency improves vection.

Vection has typically been induced in stationary observers (ie conditions providing visual only information about self-motion). Two recent studies have examined vection during active treadmill walking—one reported that treadmill walking in the same direction as the visually simulated self-motion impaired vection (Onimaru et al, 2010 Journal of Vision 10(7):860), the other reported that it enhanced vection (Seno et al, 2011 Perception 40 747–750; Seno et al, 2011 Attention, Perception, & Psychophysics 73 1467–1476). Our study expands on these earlier investigations of vection during observer active movement. In experiment 1 we presented radially expanding optic flow and compared the vection produced in stationary observers with that produced during walking forward on a treadmill at a ‘matched’ speed. Experiment 2 compared the vection induced by forward treadmill walking while viewing expanding or contracting optic flow with that induced by viewing playbacks of these
same displays while stationary. In both experiments subjects’ tracked head movements were either incorporated into the self-motion displays (as simulated viewpoint jitter) or simply ignored. We found that treadmill walking always reduced vection (compared with stationary viewing conditions) and that simulated viewpoint jitter always increased vection (compared with constant velocity displays). These findings suggest that while consistent visual–vestibular information about self-acceleration increases vection, biomechanical self-motion information reduces this experience (irrespective of whether it is consistent or not with the visual input).

Compelling illusions of self-motion, known as vection, can be produced in a stationary observer by visual stimulation alone. The role of binocular vision and stereopsis in these illusions was explored in a series of three experiments. Previous research had provided evidence of stereoscopic enhancements for linear vection in depth (e.g., Palmisano, 1996, 2002). Here we examined for the first time the effects of binocular vision and stereopsis on linear vertical vection. Vertical vection was induced by the upward or downward translation of large stereoscopic surfaces. These surfaces were horizontally oriented depth corrugations produced by
disparity modulation of patterns of persistent or short lifetime dot elements. We found that binocular viewing of such surfaces significantly increased the magnitudes and decreased the onset delays of vertical vection. Experiments utilizing short lifetime dot stereograms demonstrated that these particular binocular enhancements of vection were due to the motion of stereoscopically defined features.

Visual-vestibular conflicts have been traditionally used to explain both perceptions of self-motion and experiences of motion sickness.  However, sensory conflict theories have been challenged by findings that adding simulated viewpoint jitter to inducing displays enhances (rather than reduces or destroys) visual illusions of self-motion experienced by stationary observers. One possible explanation of this jitter advantage for vection is that jittering optic flows are more ecological than smooth displays.  Despite the intuitive appeal of this idea, it has proven difficult to test.  Here we compared subjective experiences generated by jittering and smooth radial flows when observers were exposed to either visual-only or multisensory self-motion stimulations.  The display jitter (if present) was generated in real-time by updating the virtual computer-graphics camera position to match the observer’s tracked head motions when treadmill walking or walking in place, or was a playback of these head motions when standing still.  As expected, the (more naturalistic) treadmill walking and the (less naturalistic) walking in place were found to generate very different physical head jitters.  However, contrary to the ecological account of the phenomenon, playbacks of treadmill walking and walking in place display jitter both enhanced visually induced illusions of self-motion to a similar degree (compared to smooth displays).