People are often faced with a sensation of motion when gazing at moving clouds or when a train on an adjacent track moves at a railway station (Dichgans & Brandt, 1978). Multiple senses contribute to this common visual illusion of self-motion. Optic flow stimuli induces a conflict between visual input, signalling movement of the body and vestibular input from inertial motion cues (A1). Visual-vestibular interactions also play an important role in maintaining postural stability (A4). Thus it is proposed that there is a common underlying mechanism between postural sway during quiet-stance and vection (A5).
Recent research has shown that quiet-stance postural sway can be used to predict subsequent vection strength (A5). While many different types of global optic flow can generate self-motion (A2), this relationship has only been demonstrated for radial flow (Apthorp, Nagle, & Palmisano, 2014). Therefore, does quiet-stance postural sway predict differences between multiple vection types, or is it simply a global measure distinguishing vection from non-vection?
The experience of vection describes compelling visual illusions of perceived self-motion that are induced by presenting large patterns of optic flow to physically stationary observers (Palmisano, Allison, Schira, & Barry, 2015).
Optic flow fields provide visual signals for effective navigation through the three-dimensional environment. It describes a pattern of visual motion on the retina used to rapidly estimate the direction of movement (Duffy & Wurtz, 1993). This direction is dependent on the nature of this field, differing with radial, lamellar, rotary and spiral patterns of flow (Britten, 2008).
A radial pattern refers to expanding and contracting optic flow (Apthorp et al., 2014). A lamellar pattern refers to optic flow with horizontally parallel flow (Stoffregen, 1985). A rotary pattern of optic flow describes a rotating pattern also parallel to the medial-lateral axis. A spiral pattern of optic flow combines radial and rotary patterns, with the rotary component superimposed in radially expanding flow (Nakamura, 2011).
The postural system concerns the position and orientation of body segments to organise balance and movement (Massion, 1994). Postural sway refers to readjustments in posture which can occur with medial-lateral (ML), side-to-side, or anterior-posterior (AP), back-and-forth sway (Ruhe, Fejer, & Walker, 2011). Quiet-stance postural sway refers to both eyes open and eyes closed postural sway while standing which occurs prior to the onset of vection (Apthorp et al., 2014).
A1 (vection). Self-motion perception is a multisensory experience induced by conflicts between optic flow stimuli indicating movement and vestibular input which detects no variation in body position or velocity (Lestienne, Soechting, & Berthoz, 1977).
A2 (optic flow). Radially expanding and contracting optic flow stimulates forwards and backwards linear vection, respectively (Apthorp et al., 2014). For lamellar optic flow, it generates an illusion of self-translation parallel to the direction of flow (Stoffregen, 1985). A rotary pattern induces roll vection parallel to the plane of the presented flow (Tanahashi, Ujike, Kozawa, & Ukai, 2007). Spiral optic flow induces a combination of roll and linear vection (Nakamura, 2011).
A3 (optic flow and vection). The magnitude of vection varies with the nature of the optic flow, depending on the area, velocity, depth and spatial frequency of the pattern (Palmisano, Apthorp, Seno, & Stapley, 2014). In general, more compelling vection will be induced by optic flow displays that generate significant sensory conflict (Palmisano et al., 2015).
A4 (vision and posture). Maintenance of upright posture also depends on visual-vestibular cues (Del Percio et al., 2007). The extent of reliance on visual input in particular, indicates variations in posture, with a greater dependence resulting in more postural readjustments (Apthorp et al., 2014).
A5 (postural sway and vection). Quiet-stance postural sway and vection are underpinned by the same basic mechanisms. This supports the use of quiet-stance postural sway measures to predict subsequent vection strength. (Palmisano et al., 2014).
A6 (postural sway and vection). During upright stance, ML sway involves the control of hip and trunk muscles, whereas AP sway is regulated by ankle muscles. As these are controlled separately by the postural control system, independent variations in ML and AP are predictive of sway differences between vection types (Tucker, Kavanagh, Morrison, & Barrett, 2010).
Considering that vection magnitude varies with vection type (A3), it is hypothesised that global differences in the magnitude of future vection will be predicted by changes in quiet-stance. In addition, it is proposed that vection magnitude will be stronger for individuals who rely more on their vision for postural stability.
Using local differences in sway axes, it is proposed that changes in AP sway will predict radial flow as it stimulates forwards-and-backwards self-motion (A2). For lamellar flow, which induces self-translation and roll vection generated from rotary flow (A2), it is hypothesised that ML changes will be more predictive. In addition, the combination of roll and linear vection from spiral flow may be predicted by sway in both ML and AP axes.
Vection magnitude can be operationalised through a subjective verbal vection rating. Subjects verbally rate the strength of their vection experience on a 100 point scale, with ‘0’ indicating no perceived self-motion and ‘100’ indicating complete self-motion (Apthorp et al., 2014). Though self-report measures can be susceptible to subject cognitions, subjective ratings of vection are reasonably reliably as vection is a subjective experience (Palmisano et al., 2015).
The multisensory visual-vestibular interaction for posture can be operationalised through postural sway measures. Quiet-stance postural sway, has been shown to predict subsequent vection, which makes it a viable measure of vection (Palmisano et al., 2014). Postural sway can be operationalised through the changes to the location of the centre of foot pressure (CoP) in the AP and ML direction (Ruhe et al., 2011). Larger sway amplitudes are indicative of greater postural instability. Though CoP is an indirect sway measure as it measures motor system activity, it is a practical method of measuring postural sway in standing (Ruhe et al., 2011).
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