Movement related symptoms including muscle rigidity, tremor, and bradykinesia are commonly associated with Parkinson’s disease (PD) (Berardelli et al. 2001), but non-motor symptoms, such as decrements in the sense of smell (Double et al., 2003), haptic acuity (Konczak et al., 2008) and visual perception (Lee and Harris, 1999) are also prevalent. Davidsdottir et al. (2005) and Lee and Harris (1999) administered questionnaires pertaining to various visual perceptual disorders, to individuals with PD. Many of the PD patients reported double vision, experienced changes in the detection of color and luminance contrast, misjudged spaces between objects, and perceived that vehicles and people appeared to move faster than they had in the past. These reported visual disorders may alter the perception of optic flow and as a result lead to changes in gait, such as decreased walking speed, increased stride frequency, and veering while walking.
Optic flow experienced by the observer, during locomotion, contains relevant information regarding heading direction and influences gait coordination and various walking parameters (Bruce et al., 1996; Dyre and Andersen, 1997; Prokop et al., 1997; Telford and Howard, 1996; Warren et al., 1988; Warren et al., 1991; Warren et al., 2001; Wilkie and Wann, 2006). Our research group explored the effects of systematic manipulations of optic flow speed on veering and the coordination of walking in healthy younger and older adults (Chou et al., 2009). Results supported previous findings (Himann et al., 1988; Konczak, 1994; Prokop et al., 1997) and showed that increases in optic flow speed were accompanied by decreases in walking speed, which was primarily adjusted via stride frequency. It was also demonstrated that humans veered away from the faster wall in an attempt to equalize the relative optic flow speed experienced in each hemi-field; the greater the difference between walls in terms of optic flow speed, the more veering occurred (Chatziastros et al., 1999; Chou et al., 2009; Duchon and Warren, 2002).
Visuospatial functional testing has shown differences in performance between PD patients for whom the left body-side was initially affected (LPD) as a result of the degeneration of the basal ganglia in the right hemisphere, compared to those initially affected on the right body-side (RPD) (Amick et al., 2006; Davidsdottir et al., 2008; Harris et al., 2003; Lee et al., 2001a; Schendan et al., 2009). During line bisection tasks, LPD consistently made judgments to the right of center, and RPD to the left (Lee et al., 2001b). Further research revealed that LPD underestimated the size of objects such as apertures (Lee et al., 2001b) and shapes (Harris et al., 2003) located on the left side of the visual space, their left visual hemi-field appeared compressed, whereas for RPD, the right visual field was more compressed than the left. To LPD, a unilateral compression of the visual field would cause the left border of the visual field to shift toward the right, resulting in a line bisection bias toward the right; for RPD, the opposite would be true. For a walker with PD, it is expected that veering occurs away from the perceived compressed visual field, such that LPD would walk toward the right and RPD toward the left.
Continued research revealed that PD patients perceive an egocentric reference point (ECRP) that has shifted toward the side of the brain with more extensive basal ganglia damage (Davidsdottir et al., 2008). The ECRP divides the perceived space into left and right hemi-fields with respect to the midline of the trunk (Karnath et al., 1991). A shifted ECRP among PD results in a perceived field of view that is shifted to the right for LPD and to the left for RPD and is in the same direction as the observed line bisection biases. Accordingly, a shifted ECRP may influence gait such that LPD would veer toward the right and RPD toward the left. Findings presented in the literature suggest that individuals with PD perceive a field of view that has shifted toward the side of the brain with initial basal ganglia damage. Because both predict a shift in the same direction, it is unclear whether the underlying mechanism of the shift is due to a unilateral compression of the visual field or a shifted ECRP. In either case, because of a shifted field of view it is expected that individuals with PD would walk in the same direction as the shift such that LPD would walk toward the right while RPD toward the left.
Veering during gait could also be attributed to asymmetries in the perception of optic flow speed. Research has shown that, during optic flow speed manipulations, individuals with LPD report the right visual field as moving faster than the left, while the opposite is true for RPD (Davidsdottir et al., 2008). A possible explanation for this asymmetric perception of optic flow is related to the unilateral compression of the visual field reported by Harris et al. (2003). During forward movement, texture elements within the compressed visual hemi-field travel a smaller perceived distance over the same period of time as compared to the uncompressed side. Texture elements on the compressed side would expand at a slower optic flow speed. As mentioned earlier, it has been shown that individuals walk away from the faster moving wall (Chatziastros et a., 1999; Chou et al., 2009; Duchon and Warren, 2002). Accordingly, individuals with LPD, who perceive slower optic flow in the left visual field compared to the right, should veer toward the left, away from the perceived faster moving visual field, whereas, RPD should move toward the right. These predictions are in the opposite direction of veering expected as a result of a shifted field of view due to unilateral compression of the visual field or a shifted ECRP.
In addition to a shifted field of view and/or asymmetric perceptions of optic flow speed, PD patients often show marked gait asymmetries (Johnsen et al., 2009; Lewek et al., 2009; Plotnik and Hausdorff, 2008) that may affect the coordination dynamics of walking and contribute to veering. The literature shows that individuals walk toward the side of the body with smaller step lengths (Courtine and Schiepatti, 2004). Accordingly, it is expected that individuals with PD veer toward their initially effected body-side; LPD would veer to the left and RPD right. The present study implemented virtual reality techniques to investigate the relative effects of a shifted field of view, asymmetric perceptions of optic flow speed, and gait asymmetries on heading direction, functional gait parameters, and the inter-limb coordination patterns of walking in patients with PD and healthy, age-matched control adults. Based on the outcomes of previous research it was hypothesized that:
- During the eyes open (EO), blindfolded (BF), and virtual reality (VR) walking trials of Experiment 1, findings reported in the literature would be replicated indicating that patients with PD have decreased walking speed (WS), increased stride frequency (SF), and smaller stride lengths (SL) than healthy age-matched adults.
- With symmetric and asymmetric optic flow speed manipulations, increases in optic flow speed would be accompanied by decreases in WS and SL and increases in SF for all participants. In contrast to the control group, PD patients would show an asymmetry between the initially-affected side and the secondarily-affected side in terms of SL as well as the phase and frequency relations between arm and leg movements.
- During symmetric optic flow speed manipulations, LPD patients would veer toward the right and RPD patients toward the left, indicating the presence of a shifted field of view among PD patients. When asymmetries in optic flow were presented, as in Experiment 3, all participants would use a navigation strategy that equalizes optic flow speed laterally and would walk away from the faster moving wall.