Freezing of gait and response conflict in Parkinson's disease: computational directions

In a recent study, Matar et al. (2013) found that Parkinson’s disease (PD) patients with freezing of gait significantly displayed reduced locomotive responses when passing through narrow rather than wide doors and while facing the opening of a sliding door. Freezing of gait refers to a cessation of movement despite the intention to walk forward, such that patients often feel like their feet have been “glued to the ground” (Schaafsma et al., 2003; Rahman et al., 2008). This pattern of results was not found with PD patients without freezing of gait or healthy controls. The Matar et al. study can potentially point to which environmental situations can lead to freezing of gait in PD patients. Most interestingly, these results were reported by using a “virtual” setting in which subjects navigate a realistic threedimensional environment using foot pedals (see Figure 1, also see Naismith and Lewis, 2010). The virtual reality paradigm used in the Matar et al. (2013) study is used for testing perceptual and cognitive factors underlying successful locomotion in humans. In this paradigm, forward progression in the virtual environment (left side of Figure 1) only occurs by alternating left–right sequences of footsteps (right side of Figure 1). The time taken between the footsteps (left-right or right-left) corresponds to faster or slower progression in the virtual environment, thus mimicking natural walking. In the Matar et al. study, the virtual environment contains corridors with doorways (as shown in Figure 1) and/or salient environmental stimuli that prompt locomotive responses. The Matar et al. data confirms and also extends prior results that have used real doorways (Almeida and Lebold, 2010; Cowie et al., 2010; Knobl et al., 2012). The virtual reality paradigm also allows the testing of more abstract cognitive cues on freezing of gait. Interestingly, the same study by Matar et al. also show that subjects with freezing of gait (but not other groups) slow down when instructed to “walk” when presented with word “RED” in red-font rather than “GREEN” in green font (Matar et al., 2013). Although non-significant,