VR in Stroke disorders
The objective of this module is the development and use of virtual environments for the study and the treatment of stroke disorders.
1. Description of the problem
2. Rationale of the system
In a recent review paper, "Prospects for the rehabilition of Unilateral Neglect", Robertson et al (1994) conclude with five possible avenues for rehabilitation of neglect: Dynamic stimulation, Eye-patching; Vestibular Stimulation; Optokinetic Stimulation; and Fresnel Prisms. All of these interventions can be potentially made through the use of VR technology. Dynamic stimulation and Optokinetic stimulation can be produced by moving the individual or objects within a VE, or by superimposing moving objects over the natural world with see-through HMDs. Selective provision of visual information to the hemi-fields hemi-field (retinotopic, spatiotopic or body-centric) is achievable in a HMD configuration, as is the equivalent of prismatic displacement.
Optokinetic stimulation has been shown to increase awareness of body movements in the neglected field (Vallar et al,1993) and reduction of bias (Mattingley et al, 1994). What is not clear is the mechanisms that give rise to such an improvement, or the optimal form that optokinetic stimulation should take. Vallar et al (1993) presented patients with a display where 20 dots movement across a computer monitor, Mattingley et al (1994) similarly used drifting random dot patterns. It might be expected that visual motion fields that approximated natural behaviours (locomoting, colliding, falling) would prompt the strongest stimulatory effects.
As long ago as 1907, Riddoch (1907) noted that "movement may be recognised as a special visual perception" that maybe retained despite loss of perception of static objects and scenes. He also noted that "appreciation of movement returns before the object as such is recognised, if recovery of vision is occuring". This has been more recently supported by Mestre et al (1992) who demonstrated that cortically blind patients retain perception of speed and direction from a complex (dot) flow field similar to that produced by locomotion.
So is there retention of motion perception in patients with uni-lateral neglect? Do the observations of Vallar et al (1993) reflect the general salience of motion cues and the special role of motion perception in human and animal phylogeny? These are fundamental questions that will be addressed through the use of virtual environment (VE) displays. The sensitivity of patients to features of both dynamic and static VE displays will also be used to profile differences in the ability across patients. The unilateral neglect syndrome encompasses a notoriously heterogeneous group of patients with cerebral insults. It is consequently important to establish a broad and sensitive range of measures that can discriminate the exacty degree and form of movement perception retained
Following stroke, patients who do not exhibit prolonged neglect, but present with hemiparesis attain some degree of recovery of function for the paretic limbs. In younger patients, such recovery may be rapid during the first 12 weeks post-CVA, but then follows a negatively accelerating curve toward a plateau below full function (Fugl-Meyer, 1975; Wing et al, 1990). The topology of recovery may be partially explained through the respective roles of crossed and uncrossed neural pathways and the recruitement of the latter. While neural factors may drive the initial phase of recovery a major component of later gains is the patient's re-learning of control.
A major role of therapy in the re-learning process is to stimulate the patient to explore strategies and highlight information that may guide the patient toward improved control (Wann & Turnbull, 1993). A typical example is biofeedback, where significant, but limited gains can be demonstrated by providing hemiparetic patients with visual information about specific muscular activity. This project would follow the suggestions of Wann & Turnbull (1993), by translating bio-feedback principles to action-feedback in a VE setting. In action feedback it is proposed that a hemi-paretic patient attempts to drive a VE display through movements of the paretic limb. The coupling of limb motion to patterns of expansion, contraction and flow within a VE means that irregularities in the kinematics of the limb are reflected in the "stuttering" flow of the display and thereby taps into a particularly acute facet of human visual perception. Such displays are not contrived, the visual transformation accurately reflects the dynamics of the limb.
Everyday, many of us get into a vehicle, see the optic field expand before them and smoothly control movements of their feet on the brake and accelerator in response to the flow. Novice drivers are erratic, but rapidly find the coupling between smooth limb movement and smooth ego-motion. The choice of foot/hand controls is arbitrary, the fact that patients will be regulating the flow field with their arms and will not in truth be moving does not devalue the pertinence and richness of the information being provided. Mapping a seemingly arbitrary manual/pedal action to the visual information that arises from ego-motion is something that is done habitually in a new car, riding a bicycle or using a laparoscope. The VR driving paradigm merely taps into the skills that most intact adults would display if Henry Ford had opted for manual rather than pedal controls for forward motion in earlier automobiles and in doing so it evokes one of the richest mediums for unmediated, rapid information transmission: optic transformations.
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