Visual properties of virtual target objects
: implications for reaching and grasping tasks in a virtual reality rehabilitation context

  • Vaughan Powell

Student thesis: Doctoral Thesis


Shoulder dysfunction is common and accounts for around four million days of sick leave in the UK each year. Treatment approaches aim to reduce pain, and improve range of motion (ROM). However, patient adherence to therapeutic regimes is poor, partly due to pain avoidance behaviour and lack of engaging therapeutic exercises.

Virtual rehabilitation is an area of healthcare which combines physical therapy with virtual reality (VR). It has been shown to facilitate patient engagement and reduce pain perception. Reaching and grasping tasks are well suited to VR applications, and there are a number of studies using VR to explore upper limb motion. However, current approaches for evaluating active shoulder rotation are problematic. A functional test known as Apley’s scratch test could potentially be adapted with magnetic motion tracking, to provide an objective proxy rotation measure to assess changes in ROM in a VR context.

An empirical study demonstrated that active rotational movements of the shoulder could be successfully tracked in VR, and that there was evidence of pain suppression during the motion tasks. In addition, head tracking for patients with shoulder and neck pain, was addressed using a novel eye-hand tracking approach. In addition, observations of participants revealed aberrant motion patterns that could compromise rehabilitation outcomes.

Despite providing depth cues in the VE itself, analysis suggested that a number of participants remained unsure of the spatial location of the target object which would lead to suboptimal movement behaviour. It was hypothesised that target object geometry may have a particular role in distance judgments, such that a simple icosahedron would provide richer cues with lower computational demand than commonly used spheres or detailed realistic modelled geometry. It was also proposed that providing a proximity cue using exaggerated brightness changes might improve the ability to locate and grasp the object in virtual space.

Empirical studies demonstrated that the sphere had a significantly higher duration of time-to-target required in the terminal phase of reaching. Furthermore they demonstrated that a simple icosahedron was more effectively located in virtual space than the sphere object, and performed as well as a more computationally complex “realistic” model.

The introduction of richer visual cues only in the terminal phase of reaching was ineffective as was the use of brightness as a proximity cue demonstrating that the use of richer geometry cues throughout was more influential. Nevertheless whilst target object geometry can facilitate spatial perception and influence reaching and grasping task performance, it was not associated with a reduction in arm elevation rigid system behaviour, suggesting other factors still warrant further investigation.

This thesis demonstrates that the visual properties of the VE component of a VR system, and specifically the target objects within it, should be taken into consideration when designing spatial perception or reaching tasks in VR, especially within a rehabilitation context.
Date of AwardSept 2012
Original languageEnglish
Awarding Institution
  • University of Portsmouth
SupervisorBrett Stevens (Supervisor) & Steve Hand (Supervisor)

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