# From Patient to Musician: A Multi-Sensory Virtual Reality Rehabilitation Tool for Spatial Neglect
## 2 Background
### Classification of Spatial Neglect  
USN is relatively common and has many causes, including neurodegenerative diseases, neoplasm, aneurysm, traumatic brain injury after the surgical resection of a brain tumour and stroke, where stroke is by far the most prevalent cause [4; 12; 13; 14]. Left-sided neglect is more common than right-sided neglect, as it is mostly linked to damage to the right hemisphere of the brain [15; 16]. The brain has an asymmetric attention mechanism, where the left hemisphere is responsible for processing input originating from the right side of space, whereas the right hemisphere is responsible for stimuli from both the left and partly the right. When damage occurs at the left hemisphere, the right hemisphere can partially compensate for it. However, when the right hemisphere is damaged, no compensation is possible.  
Although the existing literature is not consistent on the classification of the types of USN, two categories exist. The first categorises neglect based on the affected modality, including sensory, motor or representation neglect. Patients with sensory neglect experience a disordered awareness of sensory inputs originating from their contralesional side of space. They lack the ability to bring their attention to this side of space in the presence of external stimuli [12; 15]. This type of neglect can be further specified by the sensory modality in which it manifests itself, such as visual, auditory and tactile neglect [8]. An example of sensory neglect is when the patient only eats half the plate of food, despite still being hungry [17]. Next, motor neglect is defined as a failure to generate the desired movement in response to observed stimuli, where this failure cannot be ascribed to a primary sensor or motor deficit [8; 18]. Multiple sub-types of this type of neglect exist, such as the underutilisation of a limb opposite to the brain lesion [19]. A final type of modality-based neglect is representation neglect or imagery neglect, where a person ignores the contralesional side of images that were internally generated, such as mental representations of a task, action or movement [8; 20]. An example of this type of neglect can be seen when a patient is asked to draw an analogue clock from memory, in which the patient places all the numbers on their non-neglected side of the clock [21]. An example of this experiment is shown in Figure 1.  
The second method classifies neglect according to the distribution of abnormal behaviour in space, e.g., personal and spatial neglect [22]. Personal neglect is the lack of exploration of half of the body contralateral to the damaged hemisphere [23]. An example of personal neglect is only shaving half of their face, or putting on only one shoe. Personal neglect differs from sensory neglect as it is characterised by an unawareness of the body part itself, whereas sensory neglect refers to an unawareness of external stimuli, e.g., vision or touch [8]. Spatial neglect is defined as neglect where the abnormal behaviour is not located in personal space; this thus refers to failure to acknowledge stimuli on the contralesional side of space [23]. Spatial neglect is often used as a synonym for the neglect syndrome as a whole. Two sub-types of spatial neglect can be discerned. Peripersonal or near-space neglect is when symptoms manifest in the reaching distance of the patient, e.g., the example of eating half a plate of food. When symptoms occur in the space beyond reaching distance, e.g., inadvertently bumping into obstacles while walking, it is classified as extrapersonal or far-space neglect.  
Although these categories exist, USN remains an inherently heterogeneous syndrome, meaning that patients often experience multiple types of neglect simultaneously [7].