# From Patient to Musician: A Multi-Sensory Virtual Reality Rehabilitation Tool for Spatial Neglect
## 2 Background
### Rehabilitation Techniques for USN  
Many interventions for the rehabilitation of spatial neglect have been proposed; however, there is a need for more evidence regarding their effectiveness. Consequently, no standardised set of rehabilitation methods for USN exists [30]. The existing approaches can be categorised into top-down methods, bottom-up methods, modulation of inhibitory processes and increasing arousal. Top-down methods aim to stimulate the voluntary attention gaze of the patient towards the neglected side of space. This stimulation consists of cues and instructions from the therapist. VST is the most used form of top-down rehabilitation technique. VST aims to orientate the visual scanning of the patient back towards the neglected side by giving  
Figure 3: For the star cancellation test, all the stars have to be indicated. The difference in missed stars between the left and right side indicates potential USN. Figure 2: For the line bisection test, the middle of the line has to be indicated (**A**). A bias towards one of the extremities of the line can be an indication of USN (**B**). explicit instructions on where to look, e.g., asking them to locate the left-hand margin of the page before reading the next line [10]. Top-down methods are often criticised for their lack of specificity, as not all neglect symptoms seem to improve and non-visual sensory neglect is not addressed [35; 36]. Nevertheless, top-down approaches, and specifically VST, are still mainly used in clinical practice [37]. In an attempt to address the shortcomings of top-down methods, bottom-up methods were developed. A deficit in processing the sensory information of one side of space creates an unbalance in the patient's representation of space. Bottom-up methods aim to restore this balance by manipulating the patient's sensory environment [25; 38]. Sensory Stimulation is an example of such a method, which uses sensory input from different modalities, thereby addressing the multi-sensory aspect of USN, to eliminate this orientation bias towards the neglected side. The most well-known implementations of Sensory Stimulation are Optokinetic Stimulation (OKS), Neck-Muscle Vibration (NVM), Caloric Vestibular Stimulation (CVS) and Galvanic Vestibular Stimulation (GVS) [35]. Another well-known bottom-up technique is Prism Adaptation (PA), where patients wear prisms that shift their vision towards their non-neglected side. For left-sided neglect, the prism introduces an optical shift of several degrees to the right. At the start of the adaptation process, this leads to a right-sided error signal, as shown in Figure 4. After several iterations, compensation of the behaviour kicks in, and this error is corrected. When these prisms are then removed, a consistent deviation to the left occurs when pointing to a visual target, called the after-effect [39; 40]. A final example of a bottom-up technique is Limb Activation (LA), where patients are asked to make voluntary contralesional limb movements in their affected side of space. This, in turn, activates the corresponding areas of the brain dealing with extra- and peripersonal space [42]. Studies regarding LA show interesting results; however, long-term functional effectiveness has yet to be determined [36]. The next category are methods that address the modulation of inhibitory processes, such as Transcranial Magnetic Stimulation (TMS) and transcranial Direct Current Stimulation (tDCS). These techniques use non-invasive brain stimulation to reduce the activity of the contralesional hemisphere or increase the activity of the ipsilesional hemisphere [43]. The final category of techniques for the rehabilitation of USN includes methods that focus on the stimulation of arousal, such as Sustained Attention Training (SAT) [7]. USN has been linked with a non-lateralised attention deficit, i.e., patients often struggle to keep their attention focused for a longer time period, even in their non-affected side of space [44].