\subsection{Wheelchair - User Interface}\label{accessdevice}
The design of the input device is critical for efficient control of the wheelchair as it is the interface for the user to convey his or her intentions for movement of the wheelchair. Moreover, the user interface must take the user's abilities into consideration (Cooper et al, 2003). Joysticks operated by hand are the most common access devices for EPWs. Switches or keypads are also used. However, users with tremor or spasticity may not be able to manoeuvre the wheelchair appropriately with these devices, especially in narrow spaces (Philips et al, 2007). Such users or those with tetraplegia may opt for other input devices such as joysticks operated by chin, mouth, head or tongue, breath-controlled devices (sip and puff), tongue-touch keypads etc. (Somers, 2010). Many research groups have developed more complex wheelchair-user interfaces such as voice-controlled input (Simpson et al, 1998) and eye movement tracking (Yanco, 2000) or face direction tracking (Kuno et al, 2003). Graphical user interfaces have also been tested (Yanco, 2000; Zeng et al, 2006). Human-computer interfaces based on EMG (electromyogram) signals have also been developed. For example, Moon et al (2005) detected shoulder movements using EMG to deduce the intentions of the users. For people with severe disabilities such as locked-in syndrome, none of the above methods can be used for communicating with the wheelchair. Hence brain-computer interfaces have been developed to utilise EEG (electroencephalogram) signals for understanding the intentions of the user (Philips et al, 2007; Rebsamen et al, 2010). This is the type of user interface that is considered in this thesis and is detailed in \hyperref[BCI]{section 1.3}.