THREE BASIC APPROACHES
Although a great many different aids have been developed to provide the physically handicapped with a means of controlling other devices, their control schemes are all essentially variations on three basic approaches.
These approaches are scanning, encoding and direct selection. Each of these approaches has different advantages and disadvantages and will work better with one kind of disability than with others.
Some of the techniques such as scanning are much more powerful in that they can be used by even the most severely handicapped. Other techniques may be less powerful but more efficient, thus allowing individuals who have more control over their movements to be able to communicate in a faster manner by taking advantage of their better physical abilities.