The Inference Group at Cambridge University originally intended to create a method of entering text into PDAs and other mobile devices. The result, however, has provided a vital alternative to standard On-Screen Keyboards that are used by thousands of people with physical disabilities.
Most OSKs have a layout very similar to a regular keyboard only displayed on the screen rather than as a physical group of keys. Dasher is radically different and as a consequence can provide typing rates of up to 39 words per minute (although 20-30 wpm is more realistic).
Rather than using static keys, Dasher employs a sort of 'letter stream' where the alphabet flows towards you and you catch your letters as they pass. Actually you're really 'zooming' in to each letter and revealing more letters underneath that will go to form the next character in the word you are typing.
The program employs an intelligent prediction engine that is used to define the size of subsequent letters. For example if you zoom into a Q then by far the largest letter to come next, and consequently the easiest and quickest to select, will be U. As well as the standard alphabet Dasher also supports spaces, carriage returns and punctuation. As with some other programs, Dasher automatically learns new words and adjusts its prediction engine accordingly.
The screenshot here demonstrates how after typing 'techn' the program makes it far easiest to end the word with 'ology' and then a space ( _ ).
Dasher is designed to help people with physical difficulties and therefore this is where it is most accessible. It can be accessed using any pointing device but people are particularly excited about the use of eye tracking technologies. Although these systems are in their infancy a few are emerging that, coupled with Dasher, could provide severely disabled people with a method of producing text in a fast and comfortable manner. For those with decent head movement a head pointer could provide a cheaper, and more established, solution.
Dasher's latest release, version 4, is switch-accessible but I am unable to find any documentation on this. Playing around with the settings in the program doesn't seem to produce any completely understandable results.
A user's cognitive abilities need to be taken into account as Dasher requires a lot of concentration to learn and stick at. In fact a quick glance at the program can be very de-motivating as it looks extremely complex and difficult to use. Actually it's surprisingly easy for people with good literacy skills and concentration and can actually be quite fun.
Although the letters can be made much larger the program still requires good visual skills, especially at quickly recognising letter shapes and determining their positions relative to one another.
Dasher is free and quick to download 
from the Cambridge University Inference Group website. It is available for a variety of computer platforms including Windows, Unix (GNU/Linux), Mac OS X and Pocket PC.
Dasher is constantly evolving and adding new features. The team responsible for Dasher are very enthusiastic about its potential to assist people with physical difficulties and are frequently trying out new access devices as well as constructing their own innovative solutions.
There is lots more information about the project and its use in Special Educational Needs on the official Dasher website .
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