Speed–accuracy tradeoffs in specialized keyboards

Patients with locked-in syndrome are perceptually and cognitively aware of their environment but are unable to speak and have very limited motor capabilities. These patients frequently use a virtual keyboard with a cursor that moves over different items. The user triggers a selector when the cursor is over the desired item. For text entry such a method is excruciatingly slow, but is critical for patients who otherwise cannot communicate. We show how such keyboards can be optimally designed to maximize text entry speed while simultaneously controlling the entry error rate. The described method quantifies how different factors in keyboard design influence both entry speed and accuracy and demonstrates that different keyboard designs can greatly alter the efficiency of keyboard use. For a given text corpus and allowable average entry error proportion, the method identifies the cursor duration and character layout that minimizes average entry time. The method can easily be adapted to a variety of keyboard designs and selection devices and thereby improve the communication of locked-in syndrome patients.

► Some people must use specialized keyboards for text entry that use only a binary selection switch.
► We have identified how to optimally design such keyboards.
► The keyboards minimize text entry time and satisfy an allowable entry error proportion.