Because humans naturally employ multimodal information
channels for communication, and because multimodal interfaces
have been demonstrated to be effective [19]. Cognitive
research has shown that multimodal communication results in
increased amount of transmitted information [14]. It has been
shown that a signal with a single varying attribute can at most
transmit 2–3 bits to a human observer (for example, we can
only identify about 5–7 loudness levels of a fixed-frequency
pure tone). However, greater information transmission can
be achieved by employing signals with multiple attributes
(for example, one can easily identify hundreds of faces at
a glance of a person or a photograph, because many facialfeatures contribute to the overall appearance of a face). This
increase in transmitted information can be obtained whether
multiple modalities convey different information or encode
the same information redundantly [14]. Therefore, multimodal
interfaces facilitate more natural and efficient human–computer
interactions.