In various studies of selective listening (e.g., Morey, 1969), auding has been studied as a tracking task-the ability to follow a "moving" spoken message embedded within distracting noises, including situations in which two or more spoken messages may compete for attention. The research questions have focused upon the identification of the structured information or features which the auder may use to follow along with, attend to, or track the target message of the task. In Figure 3 we have outlined the more important features of a spoken message which research indicates an auder may use in tracking a message. We have sorted these features into acoustic, linguistic, and semantic categories.
Features a listener may use to track a spoken message
Regarding Acoustic Features, three dimensions are considered as significant factors with respect to aural message tracking: (a) spatial localization, (b) voice quality, and (c) intensity. Investigations using dichotically presented messages and shadowing tasks (Cherry, 1953; Treisman, 1960, 1964a, 1964b) have demonstrated that the structured information in the environment produced by having two ears (which produces the possibility of a sort of "triangulation" ability to localize objects in space), is a major dimension that allows for the tracking of a single message embedded in a noisy surrounding. Similarly, evidence has indicated (Cherry, 1953; Egan, Carterette, and Thwing, 1954; Talhurst and Peters, 1956; Triesman, 1964a, 1964b) that both voice quality features (ie., the acoustic spectrum in voices) and message intensity can serve as information for aural tracking. Both features exhibit a facilitory effect on the ability to selectively track dichotic messages (ie., competing messages presented one to each ear).