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Twenty operators were given a task which required answering one of two simultaneous voice messages. The task was performed under a variety of conditions produced by combinations of four experimental “aid” variables: horizontal spatial separation of the sound sources, aural shaping filters which made the tone quality different in each channel, visual cues which indicated the channel about to call the operator, and facilities to “pull down” a desired message from the initial source into a headphone or a loudspeaker near the operator's ear. It was found that the use of horizontal separation and/or the filtered messages greatly improved the operator's performance. Visual cues had no apparent effect on ability to answer the message except when used with pull down facilities. The pull down facilities aided the operator to some extent. Some speculations are advanced about the nature of recognizing and attending to a message in the presence of another message.
Spieth et al. (Sat,) studied this question.