Task-dependent costs in processing two simultaneous auditory stimuli.

A listener presented with two speech signals must at times sacrifice the processing of one signal in order to understand the other. This study was designed to distinguish costs related to interference from a second signal (selective attention) from costs related to performing two tasks simultaneously (divided attention). Listeners presented with two processed speech-in-noise stimuli, one to each ear, either (1) identified keywords in both or (2) identified keywords in one and detected the presence of speech in the other. Listeners either knew which ear to report in advance (single task) or were cued afterward (partial-report dual task). When the dual task required two identification judgments, performance suffered relative to the single-task condition (as measured by percent correct judgments). Two different tasks (identification for one stimulus and detection for the other) resulted in much smaller reductions in performance when the cue came afterward. We concluded that the degree to which listeners can simultaneously process dichotic speech stimuli seems to depend not only on the amount of interference between the two stimuli, but also on whether there is competition for limited processing resources. We suggest several specific hypotheses as to the structural mechanisms that could constitute these limited resources.