Abstract
Alternating dichotic presentation of tones was compared with monaural presentation in two different perceptual tasks. When Ss were asked to detect higher frequency target tones in lists of six background tones, they missed less than 5%. However, when the presentation was fast (lO/sec) and the tones alternated dichotically, Ss reported "hearing" only three or four tones. In two further experiments, Ss were asked to judge the number of tones or to report their spatial and temporal positions; these tasks were much less accurately performed, for the dichotic lists, than the target detection task. The differing effects of switching rate on these tasks are discussed in relation to the types of processing they require. Treisman (1971) reported and reviewed experiments in which Ss were asked to recall a list of auditory speech items. Performance was worse when the items within a list were presented alternately to the two ears than when they were presented sequentially to one or both. Further examination of the limitation involved requires investigation of its functional site, generality, and mode of operation. A precise identification of the changes in task demands associated with appearance of the effect would also be desirable. In Treisman's (1971) experiment, there was an interaction between rate and type of presentation but not between list length and type of presentation. This suggests that the limifation arises mainly during perceptual analysis rather than during storage or retrieval of the items. In the present experiment, this was tested by looking for the effect on a monitoring rather than a recall task. The limitation may occur only when speech items are used as stimuli. The present experiment tested this by employing pure tones rather than auditory speech items in a paradigm otherwise similar to Treisman's (1971) . If the perception of auditory speech and pure tones depend on the same mechanisms, the result may help to identify the level of perceptual analysis at which the limitation operates. Presence of the effect with both types of stimuli would indicate that it arises prior to speech recognition, whereas its presence only with speech signals would suggest that it occurs during speech recognition. Experiments by Lawson (1966) and Treisman and Riley (1969) lend some support to the latter possibility. They found that pure tones and changes of a speaker's voice were detected equally well in the attended and unattended channels. However, cast some doubt on Lawson's (1966) finding, in that they did find impaired detection of tones embedded in the unattended speech message when transients at the beginning and end of the tones were removed. Some limitation to attention has also been found with simultaneous tones. Moray (1970) and Moray and Fitter! found limits in Ss' ability to monitor in parallel two sequences of tones for increments in either frequency or intensity. These limits may have been due mainly to frequency rather than to dichotic separation of the tone sequences, but Moray and Fitter also found that dichotic separation increased the difficulty of their frequency-separated tones relative to binaural presentation. However, there may be factors other than the attention division requirement which affect performance in both experimental and control condi tions when stimuli are presented simultaneously. Firstly, binaural presentation may benefit from interaural summation of information, and so may not be an appropriate control when attention-sharing decrements are being sought. Secondly, there may be cross-channel inhibition with simultaneous dichotic presentation, as suggested by Sorkin, Pastore, and Pohlman (1971) . They found a small decrement with wo-channel compared to single-channel presentation. then two signals were presented at once, but little or no difference between single-and two-channel conditions on trials when no signal was present on the contralateral channel.
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