A set of experiments is described which assessed the ability of human observers to monitor two earphone channels in order to perform one or two independent frequency discriminations. Performance (d′) was significantly poorer with dichotic stimulus presentation than in monaural control conditions. A detailed analysis of the data suggested that two factors were involved in the dichotic performance deficits. The first factor was a limited ability of the observers to perceptually separate the different stimuli presented to the two earphone channels. Under certain stimulus conditions, the channel-separation factor was significant enough to produce a low performance ceiling which overshadowed additional deficits caused by the second factor. This second factor was a limited ability to time-share between two channels to perform the frequency discriminations with the same efficiency as when attention was directed toward a single channel. When overshadowed by the performance floor caused by channel-separation limitations, the time-sharing limitations may be examined only by an analysis of performance conditional upon contralateral stimulus events.
Providing the observers with stimulus parameters which make the channels more dissimilar (i.e., spectral or temporal separation) improved performance when attention was directed to only one channel. Sequential stimulus presentation was partially effective in increasing performance in selective- (or focused-) attention tasks, due to a unidirectional pattern of temporal interference, consistent with data on pitch recognition analogues to masking phenomena. The present investigation appears to make a significant step towards resolving an apparent controversy between several groups of researchers with regards to the nature of performance deficits in two-channel signal-detection paradigms
