Previous probe-signal studies of auditory spatial attention have shown faster responses to sounds at an expected versus an unexpected location, making no distinction between the use of interaural time difference (ITD) cues and interaural-level difference cues. In 5 experiments, performance on a same-different spatial discrimination task was used in place of the reaction time metric, and sounds, presented over headphones, were lateralized only by an ITD. In all experiments, performance was better for signals lateralized on the expected side of the head, supporting the conclusion that ITDs can be used as a basis for covert orienting. The performance advantage generalized to all sounds within the spatial focus and was not dissipated by a trial-by-trial rove in frequency or by a rove in spectral profile. Successful use by the listeners of a cross-modal, centrally positioned visual cue provided evidence for top-down attentional control.

The tuning of auditory spatial attention with respect to interaural level and time difference cues (ILDs and ITDs) was explored using a rhythmic masking release (RMR) procedure. Listeners heard tone sequences defining one of two simple target rhythms, interleaved with arhythmic masking tones, presented over headphones. There were two conditions, which differed only in the ILD of the tones defining the target rhythm: For one condition, ILD was 0 dB and the perceived lateral position was central, and for the other, ILD was 4 dB and the perceived lateral position was to the right; target tone ITD was always zero. For the masking tones, ILD was fixed at 0 dB and ITDs were varied, giving rise to a range of lateral positions determined by ITD. The listeners' task was to attend to and identify the target rhythm. The data showed that target rhythm identification accuracy was low, indicating that maskers were effective, when target and masker shared spatial position, but not when they shared only ITD. A clear implication is that at least within the constraints of the RMR paradigm, overall spatial position, and not ITD, is the substrate for auditory spatial attention.

Performance on a same–different ITD discrimination task was used as a metric for auditory spatial attention. In several variants of a probe-signal procedure, attention was directed as detailed below. In all experiments, performance was better for signals lateralized on the expected side of the head. In experiment 1, a lateralized auditory cue preceded the signal. Lest this functioned not simply as an attentional cue but also as a spatial landmark, experiment 2 used no cue but manipulated expectation by presenting trials more frequently on a particular side. To establish whether the effects in experiment 2 generalized to all sounds within the spatial focus or were attributable simply to greater familiarity with the expected stimulus, a rove was incorporated in the pitch (experiment 3) or the timbre (experiment 4) of the signals on each trial. Experiment 5 introduced trial-by-trial visual cueing which, necessarily endogenous, provided evidence for top–down attentional control. Further work established whether analogous effects are seen on a frequency discrimination task, and when sounds are lateralized by IID. [Work supported by UK BBSRC.]

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