There is growing evidence that spatial attention can also influence processing of sounds. When a predictive auditory spatial cue is followed by an auditory target, response latencies to the target are shorter when the two sounds appear at the same location compared to they appear at different locations. Such effects occur in tasks involving identity-based choice responses (McDonald & Ward, in press; Mondor & Zatorre, 1995), and location-based choice responses (Bédard, Massioui, Pillon, & Nandrino, 1993; Quinlan & Bailey, 1995; Spence & Driver, 1994). However, there is conflicting evidence regarding the effect of predictive spatial cueing on simple reaction time to sounds. Small cue effects have been observed in some auditory simple reaction time tasks (Bédard et al., 1993; Buchtel, Butter, & Ayvasik, 1996; Quinlan & Bailey, 1995) but not in others (Buchtel & Butter, 1988; Posner, 1978). Several researchers have argued that spatial attention typically does not affect simple auditory reactions because such responses can occur before sound localization takes place (Posner, 1978; Rhodes, 1987; Spence & Driver, 1994). The argument is based on the fact that the auditory receptor neurons topographically represent the frequency spectrum of sounds rather than their spatial locations. The location of a sound source must therefore be computed by location-sensitive neurons in more central brain areas based on differences between the ears in phase, intensity, or frequency spectrum of the sound.

In light of this consideration, several researchers have examined the effects of non-predictive spatial cueing in location-based auditory discrimination tasks (McDonald & Ward, in press; Spence & Driver, 1994; Ward, 1994). However, the results of such tasks can be weakened by non-attentional explanations for the observed cue effects. The most serious problem is that the usual facilitatory effect of spatial cueing can often be attributed to response-level effects. This is particularly problematic when both the cue and target appear on the left or right side of fixation and subjects are required to make left-right discrimination responses based on the location of the target. In this situation, subjects might response faster on valid-cue trials than on invalid-cue trials simply because the cue activated the correct response (cf. the Simon effect; Simon & Small, 1967).

We developed the "implicit" spatial discrimination task to eliminate the possibility of response- priming by the cue (McDonald & Ward, in press). In this task, the decision to make a go/no-go response is based on the spatial location of the target stimulus. For example, listeners might respond, by pressing a single button, to targets emanating from either of two peripherally-located speakers but not to targets emanating from a centrally-located speaker. The rationale for this technique is that the requirement to respond to targets appearing at certain locations ensures the use of location-sensitive neurons in making responses, but it does not allow cues to activate responses differentially on valid-cue and invalid-cue trials. Using this technique, we found that a non-predictive auditory spatial cue does influence responses to an auditory target, enhancing performance on valid-cue trials at short cue-target intervals and inhibiting performance on valid-cue trials at long cue-target intervals, However, the same cue did not influence performance under identical stimulus conditions when the decision to respond was based on non-spatial criteria. These results demonstrate that performance enhancements arising from stimulus-driven attention orienting do occur in the auditory domain but only when the decision to respond is based on the spatial location of the target.