We used attentional cueing procedures analogous to those employed by Green and colleagues (2005
to investigate the claim that the ADAN component is absent when shifts of attention are signalled by auditory cues. The central finding was that an ADAN was reliably elicited regardless of whether probes were presented infrequently (as in Green et al., 2005
), or more frequently, so that stimuli at uncued locations were less likely to be task-relevant. The presence of an ADAN in the frequent probe condition demonstrates that this component can be observed under purely unimodal auditory attention conditions. Its presence in the infrequent probe condition is surprising, as it is inconsistent with results by Green et al. (2005
, Experiment 4), who found no ADAN under virtually identical conditions. One possible explanation for this discrepancy is that Green et al. (2005)
quantified the ADAN exclusively for electrodes F5/6. In the present experiment, the ADAN was very small at F5/6 in the infrequent probe condition (), and in fact non-significant during the critical 300-500 ms interval. However, Green & McDonald (2006)
found no reliable ADAN for auditory cues (upward or downward frequency sweeps) followed by visual targets even when all four anterior electrode pairs included in the present analysis were considered. The fact that the ADAN observed in the present experiment, although reliable, was considerably smaller than the ADAN observed previously for auditory cues signalling the location of task-relevant visual or tactile events (Eimer & van Velzen, 2002
; Eimer et al., 2003
; Van Velzen et al., 2006
) suggests that this component may be attenuated in purely unimodal auditory attention tasks, which could have contributed to the negative results reported by Green and colleagues.
It is unlikely that the ADAN observed here resulted from undetected lateral eye movements. As can be seen in the grand-averaged HEOG waveforms (), eye movement rejection and the exclusion of three participants with poor gaze control ensured that there were no systematic eye gaze deviation during the cue-target interval. The presence of ADAN components in the present experiment demonstrates that this component is triggered in response to auditory cues in a unimodal auditory attention task, and is consistent with previous observations that an ADAN is elicited by auditory cues during visual or tactile attention tasks (Eimer & Van Velzen, 2002
; Eimer et al. 2003
), and is also reliably observed in the congenitally blind when auditory cues direct attention to the location of task-relevant tactile events (Van Velzen et al., 2006
). Green and colleagues have argued against an interpretation of the ADAN in terms of supramodal attentional control because this component was absent with auditory cues, and suggested that the ADAN is linked to the control of attention in visual (and possibly tactile) space. The present demonstration that this component can be reliably elicited in a unimodal cued auditory attention task provides strong evidence against this argument, and thus supports a supramodal interpretation.
The present results and the results by Green et al. (2005)
and Green & McDonald (2006)
converge with respect to the posterior LDAP component. This component was consistently present, demonstrating that the LDAP is not confined to visual and tactile attention tasks, but is also triggered in a unimodal auditory context. While this is consistent with the hypothesis that this component is linked to supramodal attentional control, other interpretations remain possible. The fact that the LDAP is eliminated during shifts of tactile attention in the congenitally blind (Van Velzen et al., 2006
) suggests that this component may be associated with the availability of visually mediated spatial information. Given the superior spatial acuity of vision, visual-spatial coordinates may be used whenever available to anchor shifts of attention in external space, even when other modalities are task-relevant, and the LDAP might emerge as a result of such visually mediated attention shifts.
In summary, the present study has shown that an ADAN can be observed during shifts of auditory attention that are triggered by symbolic auditory cues. This demonstrates that this component is not specific to tasks where visual or tactile stimuli are relevant, and is consistent with the hypothesis that the ADAN reflects supramodal attentional control mechanisms.