Impoverished auditory cues limit engagement of brain networks controlling spatial selective attention

Abstract

Spatial selective attention enables listeners to process a signal of interest in natural settings. However, most past studies on auditory spatial attention used impoverished spatial cues: presenting competing sounds to different ears, using only interaural differences in time (ITDs) and/or intensity (IIDs), or using non-individualized head-related transfer functions (HRTFs). Here we tested the hypothesis that impoverished spatial cues impair spatial auditory attention by only weakly engaging relevant cortical networks. Eighteen normal-hearing listeners reported the content of one of two competing syllable streams simulated at roughly +30° and -30° azimuth. The competing streams consisted of syllables from two different-sex talkers. Spatialization was based on natural spatial cues (individualized HRTFs), individualized IIDs, or generic ITDs. We measured behavioral performance as well as electroencephalographic markers of selective attention. Behaviorally, subjects recalled target streams most accurately with natural cues. Neurally, spatial attention significantly modulated early evoked sensory response magnitudes only for natural cues, not in conditions using only ITDs or IIDs. Consistent with this, parietal oscillatory power in the alpha band (8-14 ​Hz; associated with filtering out distracting events from unattended directions) showed significantly less attentional modulation with isolated spatial cues than with natural cues. Our findings support the hypothesis that spatial selective attention networks are only partially engaged by impoverished spatial auditory cues. These results not only suggest that studies using unnatural spatial cues underestimate the neural effects of spatial auditory attention, they also illustrate the importance of preserving natural spatial cues in assistive listening devices to support robust attentional control.

Keywords: Auditory spatial selective attention; Electroencephalography; Head-related transfer functions.

Fig. 1.

Auditory spatial attention task with two competing streams was used to assess the consequence of impoverished auditory spatial cues on neural proxies of attention control. An auditory cue was presented first from the location of the upcoming target stream, processed by the same spatialization scheme as the upcoming mixture. Following the cue, the competing streams began, one from around +30° the other from around −30° azimuth. Listeners were asked to recall the syllable sequence presented from the cued side. The first syllables of both streams were temporally aligned; however, the latter two syllables in the competing streams were staggered, enabling us to isolate neural responses to each. Feedback was provided after every trial.

Fig. 2.

Listeners’ (N = 18) recall performance was evaluated for every syllable and every spatialization condition. Sounds were spatialized either based on generic ITDs, individualized IIDs, or the natural combination of ITDs, IIDs, and spectral cues in individualized HRTFs. Behavioral advantages of having more consistent spatial information were statistically significant but small in absolute terms. * P < .05; ** P < .001; ***P < .0001

Fig. 3.

P1 amplitudes were only modulated by the attended direction in the HRTF condition, whereas N1 amplitudes were modulated equally strongly across spatialization conditions (N = 18). A. ERP waveforms at fronto-central electrodes were compared between the attended target stream and the unattended distractor stream for every spatialization condition. The P1 time range was defined as 50 ms to 100 ms, and the N1 time range as 100 ms to 180 ms. B. Most topographies of both ERP components show maxima at the fronto-central sites (black dots) used for evaluation. C. The modulation strength of ERP components was assessed by the amplitude differences between attended and unattended streams. * P < .05; ** P < .01

Fig. 4.

Within-subject differences in alpha-band GFP are larger in the HRTF condition, especially during the preparatory time window (after the sound cue but before the first syllables of the competing streams). A. Waveforms of the average (± SEM) GFP differences are shown during the baseline period, preparatory phase, and stimulus phase with stream competition. B. The temporal average of the preparatory alpha GFP difference is larger for the HRTF condition. ** P < .01

Fig. 5.

Attentional modulation of alpha activity was lateralized to the hemisphere ipsilateral to the target stream only in the HRTF condition. AMI topographies and hemispheric averages are shown for every spatialization condition (N = 18). * P < .05