Up- and down-tuning in attention: neuro-cognitive mechanisms of target selection and distractor suppression

In principle, selective attention results from target enhancement and distractor suppression. However, it is unclear whether the human neurocognitive system implements a mechanism to suppress distraction that is independent of target enhancement. Neural oscillatory power in the alpha frequency band (~10 Hz) has been implicated in the selection of targets, but there is lack of empirical evidence for its involvement in the suppression of distractors. I will present evidence from electroencephalography (EEG) in support of the hypothesis that alpha power directly relates to distractor suppression and thus operates independently from target selection. In an auditory spatial pitch discrimination task, we modulated the location (left vs right) of either a target or a distractor tone sequence, while fixing the other in the front. When the distractor was fixed in the front, alpha power relatively decreased in the hemisphere contralateral to the target and increased ipsilaterally. Most importantly, when the target was fixed in the front, alpha lateralization reversed in direction for the suppression of distractors on the left versus right. These data show that target-selection–independent alpha power modulation is involved in distractor suppression. While both lateralized alpha responses for selection and for suppression proved reliable, they were uncorrelated and distractor-related alpha power emerged from more anterior, frontal cortical regions. Lending functional significance to suppression-related alpha oscillations, alpha lateralization at the single-trial level was predictive of behavioral accuracy. I will argue that these results fuel a renewed look at neurobiological accounts of selection-independent suppressive filtering in attention.

Natural listening often comes with the challenge of attending one auditory foreground source in the presence of distracting background sources that need to be suppressed. Such scenarios can be experimentally simulated by presenting different sound sources and asking listeners to perform a task that requires selective attention on one of these sources. For a balanced investigation, the attended sound source is changed throughout the experiment. It turns out that some sources are easier to attend than others. For instance, when using sequences of three repeatedly presented tones (e.g. low, middle and high frequency tone: ‘ABC’ pattern), the middle tones (‘B’) are rarely reported as foreground source, and it is particularly difficult to perform a task on them. Here we report a combined behavioral-EEG study that was designed to reveal underlying mechanisms of this so-called middle-stream deficit. We replicated an impairment in behavioral performance for a selective-attention task on the middle tones compared to the outer tones. We then examined how the middle-stream deficit could be overcome. In line with our hypotheses, the performance deficit decreased with either a larger frequency separation between the tones or a higher intensity of the middle tones. Corresponding EEG data during passive listening shed light on bi-directional inhibition from the outer tones as a candidate mechanism for the middle-stream deficit and for its reduction by the applied manipulations. Findings will be discussed in light of current theories of auditory foreground-background formation and selective attention.

Listening to speech in adverse, noisy environments can be a difficult task for listeners with normal-hearing (NH), but significantly more so for hearing-impaired (HI) individuals. Here we investigate selective attention to speech and how different factors may influence it in normal-hearing and hearing-impaired individuals. In our investigation we use electroencephalography (EEG) and auditory attention decoding approach which relies on the fact that neural signals synchronize to continuous, running auditory stimulus, more so to attended than to ignored one. Our NH and aided HI participants listened to an audiobook presented in noise. We manipulated noise level (easy vs. hard listening), but also participants’ motivation by providing a monetary reward. Both participant groups performed better in easier listening condition, which was reflected in faster EEG impulse responses to speech. NH participants behaviorally performed better than HI, but increase in motivation indeed improved performance of HI group. In a separate study we also showed on neural level that selective attention abilities of HI participants improve with help of visual cues, such as speaker's lip movements. On behavioral level we showed that this increase is related to the amount of hearing loss. We suggest that auditory selective attention should be investigated in context of cues and scenarios that are common in every-day life as we do observe their influence on cortical speech tracking. In HI individuals these effects are even stronger than in NH and ecological approaches are needed to fully explain stream segregation and selective attention mechanisms.

Retroactive cuing of information after encoding improves working memory performance. However, there is an ongoing debate on the contribution of target enhancement vs. distractor inhibition attentional sub-processes to this behavioral benefit. We investigated the electrophysiological correlates of retroactive attentional orienting by means of oscillatory EEG parameters. In order to disentangle excitatory and inhibitory attentional processes, the to-be-memorized information was presented in a way that posterior hemispheric asymmetries in oscillatory power could be unambiguously linked to lateral target vs. distractor processing. We found an increase of posterior alpha power (8-14 Hz) contralateral to the position of non-cued working memory content and a decrease of alpha power contralateral to cued positions. These effects were insensitive to the number of cued or non-cued items, supporting their relation to the spatial orienting of attention. Importantly, only the alpha power increase contralateral to non-cued positions differed reliably from the asymmetry in a neutral control condition, highlighting the importance of an inhibitory mechanism for the retroactive focusing of attention. Furthermore, the alpha power asymmetries relative to the positions of cued and non-cued items predicted the individual susceptibility to interference by irrelevant information during working memory retrieval. These findings suggest that spatially specific modulations of posterior alpha power are related to enhancing vs. inhibiting the spatial context of information stored in working memory, thereby guaranteeing a target-oriented retrieval process.

Salient yet irrelevant objects often interfere with daily tasks by capturing attention against our best interests and intentions. Recent research showed that through implicit learning, distraction by a salient object can be reduced by suppressing the location where this distractor is likely to appear. Here, we investigated whether suppression of such high probability distractor locations is an all-or-none phenomenon or specifically tuned to the degree of interference caused by the distractor. In two experiments, we varied the salience of two task-irrelevant singleton distractors each of which was more likely to appear in one specific location in the visual field. We show that the magnitude of interference by a distractor determines the magnitude of suppression for its high probability location: the more salient a distractor, the more it becomes suppressed when appearing in its high probability location. We conclude that distractor suppression emerges as a consequence of the spatial regularities regarding the location of a distractor as well as its potency to interfere with attentional selection.