The effects of selective attention and speech acoustics on neural speech-tracking in a multi-talker scene

Abstract

Attending to one speaker in multi-speaker situations is challenging. One neural mechanism proposed to underlie the ability to attend to a particular speaker is phase-locking of low-frequency activity in auditory cortex to speech’s temporal envelope (“speech-tracking”), which is more precise for attended speech. However, it is not known what brings about this attentional effect, and specifically if it reflects enhanced processing of the fine structure of attended speech. To investigate this question we compared attentional effects on speech-tracking of natural vs. vocoded speech which preserves the temporal envelope but removes the fine-structure of speech. Pairs of natural and vocoded speech stimuli were presented concurrently and participants attended to one stimulus and performed a detection task while ignoring the other stimulus. We recorded magnetoencephalography (MEG) and compared attentional effects on the speech-tracking response in auditory cortex. Speech-tracking of natural, but not vocoded, speech was enhanced by attention, whereas neural tracking of ignored speech was similar for natural and vocoded speech. These findings suggest that the more precise speech tracking of attended natural speech is related to processing its fine structure, possibly reflecting the application of higher-order linguistic processes. In contrast, when speech is unattended its fine structure is not processed to the same degree and thus elicits less precise speech tracking more similar to vocoded speech.

Effects of Spectral Degradation on Attentional Modulation of Cortical Auditory Responses to Continuous Speech

“…examined the neural responses to vocoder speech embedded in stationary broadband noise and found that delta band neural response enhanced with increasing spectral resolution and this enhancement correlated with speech comprehension scores. Rimmele et al (2015) further demonstrated that spectral resolution influences not only neural tracking of speech but also differentially influences the neural tracking of attended and unattended speech streams, in the case when the attended and unattended speech streams had different spectral resolutions. Different from these studies, we (1) systematically examined the effect of spectral degradation on selective attention in a competing background by varying the number of vocoder channels and (2) investigated the relationship between neural modulation responses and speech comprehension ability.…”

High-frequency neural activity predicts word parsing in ambiguous speech streams

“…If the present LFO modulations cannot be explained by shifts in the parsing windows for speech segmentation, they may alternatively reflect an attentional modulation of acoustic processing, i.e., an enhanced neural excitability to particular acoustic features as has previously been reported for various kinds of sound stimuli (Besle et al 2011;Cravo et al 2013;Gomez-Ramirez et al 2011;Lakatos et al 2008;Rimmele et al 2015;Schroeder and Lakatos 2009a;Stefanics et al 2010;Zion-Golumbic et al 2013). One possibility is that the observed top-down effects reflect the processing of nonlexical speech information relevant for speech segmentation.…”

“…Delta oscillations were shown to delineate the perceived linguistic structure (phrases and sentences) within continuous speech, suggesting that LFO may be actively relevant for the parsing of smaller linguistic units such as words or syllables, which is the focus of this study. So far, the strength of LFO entrainment has been reported to systematically correlate with speech intelligibility (Ahissar et al 2001;Ding and Simon 2013;Doelling et al 2014;Gross et al 2013;Peelle et al 2013;Rimmele et al 2015), implying that LFO may be relevant for word segmentation. However, and importantly, speech intelligibility was also confounded with changes in the acoustic properties of the speech signal, leaving open the possibility that the observed modulations of LFO were driven by acoustic cues.…”

“…LFO are known to be under top-down attentional control: both attention and stimulus expectation can modulate the phase of entrained neural oscillations bringing periods of high neural excitability in phase with stimulus presentation, thereby facilitating the detection of the attended sensory inputs (Besle et al 2011;Cravo et al 2013;Gomez-Ramirez et al 2011;Lakatos et al 2008;Schroeder and Lakatos 2009a;Stefanics et al 2010). In complex auditory environments, the control of neural oscillations by attention has been shown to be beneficial for speech processing, as well (Rimmele et al 2015;Zion Golumbic et al 2013), suggesting that when speech perception is under attentional or volitional control, LFO may correlate with the outcome of word comprehension.…”

“…In particular, two main oscillatory regimes are deemed fundamental for the encoding of speech. First, low-frequency neural oscillations in the delta to theta range (2-8 Hz) have been shown to follow natural speech rhythms, enabling the tracking of the temporal structure of acoustic speech features such as syllables and words (Ahissar et al 2001;Ding and Simon 2013;Doelling et al 2014;Gross et al 2013;Poeppel 2007, 2012;Millman et al 2013;Peelle and Davis 2012;Rimmele et al 2015;Zion Golumbic et al 2013). Second, high-frequency neural activities, including the beta (20 -30 Hz) and gamma bands (Ͼ40 Hz), have been hypothesized to encode the finegrained properties of the speech signal such as phonetic features (Ghitza 2011;Giraud and Poeppel 2012;Poeppel 2003;Poeppel et al 2008).…”