Attentional gain control of ongoing cortical speech representations in a "cocktail party"

Abstract

Normal listeners possess the remarkable perceptual ability to select a single speech stream among many competing talkers. However, few studies of selective attention have addressed the unique nature of speech as a temporally extended and complex auditory object. We hypothesized that sustained selective attention to speech in a multitalker environment would act as gain control on the early auditory cortical representations of speech. Using high-density electroencephalography and a template-matching analysis method, we found selective gain to the continuous speech content of an attended talker, greatest at a frequency of 4-8 Hz, in auditory cortex. In addition, the difference in alpha power (8-12 Hz) at parietal sites across hemispheres indicated the direction of auditory attention to speech, as has been previously found in visual tasks. The strength of this hemispheric alpha lateralization, in turn, predicted an individual's attentional gain of the cortical speech signal. These results support a model of spatial speech stream segregation, mediated by a supramodal attention mechanism, enabling selection of the attended representation in auditory cortex.

Figure 1.

Trial structure. A cue is presented to indicate the to-be-attended direction, followed by the onset of one of the sentences (Single Talker condition) or both simultaneously (Selective Attention condition). The expanded, inset box depicts a subject attending to sentence A while the EEG waveform is recorded. Upon hearing the final word, the participant then judges whether the attended sentence was congruent or incongruent.

Figure 2.

EEG Waveform analysis. a, Data from the 128-electrode array were reduced to two source waveforms in the left and right hemispheres, based on early-onset (N1) response to the first word of each sentence. Sources were localized in or near early auditory cortex. b, A diagram of the regression analysis used to produce the discrimination index. For a given subject, each trial EEG waveform is matched to a group average template for the same (“Within”) or a different (“Across”) sentence. A positive discrimination index means that the EEG signal can distinguish which sentence was presented or attended.

Figure 3.

Gain control of speech representation across frequency. The DI is shown for three comparisons, across frequency. For Speech Encoding, a significant DI means that the EEG can distinguish sentence A from sentence B when presented alone. For Attentional Gain, a significant DI means that in a multitalker environment, attention modulates the EEG to distinguish which sentence was attended. In Generalized Attentional Gain, a significant DI means that to distinguish sentences, multitalker attention modulates the EEG signal that represents speech when presented alone. That is, attention changes the gain of the single-talker EEG signal.

Figure 4.

Spatial attention and sentence content selection. a, Scalp topographic maps over time of alpha activity following the onset of the sentences (left minus right cued conditions). The superimposed line plots represent the normalized time course of the Single Talker and Selective Attention alpha lateralization indices. b, Topographic maps of the discrimination indices following the onset of the sentences. The superimposed line plots represent the normalized time course of the Speech Encoding and Generalized Attentional Gain discrimination indices, averaged from the two source waveforms.

Figure 5.

Early alpha lateralization predicts generalized attentional gain. A scatter plot of the average early Selective Attention alpha lateralization index versus late Generalized Attentional Gain discrimination index for each individual participant, with the line of best fit. Parietal alpha lateralization predicts how strongly attention modulates the selected speech representation.