Attempts to identify physiological correlates of listening effort have mainly focused on peripheral measures (e.g. pupillometry) and auditory-evoked/event-related potentials. Although nonauditory studies have suggested that sustained time-frequency electroencephalographic (EEG) features in the θ-band (4-7 Hz) are correlated with domain-general mental effort, little work has characterized such features during effortful listening. Here, high-density EEG data was collected while listeners performed a sentence-recognition task in noise, the signal-to-noise ratio (SNR) of which varied across blocks. Frontal midline θ (Fmθ), largely driven by sources localized in or near the medial frontal cortex, showed greater power with decreasing SNR and was positively correlated with self-reports of effort. Increased Fmθ was present before speech onset and during speech presentation. Fmθ power also differed across SNRs when including only trials in which all words were recognized, suggesting that the effects were unrelated to performance differences. Results suggest that frontal cortical networks play a larger role in listening as acoustic signals are increasingly masked. Further, sustained time-frequency EEG features may usefully supplement previously used peripheral and event-related potential measures in psychophysiological investigations of effortful listening.

In listening tasks where a target speech signal is spatially separated from a masking voice, listeners can often gain a substantial advantage in performance by attending to the ear with the better signal-to-noise ratio (SNR). However, this better-ear strategy becomes much more complicated when a target talker located in front of the listener is masked by interfering talkers positioned at symmetric locations to the left and right of the target. When this happens, there are no long-term SNR advantages at either ear and the only binaural SNR advantages available are the result of complicated better-ear glimpses that vary as a function of frequency and rapidly switch back and forth between the two ears according to the natural fluctuations in the relative levels of the two masking voices. In this study, a signal processing technique was used to take the better-ear glimpses that would ordinarily be randomly distributed across the two ears in a binaural speech signal and move them all into the same ear. This resulted in a monaural signal that contained all the information available to an ideal listener using an optimal binaural glimpsing strategy. Speech intelligibility was measured with these optimized monaural stimuli and compared to performance with unprocessed binaural speech stimuli. Performance was similar in these two conditions, suggesting that listeners with normal hearing are able to efficiently extract information from better-ear glimpses that fluctuate rapidly across frequency and across the two ears.

Recent studies have related enhancements of theta- (∼4-8 Hz) and alpha-power (∼8-13 Hz) to listening effort based on parallels between enhancement and task difficulty. In contrast, nonauditory works demonstrate that, although increases in difficulty are initially accompanied by increases in effort, effort decreases when a task becomes so difficult as to exceed one's ability. Given the latter, we examined whether theta- and alpha-power enhancements thought to reflect effortful listening show a quadratic trend across levels of listening difficulty from impossible to easy. Listeners (n = 14) performed an auditory delayed match-to-sample task with frequency-modulated tonal sweeps under impossible, difficult (at ∼70.7% correct threshold), and easy (well above threshold) conditions. Frontal midline theta-power and posterior alpha-power enhancements were observed during the retention interval, with greatest enhancement in the difficult condition. Independent component-based analyses of data suggest that theta-power enhancements stemmed from medial frontal sources at or near the anterior cingulate cortex, whereas alpha-power effects stemmed from occipital cortices. Results support the notion that theta- and alpha-power enhancements reflect effortful cognitive processes during listening, related to auditory working memory and the inhibition of task-irrelevant cortical processing regions, respectively. Theta- and alpha-power dynamics can be used to characterize the cognitive processes that make up effortful listening, including qualitatively different types of listening effort.

In many multitalker listening tasks, the degradation in performance that occurs when the number of interfering talkers increases from one to two is much larger than would be predicted from the corresponding decrease in the signal-to-noise ratio (SNR). In this experiment, a variety of contextually-relevant speech maskers, contextually-irrelevant speech maskers and non-speech maskers were used to examine the impact that the characteristics of the interfering sound sources have on the magnitude of this "multimasker penalty." The results show that a significant multimasker penalty only occurred in cases where two specific conditions were met: 1) the stimulus contained at least one contextually-relevant masker that could be confused with the target; and 2) the signal-to-noise ratio of the target relative to the combined masker stimulus was less than 0 dB. Remarkably, in cases where one masker was contextually relevant, the specific characteristics of the second masker had virtually no impact on the size of the multimasker penalty. Indeed, when the results were corrected for random guessing, there was essentially no difference in performance between conditions with three contextually-relevant talkers and those with two contextually-relevant talkers and one irrelevant talker. The results of a second experiment suggest that the listeners are generally able to hear keywords spoken by all three talkers even in situations where the multimasker penalty occurs, implying that the primary cause of the penalty is a degradation in the listener's ability to use prosodic cues and voice characteristics to link together words spoken at different points in the target phrase.