Electromyographic correlates of effortful listening in the vestigial auriculomotor system

Abstract

Recently, electromyographic (EMG) signals of auricular muscles have been shown to be an indicator of spatial auditory attention in humans, based on a vestigial pinna-orienting system. Because spatial auditory attention in a competing speaker task is closely related to the more generalized concept of attentional effort in listening, the current study investigated the possibility that the EMG activity of auricular muscles could also reflect correlates of effortful listening in general. Twenty participants were recruited. EMG signals from the left and right superior and posterior auricular muscles (SAM, PAM) were recorded while participants attended a target podcast in a competing speaker paradigm. Three different conditions, each more difficult and requiring a higher amount of effortful listening, were generated by varying the number and pitch of distractor streams, as well as the signal-to-noise ratio. All audio streams were either presented from a loudspeaker placed in front of the participants (0°), or in the back (180°). Overall, averaged PAM activity was not affected by different levels of effortful listening, but was significantly larger when stimuli were presented from the back, as opposed to the front. Averaged SAM activity, however, was significantly larger in the most difficult condition, which required the largest amount of effort, compared to the easier conditions, but was not affected by stimulus direction. We interpret the increased SAM activity to be the response of the vestigial pinna-orienting system to an effortful stream segregation task.

Keywords: auricular muscles; effortful listening; electromyography (EMG); objective measures; superior auricular muscle.

Figure 1

Experimental setup indicating the positions of the loudspeakers around the participant inside the 3 × 3 × 3 m cubicle made of heavy stage molton. Note that the experimenter remained outside of the cubicle.

Figure 2

Positions of the five electrodes used to cover the SAM.

Figure 3

Averaged values of the normalized perceived listening effort (LE) and target lost ratings, as well as percentages of correctly answered questions and topic recall scores. Both, LE scores and target lost values significantly increase when the paradigms become more effortful. The differences between easy and medium are much larger when considering the target lost, than the LE scores. Question and topic scores are primarily used to indicate that participants attempted to solve all paradigms, and did not give up or disengage during the difficult condition. P-values were obtained using Bonferroni corrected paired t-tests (df = 19). Black dots outside of the boxplots indicate outliers.

Figure 4

Left: Time-resolved normalized activity of the superior auricular muscle (SAM) depending on the three effortful listening conditions. There appears to be a trend that the contrast between the difficult and easy/medium conditions increases with time, and diminishes in the last few seconds. Right: Averaged and normalized SAM activity according to the effortful listening conditions. SAM activity was significantly larger during the difficult condition than during the easy and medium conditions. P-values were obtained using Bonferroni corrected paired t-tests (df = 19).

Figure 5

Boxplots of the normalized energy of the posterior auricular muscle (PAM), depending on the stimulus direction and time. PAM activity was significantly larger when stimuli were presented from the loudspeaker located behind the participants, than from the loudspeaker in the front (top left). For data from the second halves of the trials, the same effect was observed (top right). When comparing PAM activity from the first halves to the second halves of the trials, there was no significant difference when stimuli were presented from the front (bottom left), but activity was significantly larger during the first half, when stimuli were presented from the back (bottom right). P-values were obtained using Bonferroni corrected paired t-tests (df = 19). Black dots outside of the boxplots indicate outliers.

Figure 6

Boxplots of the normalized energy of the superior auricular muscle (SAM), posterior auricular muscle (PAM), and M. masseter, arranged to be in presentation order for every participant during the experiment. There were no significant differences associated with the presentation order, indicating that there were no fatiguing or habitation effects. Black dots outside of the boxplots indicate outliers.