Microsaccades and Pupillary Responses Represent the Focus of Auditory Attention

Previous studies have demonstrated that microsaccades and pupillary responses reflect visual covert attention (Engbert & Kliegl, 2003; Mathôt et al., 2013). However, it is unclear whether the effect is specific to vision and what processes are potentially underlying. In the current study, we investigated the sensitivities of microsaccades and pupillary responses to covert spatial attention in the auditory domain. In two experiments, participants listened to two different auditory streams presented dichotically through headphones: environmental sounds in experiment 1 and spoken sentences, one by a female and the other by a male, in experiment 2. Before the target presentation, the voice cue ‘left’ or ‘right’ (in expt. 1 and 2) or ‘female’ or ‘male’ (in expt. 2) was given to define the target sequence. The task was to report the content of the sequence presented in the cued ear after the target presentation. The visual display consisted of a luminance disparity between the left and right visual fields. Participants maintained their fixation at a central point for eye-tracker recording throughout the session from the cue presentation to task-related response. Results showed a decrease in the microsaccade rate corresponding to the cue and target appearances, indicating mental effort and/or attention engagement. There was a transient bias in the microsaccade direction towards the target location within one second after the target onset, but no such bias was found that locked to the cue presentation. In contrast, pupil size changed with the luminance of the attended direction as long as the target location was identified (regardless of whether the target appeared or not), and the effect lasted for several seconds until the response. The overall results indicate that both the microsaccades and pupillary responses reflect an endogenous shift of auditory attention, with processes differing in operating time courses.