Abstract
The pupil dilates in response to salient audiovisual stimuli. The louder the sounds, the stronger the pupil dilation (PD) evoked (Liao et al., 2016). Microstimulation of the superior colliculus (SC) elicits the corresponding PD not only in response to the stimulation intensity but also to the stimulation site’s eccentricity, reflecting the topographic representation (Wang & Munoz, 2021). Considering that the SC also represents auditory space, we can expect that the sound's location influences the degree of sound-induced PD. In two experiments, we presented a white noise burst (55 or 80 dBA) via loudspeakers located at different eccentricities: 0°,±30°,±60°,±90°,±150°,and 180° of azimuth in the horizontal plane (0° indicated the front; 180° indicated the back; positive values indicated the right; and negative values indicated the left). Participants looked at a fixation point on the screen while an infrared eye-tracking camera recorded their pupillary responses. We monitored the participants’ attention under the following conditions: They were asked to detect fixation disappearance (focus on visual stimuli) or a 1-kHz tone oddball (attend to auditory stimuli) and press a button immediately after detection. These attention-controlled events were uncorrelated with the noise burst’s intensity or location. To avoid decision-evoked PD (Einhäuser et al., 2016), trials containing these events or the participant’s false responses were removed from analyses. We observed larger PD in response to 80-dBA than 55-dBA sounds, consistent with our previous finding. Critically, PD was more prominent when the noise burst appeared to deviate from the middle line and when the noise burst appeared in front of the participant than in the back, consistent with the location-representation hypothesis. We did not find any interactions between loudspeaker location and attention, suggesting that the location-represented PD is stimulus-driven and free from task demand. The auditory space map in the SC may contribute to modulating sound-induced PD.