Pupillometry in auditory multistability

Bimodal moment-by-moment coupling in perceptual multistability

Multistable perception occurs in all sensory modalities, and there is ongoing theoretical debate about whether there are overarching mechanisms driving multistability across modalities. Here we study whether multistable percepts are coupled across vision and audition on a moment-by-moment basis. To assess perception simultaneously for both modalities without provoking a dual-task situation, we query auditory perception by direct report, while measuring visual perception indirectly via eye movements. A support-vector-machine (SVM)-based classifier allows us to decode visual perception from the eye-tracking data on a moment-by-moment basis. For each timepoint, we compare visual percept (SVM output) and auditory percept (report) and quantify the co-occurrence of integrated (one-object) or segregated (two-objects) interpretations in the two modalities. Our results show an above-chance coupling of auditory and visual perceptual interpretations. By titrating stimulus parameters toward an approximately symmetric distribution of integrated and segregated percepts for each modality and individual, we minimize the amount of coupling expected by chance. Because of the nature of our task, we can rule out that the coupling stems from postperceptual levels (i.e., decision or response interference). Our results thus indicate moment-by-moment perceptual coupling in the resolution of visual and auditory multistability, lending support to theories that postulate joint mechanisms for multistable perception across the senses.

Smooth pursuit inhibition reveals audiovisual enhancement of fast movement control

The sudden onset of a visual object or event elicits an inhibition of eye movements at latencies approaching the minimum delay of visuomotor conductance in the brain. Typically, information presented via multiple sensory modalities, such as sound and vision, evokes stronger and more robust responses than unisensory information. Whether and how multisensory information affects ultra-short latency oculomotor inhibition is unknown. In two experiments, we investigate smooth pursuit and saccadic inhibition in response to multisensory distractors. Observers tracked a horizontally moving dot and were interrupted by an unpredictable visual, auditory, or audiovisual distractor. Distractors elicited a transient inhibition of pursuit eye velocity and catch-up saccade rate within ∼100 ms of their onset. Audiovisual distractors evoked stronger oculomotor inhibition than visual- or auditory-only distractors, indicating multisensory response enhancement. Multisensory response enhancement magnitudes were equal to the linear sum of responses to component stimuli. These results demonstrate that multisensory information affects eye movements even at ultra-short latencies, establishing a lower time boundary for multisensory-guided behavior. We conclude that oculomotor circuits must have privileged access to sensory information from multiple modalities, presumably via a fast, subcortical pathway.

Temporal dynamics of auditory bistable perception correlated with fluctuation of baseline pupil size

A dynamic neural network change, accompanied by cognitive shifts such as internal perceptual alternation in bistable stimuli, is reconciled by the discharge of noradrenergic locus coeruleus neurons. Transient pupil dilation as a consequence of the reconciliation with the neural network in bistable perception has been reported to precede the reported perceptual alternation. Here, we found that baseline pupil size, an index of temporal fluctuation of arousal level over a longer range of timescales than that for the transient pupil changes, relates to the frequency of perceptual alternation in auditory bistability. Baseline pupil size was defined as the mean pupil diameter over a period of 1 s prior to the task requirement (i.e., before the observation period for counting the perceptual alternations in Experiment 1 and reporting whether participants experienced the perceptual alternations in Experiment 2). The results showed that the baseline pupil size monotonically increased with an increasing number of perceptual alternations and its occurrence probability. Furthermore, a cross‐correlation analysis indicates that baseline pupil size predicted perceptual alternation at least 35 s before the behavioral response and that the overall correspondence between pupil size and perceptual alternation was maintained over a sustained time window of 45 s at minimum. The overall results suggest that variability of baseline pupil size reflects the stochastic dynamics of arousal fluctuation in the brain related to bistable perception.