Spontaneous perceptual reversals are accompanied by systematic changes in pupil size but not respiration phase

Humans tend to align their respiration with important events, in line with the notion that respiration serves as a tool to allocate (neuro-)physiological resources. This respiration alignment is consistently reported in laboratory studies relying on sequences of stimulus-driven experimental trials. However, whether such respiration alignment also occurs in the absence of external cues - such as during spontaneous perceptual changes - remains unknown. However, pupil size is known to change around spontaneous changes in perception, and pupil size supposedly covaries with respiration phase, suggesting that indeed respiration may align to spontaneous changes in perception. We investigate the link between respiration, pupil size, and spontaneous perceptual changes while participants (n=21) reported spontaneous perceptual reversals while viewing an ambiguous Necker cube. We confirmed the biphasic modulation of pupil size around reversals (constriction followed by dilation) and the general variation of pupil size with respiration phase. However, this coupling of pupil size and respiration phase broke down around perceptual reversals. Crucially, our data reveal no clear alignment of respiration phase to perceptual reversals although respiration frequency was predictive of the perceptual stability duration. These findings suggest that the temporal predictability of external events is critical for the robust respiration alignment to cognitive events.