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 temporally structured, stimulus-driven experimental trials. Whether this alignment also occurs in the absence of external cues - such as during spontaneous changes of perception - remains unknown. Notably, pupil size is known to change around spontaneous changes in perception, and pupil size covaries with respiration phase, suggesting that respiration may also align to spontaneous changes in perception. We here investigate the three-fold relations between respiration, pupil size, and spontaneous perceptual changes in a paradigm where participants (N = 20) reported spontaneous perceptual reversals of an ambiguous Necker cube. We confirmed the biphasic modulation of pupil size around reversals (constriction followed by dilation) and the covariation of pupil size with respiration phase. However, the alignment of respiration phase to perceptual reversals was much weaker than previously reported for stimulus-driven experimental trials. These findings suggest that changes in perception alone, whether exogenously or endogenously driven, might not be the key driving force of respiration alignment, but the temporal predictability of external events likely facilitates this alignment.