A substantial body of acoustic and behavioural evidence points to the existence of two broad
categories of laughter in humans: spontaneous laughter that is emotionally genuine and
somewhat involuntary, and volitional laughter that is produced on demand. In this study, we
tested the hypothesis these are also physiologically distinct vocalisations, by measuring and
comparing them using real-time MRI (rtMRI) of the vocal tract. Following Ruch & Ekman
(2001), we further predicted that spontaneous laughter should be relatively less speech-like
(i.e. less articulate) than volitional laughter. We collected rtMRI data from five adult human
participants during spontaneous laughter, volitional laughter, and spoken vowels. We report
distinguishable vocal tract shapes during the vocalic portions of these three vocalisation types,
where volitional laughs were intermediate between spontaneous laughs and vowels.
Inspection of local features within the vocal tract across the different vocalisation types offers
some additional support for Ruch and Ekman’s (2001) predictions. We discuss our findings in
light of a dual-pathway hypothesis for the neural control of human volitional and spontaneous
vocal behaviours, identifying tongue shape and velum lowering as potential biomarkers of
spontaneous laughter to be investigated in future research
