When we started discussing our ideas, one of us (Florencia Assaneo) was placed at NYU in the US, while the other one (Johanna Rimmele) at the Max Planck Institute in Germany. Johanna just had published an opinion paper in Trends in Cognitive Sciences, where the authors claim that timing from the motor system can phase-reset neuronal oscillatory activity in auditory cortex during auditory perception. In the case of rhythmic motor cortex activity, motor cortex would align the excitability cycles of auditory cortex neurons resulting in motor-to-auditory entrainment. The idea is intriguing, as it mechanistically explains the impact of motor timing on auditory perception (a view supported by previous research, e.g. suggesting that motor timing such as moving our body when we dance, or tapping when we listen to music can make our hearing more precise).
From this proposal we draw the conclusion that the timing of our own speech production should have an impact on how we perceive speech. We were surprised when we realized that, although many studies show effects of motor timing on sound perception and neuronal processing in auditory cortex, we could not find behavioral evidence for this claim in the speech domain.
If the claims of Johanna and co-authors were true, we wondered, how would the surprising findings by Florencia et al. -now published in Nature Neuroscience- fit into the picture? Florencia and colleagues found that only parts of the population are spontaneously synchronizing their speech production to the perceived speech. Importantly, individuals with spontaneous speech-motor synchronization had stronger connections between the frontal-motor and auditory cortex in the brain, suggesting that the structural connection strength scales with the spontaneous synchronization behavior.
We developed (teaming up with David Poeppel) the hypothesis that whether motor timing affects the entrainment of auditory cortex activity to the speech signal, varies across individuals depending on how strongly each individual’s auditory and motor cortex were connected. Understanding the systematics of individual variance provides insides into the plasticity of such a mechanism, including possibilities on how to improve perception. Individual variance, we thought, might also explain controversial evidence regarding auditory entrainment phenomena.
To cut a long story short, our findings of this project provide evidence for both, the claims raised in the Trends in Cognitive Science and in the Nature Neuroscience paper. In line, with the former, we provided behavioral evidence that speech production can entrain speech perception, resulting in perceptual advantages in certain phases of the speech motor rhythm. In line with the latter, we found evidence that such a motor-to-auditory entrainment effect occurred particularly in individuals with an estimated strong auditory-motor cortex connection. In a neural computational model (winning Yonathan Sanz Perl placed in Argentina for our project), we showed that our findings indeed could be explained by scaling the bidirectional coupling of neuronal oscillators in auditory and motor cortex with the estimated individual connection strength.
Fun effect: The collaboration also resulted in alignment of US and German zoom times.