abstract Stuart Washington

Stuart Washington (Georgetown University, USA and University of Antwerp, Belgium)

Lessons on Hemispheric Specialization for Speech and Music as Learned from Bats

Evidence suggests that the degree of left hemispheric specialization in the auditory cortex (AC) for processing social communication calls, including human speech sounds, is dictated by acoustic structure and not semantic content. Specifically, the relatively greater precision with which the left AC processes time-critical (temporal) information enables it to detect the rapid frequency modulations (FMs) that comprise social calls, which are analogous to formant-transitions in speech. The right AC, on the other hand, has greater precision at processing frequency-related (spectral) information that enables it to track prosodic variation and pitch. Elements of this Asymmetric Sampling in Time have been identified not only in human AC but also in the Doppler-shifted constant frequency processing (DSCF) subregion of mustached bat AC. In this talk, I will use published observations and theorems to suggest how an idealized version of the classic left hemispheric specialization for speech processing, characteristic of human AC, evolved in the DSCF area. I will review how DSCF neurons are tuned to the tonal component of the returning, Doppler-shifted, second harmonic of the mustached bat’s biosonar signal (echo‐CF2) to calculate the relative velocities of targets, including prey. Precise velocity calculations based on the echo-CF2 are thus ethologically advantageous to the mustached bat but can only be achieved by refined frequency discrimination.  The Acoustic Uncertainty Principle dictates that refining frequency discrimination comes at the expense of temporal precision, and refined temporal precision is necessary for detecting and processing rapid FMs in social communication calls of this species. I argue that these ethological pressures forced right hemispheric DSCF neurons to develop greater spectral precision, which enables them to precisely track target velocity and other frequency variations while debilitating their processing of short, rapid FMs. Left hemispheric DSCF neurons, on the other hand, developed greater temporal precision and can thus process the short, rapid FMs used in social calls but perform frequency discrimination relatively poorly. I will address some lingering questions related to sex differences and spectral energy as well as suggest how fMRI can be used to address this topic. Ultimately, I suggest that left hemispheric specialization for social calls and rapid FMs in mustached bats may help us to unravel fundamental phonological mysteries related to hemispheric differences in processing human speech and music.