What are the brain specializations underlying human vocal control?

Thesis

One uniquely human experience is our ability to communicate via speech and language. Specialized laryngeal motor control has been suggested to be a critical adaptation in the human brain that enabled speech. The aim of this thesis was to study the human brain specializations underlying laryngeal motor control and to study evolutionary principles of brain organization within the primate lineage. I investigated these questions using magnetic resonance imaging and comparative primate neuroanatomy.

To characterize the representations of the larynx in the human brain, I used anatomical and functional studies. First, I mapped the motor representations of the larynx and other effectors using fMRI. I designed a novel functional paradigm to isolate brain activity related to laryngeal activity during vocalization whilst controlling for breathing. Then, I characterized the cortical microstructure underlying the activated regions using structural and quantitative neuroimaging. The results revealed the existence of a dorsal larynx representation in primary motor cortex and a ventral larynx representation presumed to be in premotor cortex as indexed by lower cortical thickness and lower myelin content.

Next, I examined whether variability in cortical folding explains inter-individual variability in the location of the larynx representations. I provided a systematic description of the sulcal patterns in the central and subcentral region. I showed that the anterior subcentral sulcus typically consists of two distinct segments, as opposed to being one continuous sulcus as previously described in a brain atlas. I demonstrated a tight link between morphological landmarks in the central and subcentral region and functional activation maxima for several movement types. This result indicates that inter-individual variability in brain morphology might have contributed to controversies and inconsistent reports in the case of the ventral larynx representation.

Furthermore, I set out to dissociate different scenarios of cortical brain reorganization during primate evolution. Using a surface-based cross-species registration, I studied if changes to the relative location of brain areas and cortical expansions went hand in hand with changes to connectivity profiles. The registration was based on homologous maps of cortical myelin in humans, chimpanzees and macaques to model local cortical expansions and relocations of brain areas. Then we applied this registration to surface tract maps of a set of homologous white matter tracts. Our results demonstrated that several tracts can be predicted well by cortical expansion and relocation, but that the arcuate fasciculus underwent additional evolutionary modifications affecting the temporal lobe connectivity pattern.

Authors: Eichert, N

• DOI: 10.5287/ora-qomvnabkr
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: larynx; sulcal morphology; quantitative neuroimaging; speech; neuroanatomy; evolution; white matter; fMRI
• Subjects: Speech; Evolution; Anatomy; Brain; Larynx