Differential Myeloarchitecture of Subfields of the Mammalian Auditory Cortex: A Comparative Approach

The mammalian auditory cortex can be parcellated into multiple functional subfields, with each subfield making a distinct contribution to sound processing. For example, primary fields consisting of primary auditory cortex and anterior auditory field are the first to receive information from the thalamus, and the neurons in each of these fields have different properties in terms of latency and response duration. Non-primary auditory fields consist of secondary auditory cortex, which is involved in object recognition and emotional conditioning, while dorsal auditory fields are more responsive during locomotion and spatial tasks. What is currently unknown is how the structure of each auditory cortical subfield relates to function. Thus, it is imperative to understand how anatomical substrates that make up each field contribute to function.

In this review, we suggest that myelin may serve as a structural anchor for the organization of auditory cortical subfields. Myelination patterns among mammals that have been studied (primates, carnivores, rodents, and bats) show that primary auditory cortical fields are more heavily myelinated than non-primary auditory cortical fields, and upper cortical layers are less myelinated than middle and deep layers. Myelin also demonstrates experience-dependent plasticity and can be measured with a variety of invasive and noninvasive methods, and demyelination has been linked to cognitive decline. Conversely, the archicortex is lightly myelinated, and we speculate that because myelin inhibits axonal and synaptic plasticity, it would not be advantageous for the archicortex to be myelin-dense, as it has greater requirements for flexibility for learning and memory.

This is the first review, to our knowledge, that uses a comprehensive comparative approach across mammals to determine the distribution of myelin across auditory cortical subfields. We argue that a detailed map of the myeloarchitecture of the auditory cortex must be directly aligned to the functional maps of the auditory cortex to account for individual variability and identify subfields accurately. Furthermore, myelin maps need to be compared with other anatomical markers as well to improve our understanding of the role of myelin. Finally, a detailed histological myelin map can serve as a ground truth for comparisons to noninvasive measures of myelin.