by Barrett St. George
{Editor’s note: I have asked Barrett St. George, an Au.D. – Ph.D. student in our lab to write this commentary on the neuroanatomy projects in our lab. Several topics are covered and I thought it would be of interest to our Pathways/HHTM readership. – FM}
A detailed understanding of the central auditory nervous system (CANS) is fundamental to both clinical audiology and audiological research. However, some brain structures relevant to audition have considerable individual anatomical variability which inherently complicates their identification. Dr. Frank Musiek’s Neuroaudiology Lab at the University of Arizona has been focusing much of its research on the anatomy of the CANS. Specifically, the Neuroaudiology lab is describing in detail, the natural anatomical variability of cortical areas involved in auditory processing. The lab is capitalizing on open-access neuroimaging databases (e.g., OASIS Brains), and using state-of-the-art neuroimaging software such as Freesurfer, MRIcron and BrainVisa Anatomist to reconstruct three-dimensional brain images for visualization, manipulation and precise measurement of specific auditory brain structures.
Most clinical brain imaging research, including fMRI, and lesion localization typically convert brain images into standardized or “normalized” space. For example, functional imaging studies use a technique that involves identifying regions of activation across several brains, and then plotting those regions of activation on a standardized, or averaged brain template. Although quite useful for relative comparison between different groups of brains, the underlying problem with this type of approach is that it fails to address differences in individual morphological variability of certain cortical structures. The University of Arizona’s Neuroaudiology Lab is attempting to bring awareness to this issue by describing the morphological variability of auditory cortex and auditory association areas in the human brain including planum temporale, the angular gyrus, and even the Sylvian fissure. Their descriptive research hopes to facilitate more accurate interpretations of studies that rely on structure-function correlates of auditory brain regions. The research is described in further detail below.
The Angular Gyrus (AG) is an association area of the human cerebral cortex that plays a role in several processes, including auditory function. The Neuroaudiology lab conducted an intensive literature review regarding the anatomical locus of the AG. In their literature review, two commonly used/cited techniques to locate the AG were identified: the “parallel” and “count-back” methods (Brodmann, 2007; Ribas, 2010). However, the researchers found that these different methods are in fact imprecise and possibly misleading due to the gyral and sulcal variability of the human cortex. The reliability of the parallel and count-back methods for locating the AG was assessed in 20 human brains (40 hemispheres) to gain a better understanding of the locational variability of the AG. Results showed that there was worse reliability between the parallel and count-back methods for locating the AG in the left hemisphere, compared to the right. However, even in the right hemisphere, reliability was only 80%. This places the traditional macroscopic methods for locating the AG in question and opens the door for the development of new techniques to define this area of human auditory neuroanatomy.
The morphological variability of the Sylvan fissure was also investigated. The Sylvian fissure has been used as a landmark for measuring the size of auditory structures concealed within it that lie along the superior temporal plane (e.g., planum temporale). The Sylvian fissure also by definition, defines the location of the supramarginal gyrus – another auditory area in the brain (Brodmann, 2007). The Sylvian fissure is sometimes described as having a posterior upward branch, otherwise known as the posterior ascending ramus (PAR). Previous studies measuring the Sylvian fissure have inconsistently included this branch, despite recent work suggesting its inclusion may influence resulting laterality calculations. Moreover, these studies have not reported clear criteria for what constitutes an ascending ramus. The University of Arizona’s Neuroaudiology lab has developed empirically motivated anatomical criteria for what constitutes a true PAR. 58 brains (116 hemispheres) of healthy, right handed individuals were examined and the Sylvian fissures were measured. Using anatomical criteria derived from the normal distribution of length and angle of posterior Sylvian fissure, the Neuroaudiology lab members found that PARs were more pronounced and occurred more often in the right hemisphere, compared to the left. This finding lends support to the typically seen structural asymmetry of planum temporale in the human brain (discussed in further detail later).
While examining those same 58 brains, the researchers identified two different branching patterns of the posterior Sylvian fissure that previously, had not been described in the literature. Hence there were no systematic attempts to explore how regularly these types of branching patterns occur in healthy adults. A new goal of this overarching study was to investigate the prevalence of atypical morphological patterns in the branches of the posterior Sylvian fissure. The research focused on two branching patterns as defined by the authors, including (a) a false ascending ramus (FAR), defined as a ventral, discontinuous extension of the post-central sulcus joining the Sylvian fissure and (b) a trifurcation in the posterior Sylvian fissure, characterized by an ascending ramus, descending ramus, and FAR, all branching from the same location. Of the 58 brains examined, 25 (43%) exhibited an FAR in at least one hemisphere, which was more common in the left hemisphere compared to the right. Trifurcations were observed in 8 (14%) of brains examined, yet again occurring more often in the left hemisphere compared to the right. The results of this study suggest that atypical branching patterns of the posterior Sylvian fissure are not rare and should therefore be appreciated if structures of posterior perisylvian cortex (e.g., supramarginal and angular gyri) are to be accurately localized.
The University of Arizona’s Neuroaudiology Lab has also re-examined the anatomy of the planum temporale (PT). The PT is a workhorse for auditory processing; it is responsible for processing the spectro-temporal characteristics of speech (Griffiths and Warren, 2002; Price, 2010) and thus been studied extensively in both healthy and disordered populations (Shapleske et al., 1999). The PT demonstrates the most striking hemispheric asymmetry out of all structures in the human brain (Toga and Thompson, 2003), often larger in the left hemisphere compared to the right hemisphere. This anatomical asymmetry has been related to the left-hemisphere command of language function (Geschwind & Levitsky, 1968). Lying along the superior temporal plane, the PT is hidden within the Sylvian fissure. It is bounded anteriorly by Heschl’s gyrus, and posteriorly by the posterior ascending ramus (PAR) of the Sylvian fissure. However, these two anatomical boundaries are highly variable in their morphology. PARs vary significantly in both length and angle, and sometimes are absent (St. George et al., 2016). Furthermore, Heschl’s gyrus often demonstrates a partial or complete duplication (Marie et al., 2015).
Although previous literature attempting to quantify the anatomy of PT has considered some of these anatomical complexities, the effects of new research refining its boundaries with primary auditory cortex and the PAR have been largely neglected (Da Costa et al., 2011; De Martino et al., 2015; Moerel et al., 2014; St George et al., 2016). Accounting for changes in the boundaries of Heschl’s gyrus/gyri and newly developed anatomical criteria considering the angle and length of the PAR, the Neuroaudiology lab re-examined the anatomy of PT in 28 different brains (56 hemispheres). The authors found that PT surface area was significantly larger in hemispheres with single Heschl’s gyrus morphology compared to hemispheres exhibiting Heschl’s gyrus duplications. Furthermore, PT surface area was significantly larger in hemispheres without PAR compared to hemispheres with PAR. The results of this study confirm that the anatomy of PT depends on the morphology of neighboring perisylvian structures (including the Sylvian fissure itself), which helps to explain its structural asymmetry between right and left hemispheres.
Conclusion
With ever-increasing in-vivo brain imaging studies flooding the clinical and basic research domains, now it is more important than ever for auditory researchers to appreciate the structural variability of the CANS. This is not only paramount for accurate interpretation of functional imaging research, but also lesion effects (e.g., stroke/aphasia) and evoked potential mapping of auditory stimuli. Taken as a whole, the recent research coming out of the University of Arizona Neuroaudiology lab shows that individual neuroanatomical variability cannot be overlooked in the human cortex, and new approaches for examining and interpreting brain imaging research should be considered.
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- St George, B., DeMarco, A.T., & Musiek, F. (2016, April 15). Revisiting anatomical variability along the Sylvian fissure: Its impact on central auditory research. 28th annual AudiologyNOW! Meeting, Phoenix, AZ.
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