by Aaron Whiteley & Frank Musiek, University of Arizona
Intro. & Review of Part 1: In part 1 an overview of the research in validating the 2/3 rule in the location of Heschl’s gyrus was discussed. The data showed that, within measurement limitations the 2/3 rule can be used to accurately locate Heschl’s gyrus in the human brain. In part 2 we will comment on our views of how the 2/3 rule can be applied in various clinical and research situations and in general, how morphometry in neuroanatomy remains a critically important aspect in our knowledge of the auditory system.
The 2/3 rule, morphometry and history
The 2/3 rule is a morphometric index. In our research a variety of measurements were completed to answer the question as to the value of the 2/3 rule. This rule takes it’s place along side many other morphometric studies of the brain that have proven critical in our understanding of neuroanatomy and physiology. Despite this, morphometric studies, historically have endured a rather rocky road. Early studies by anatomist such as Brodmann, von Economo, Ades, and Woolsey just to name a few used morphometric studies to advance our knowledge. However, later on in the 1970s and 1980s morphometric studies decreased as funding for this kind of research was not gaining the attention or research following it experienced earlier.
This resulted in many anatomists turning toward cell (molecular) studies and gross anatomical studies —especially of the human brain were not pursued and knowledge gain in this arena was slowed. This happening took place with many questions about gross neuroanatomy waiting for answers.
Interestingly, in about the 1990s with imaging and functional imaging becoming popular in the clinic and research labs, previously unanswered anatomy questions were being posed that morphometric and gross anatomical studies could likely answer. Since then, there has been a re- emergence of gross anatomy and related morphometric studies.
Asymmetries of the brain
Another way that the 2/3 rule could prove helpful would be in the determination of asymmetries in the brain. By helping define the auditory cortex the 2/3 rule in difficult cases could contribute to the accuracy of interhemispheric comparisons—an important research effort in the area of auditory science.
The classic case in point was that of Geschwind and Levitsky in 1968 that revealed that the planum temporale was larger on the left side than right side of the brain. This asymmetry was also shown for Heschl’s with the left side being larger than the right (Musiek and Reeves, 1990). This finding was corroborated in the present study and in addition, revealed that the superior temporal plane was also larger on the left than right side of the brain.
Despite this asymmetry, the 2/3 rule held up in locating Heschl’s gyrus on either side. Measured percentages for the distance to Heschl’s for the right and left sides using the half-way or medial track hovered around 66-68% of the length of the temporale plane. Also there were no gender differences.
In a very practical way, the 2/3 rule could prove helpful in pathology in defining the locus of Heschl’s gyrus. For example, in degenerative, vascular, mass lesion or congenital disorders that effect the auditory cortex there is often considerable distortion along the Sylvian fissure. This in turn can make the determination of involvement of auditory cortex challenging – especially if it is difficult to visualize on imaging studies.
However, even when Heschl’s is obscured or relocated due to the disorder applying the 2/3 rule can be of significant help. It would at least provide an indication of where structures such as Heschl’s should be and therefore lend comparative knowledge to the examiner. This kind of information could be useful to clinicians as well as researchers studying anatomical pathology etc.
As alluded to earlier in this commentary morphometric anatomy is a useful approach to helping students learn auditory anatomy. As a practice in teaching neuroanatomy, directing students to make measurements of critical anatomical features forces, in motivational way, the students to learn the anatomy relevant to the measurements. Also, morphometric approaches expose the students to concepts in connectivity and comparative anatomy.
Developing a sense of the absolute and relative size of various anatomical structures is something can be transferred to clinical situations for the audiologist. Using such morphometry as the 2/3 rule in reviewing CT scans of MRIs can be of value in understanding pre and post-surgical approaches and their potential effects on audiological tests.
Readings for Part 1 and 2
- Geschwind, N. & Levitsky, W. (1968) Human brain left-right asymmetries in temporal speech region. Science, 161 186–187.
- Musiek, F. E. (1986). Neuroanatomy, neurophysiology, and central auditory assessment. Part II: The cerebrum. Ear and Hearing, 7(5), 283-294.
- Penhune, V. B., Zatorre, R. J., MacDonald, J. D., & Evans, A. C. (1996). Interhemispheric anatomical differences in human primary auditory cortex: Probabilistic mapping and volume measurement from magnetic resonance scans. Cerebral Cortex, 6, 661-672.