by Frank E. Musiek, Ph.D.
Most audiologists are familiar with at least two major pathways in the brain. One is the ascending (and descending) auditory brainstem pathway and the other is the ascending (and descending) vestibular brainstem pathway. In the cerebrum there are also pathways of importance to audiology. These include, in our view, the superior longitudinal fasciculus (SLF), the arcuate fasciculus (AF) and the inferior longitudinal fasciculus (ILF) (see figure 1).
The ascending auditory brainstem pathway is often considered the lateral lemniscus pathway (LLP), not to be confused with the lateral lemniscus nuclei (LLN) which are located in the in the more rostral aspect of the pons. The LLP originates at the cochlear nucleus in the caudal pons and courses bilaterally up the lateral aspect of the pons. The LLP terminates at the inferior colliculus in the midbrain. This ascending pathway carries impulses from the auditory nerve which projects into the cochlear nucleus. Though audiologists are formally educated regarding this pathway, it became trendy with the introduction of the auditory brainstem response (ABR) in the 1970s. This clinical procedure forced audiologists to better understand auditory brainstem pathway in order to properly interpret ABR results.
The ascending vestibular brainstem pathway is actually known as the medial longitudinal fasciculus (MLF) which also contains descending fibers. This bilateral pathway starts at the vestibular nuclei in the caudal lateral pons (immediately close to the cochlear nuclei) and runs a course that is near midline up the pons and midbrain terminating at the interstitial nucleus of Cajal, which is located at the lateral aspect of the third ventricle. Like the auditory brainstem pathway, the MLF became interesting to audiology when the profession starting performing ENGs/VNGs. The MLF primarily receives input from the vestibular nerves carrying information from the vestibular periphery. The MLF is critical to eye control the keystone of vestibular evaluation.
As mentioned, the auditory and vestibular ascending brainstem tracts garnered attention from audiologists in part, due to the clinical correlations regarding assessments derived from the ABR and ENG/VNG. Unfortunately, this has not been the case in the cerebral pathways, even though they are also critical to interpretation of tests used in central auditory assessment.
One of the largest pathways in the brain is the SLF which courses from the occipital lobe to the frontal lobe. It is sometimes portioned into three separate segments but defining these is beyond the scope of this overview. Its course runs (posterior – anteri
or) in the cerebral white matter near the lateral ventricles. This pathway is likely joined by auditory fibers from the superior temporal gyrus and possibly the insula. In addition, the SLF connects the angular gyrus which has multiple sensory fibers (including auditory) and the supra-marginal gyrus with fibers from the posterior temporal plane. The SLF in its most posterior location in the occipital lobe has fibers that splay out covering a major portion of this lobe. Another major pathway is the arcuate fasciculus which some consider a part of the SLF and courses along with it —for the most part. However, the AF at its posterior origin appears to be close to the posterior, superior temporal gyrus and courses posteriorly and rostrally around the supra-marginal gyrus before joining the SLF running towards the frontal lobe. It is difficult to know which may be involved or responsible for various dysfunctions because the AF and SLF are highly similar in their pathways. Therefore, we will not make a specific attribution between these two pathways in regard to the effect of various disorders. Gross anatomy dissection reveals that these two tracts are difficult to differentiate. Some recent studies using functional imaging may provide some differentiation but much of the challenge in this regard remains.
The next major pathway is the ILF which connects the occipital lobe to various regions of the temporal lobe. More specifically, this pathway as it leaves the occipital area progresses toward the middle temporal gyrus. This major pathway courses inferior to the SLF and runs semi-parallel to it in a posterior to anterior manner. However, the more medial aspect of the ILF runs more superiorly near to the primary auditory cortex. This rather direct connection between visual and auditory regions lends itself to shared and or combined processes between these anatomical areas. The ILF, especially in humans, is often considered part of a larger pathway – the occipitotemporal fasciculus by some investigators.
Of interest clinically, is when the major pathways in the brain are disrupted or not developed properly. It is well known by audiologists, at least in general, that when the ascending auditory (lateral lemniscus) and vestibular (medial longitudinal fasciculus) brainstem pathways are compromised there are problems related to central auditory and vestibular function. These problems are revealed by auditory and vestibular tests that focus on the central systems. When SLF, AF, and ILF are disrupted the effects may not be quite as well understood but are certainly significant. When the SLF/AF are compromised, signals from the auditory cortex to speech areas in the frontal lobe (Broca’s area) are not preserved and can result in an aphasia known as conduction aphasia. This is typified by patients that understand words said to them but may not be able to repeat them. There also has been interesting data showing those with tone deafness may not have normal structure (perhaps absent) of the right SLF (Loui, Alsop, Schlaug (2009).
Dysfunction of the ILF is likely highly significant by considering the size of this pathway; however, our understanding of it is still in its infancy. Disruption of the ILF may impair object recognition and could be tied into hallucinations. There is also accumulating evidence that the ILF is part of a complex network involved in reading where auditory and visual processes need to be shared and or combined. The ILF may also play a role in mapping sound onto meaning (Science Direct).
- Loui, P; Alsop, D; Schlaug, S (2009). “Tone Deafness: A New Disconnection Syndrome?”. Journal of Neuroscience. 29 (33): 10215–10220.
- Musiek, F. and Baran, J. (2007) The Auditory System: It’s Anatomy, Physiology, and Clinical Correlates, Plural, San Diego
- Seghier, M. L. (2012). “The angular gyrus: multiple function ad multiple subdivisions”. Neuroscientist. 19: 43–61.