Frank Musiek, Ph.D.
The term thalmo-cortical pathway (TCP) is an anatomical one and one of import to audiologists. However, it seems to be a term that is not commonly used in neuroanatomy or audiology. Therefore, consistent with Pathways articles, to focus on some lesser known but important topics, a brief account of the TCP is presented.
Some Neuroanatomy
The auditory TCP involves the medial geniculate body (MGB) as its origin. More specifically, the starting point is the ventral portion of the MGB which has more fibers dedicated to audition than the other segments of the MGB. The other divisions of the MGB includes the medial and dorsal segments. These segments also are “heavy” in auditory fibers but are also polysensory. The MGB is located on the inferior aspect (or ventral side if you wish) of the thalamus which is positioned deep in the brain. Auditory fibers projecting from the ventral MGB courses from each thalamus laterally to the inferior portion of the internal capsule and then a bit superiorly and laterally to the core of the auditory cortex. The medial segment of the MGB projects to the insula again via the internal capsule and then using a different route, courses to the amygdala. The dorsal segment of the MGB courses to the insula and association auditory areas via the internal capsule. The medial and dorsal segments also contribute to what is known as the “non- classical” auditory pathway. The TCP, of course, projects ipsilaterally on both sides of the brain and could be considered a bit of a bottleneck. This is because many fibers are “funneled” into a portion of the internal capsule. Therefore, if this area of the internal capsule damaged it could prevent much of the auditory input into the auditory cortex and result in marked deficits in auditory processing.
Some Physiology
A great deal of physiological investigation has been done on the structures along the TCP —specifically the MGB and auditory cortex (see Musiek and Baran, 2020). Only a few points of physiology will be offered here and those will be towards the MGB and auditory cortex as key components of the TCP. The MGB has a definite tonotopic arrangement based mostly on animal studies. The auditory cortex tonotopicity has been well documented in animals but has under gone recent changes as more new data has emerged in this regard (see Musiek et al. 2020). In regard to intensity coding the MGB has yielded both monotonic and non-monotonic patterns similar to the auditory cortex. The cortex however, also reveals some intricate inhibitory- excitatory actions in coding intensity that can influence the perception of loudness. Both the MGB and auditory cortex demonstrate good temporal processing capability. For example, the MGB has some neurons that can phase lock at high relatively high frequencies (near 1000 Hz) but most neurons phase lock at much lower frequencies (~ 200 Hz). The auditory cortex, in general, has much lower frequency phase locking capability but some cortical fibers do demonstrate sharp temporal responses to transient like stimuli. Temporal resolution, often measured via gap detection procedures has been shown good capability for both the MGB and auditory cortex in animals. Damage to the MGB and or auditory cortex compromises localization. As shown in animal studies, both the MGB and auditory cortex (i.e. TCP) have excitatory and inhibitory cells sensitive to inter-aural timing and intensity differences – the bases for localization.
Some Audiology Relevance
Perhaps the TCP is best known in audiology as a major generator for the middle latency evoked response (MLR). The TCP likely contributes to the Na and Pa waves of the MLR which are considered the main wave components for clinical and research use. The TCP is rostral to the generators of the ABR hence brings into play a segment of the auditory pathway important to evaluate. That is, the TCP cannot be assessed by ABR but represents an important tract of auditory fibers for which neural integrity should be evaluated if deemed necessary. Therefore, the MLR (including the 40 Hz potential) becomes an important electrophysiologic procedure for the evaluation of the TCP. However, the MLR is not the only audiologic diagnostic procedure that can be sensitive to compromise of the TCP. There are a variety of behaviorally based hearing tests that can be utilized to assess the integrity of the TCP yet on the other hand some tests will be of little value in evaluating this anatomical region of the central auditory system.
As one would expect, pure-tone thresholds and speech recognition in quiet would likely show little effect of compromise of the TCP. Likewise, expected results from the acoustic reflex, startle response, and as mentioned the ABR would likely not be influenced by lesions of the TCP. All three of these auditory tests are mediated in anatomical regions below the TCP. Dichotic listening, filtered speech, speech in speech or noise competition, compressed speech and similar low redundancy speech tasks will generally show contralateral deficits to the hemisphere involved. Pattern perception tests such as frequency (pitch) and duration patterns will often show a bilateral deficit for a verbal response if the TCP is compromised. The gaps in noise (GIN) procedure will likely show increased gap durations for various disorders affecting the TCP, but laterality for this test can be varied (ipsilateral or contralateral findings) (see Musiek and Chermak, 2014).
Summary
The TCP is a pathway from approximately the MGB to auditory cortex has import to audiology from both a neurobiological and audiological perspective. It’s integrity can be evaluated by both
behavioral and electrophysiologic means. Proper interpretation of these procedures however, is linked to at least a cursory understanding of the TPC anatomy and physiology.
Relevant readings:
- Musiek, F.E. & Baron, J.A. (2020). The Auditory System, Anatomy, Physiology and Clinical Correlates. 2nd Edition, Plural publishing, San Diego.
- Musiek, F., & Chermak, G. (2014) 2nd Edition Handbook of Central Auditory Processing Disorder, Vol. I: Auditory Neuroscience and Diagnosis. San Diego, CA. Plural Publishing.
