By Frank Musiek, Ph.D.
The insula (meaning island and sometimes called the Island of Reil) is located medial to the temporal lobe. If one were to pull away the temporal lobe in a lateral manner, the insula would be observed behind it. It looks like another cortex and is situated of course, close to the temporal lobe – the lobe of hearing. Most audiology trained individuals get little if any exposure to the Insula’s anatomy or physiology. One reason is that in many accounts, such as books on hearing, little is said about the insula.
Though there were articles about hearing and the insula many years ago, such as Woolsey’s work on the cat back in the late 1950’s, it has not been until recently that hearing functions have been linked to the insula. In a rather general sense, the insula has been a bit of a mystery in regard to what it does. If one were to access a website, it would say that the insula is involved in self awareness, perception, cognition etc. – many things. However, most of these computer entries would also say that they really aren’t sure about what the insula does or how it does it. Our argument here in this short essay, is that the insula plays a role in hearing—likely higher level hearing processes. That is not to say that it isn’t involved in other brain functions, it surely is – but hearing is a function that can be linked to processes belonging to that of the insula.
One of the first times I thought about hearing possibly being tied into the insula was early in my career when I was at Dartmouth (Dartmouth – Hitchcock Medical Center). I was talking with Alex Reeves who, at that time was head of neurology and a fine neuroanatomist. Knowing that I was interested in hearing, he mentioned some extensive work that he and his colleagues did when he was at NIH. Much of this work involved the insula in animals (I believe the cat). Alex said that he was moderately convinced that the insula played a role in hearing. He gave me a run down on his research and a reference to read. The article essentially backed up what he said (Sudakov, McLean, Reeves and Marino, 1971). Alex Reeves urged me to look at the insula and hearing because he thought there was something more to learn there.
A study after the Sudakov work was timely and enlightening. Collavita, Szeligo, and Zimmer (1974) completed an animal study where the insula was ablated. After ablation, the animal who had learned to recognize auditory patterns lost that ability. This gave rise to the idea that perhaps the insula was duly involved in auditory functions and specifically, that of temporal processing. Later in this article we will visit this concept again.
A number of years after Dr. Reeves piqued my interest in the insula and its probable link to hearing, I saw a case of a person who had incurred a stroke of the insula. They had extreme difficulties hearing in background noise and demonstrated both behavioral and electrophysiological deficits on a central auditory test battery (Gollegly and Musiek, 1993). At about the same time, deficits on dichotic listening tasks for individuals with insular lesions was reported (Hyman and Tranel, 1989). As with auditory cortex lesions, insular lesions created deficits in the ear contralateral to the hemisphere involved. The argument for the insula and its role in auditory processing was gaining momentum; however, one of the most compelling reports supporting the insula as an auditory entity was yet to come. Interestingly, this compelling report was not to originate from an experimental investigation but rather a single case report.
In 1995, Habib, Daquin, Milandre et al. published an interesting case report of middle aged patient who, after incurring a stroke of one insula and then a few days later a stroke of the other insula, was rendered essentially centrally deaf (auditory agnosia by report). This happened with Heschl’s and planum temporale staying intact! This of course implicated the insula as the mediator of these severe central auditory symptoms. Testing revealed a primarily intact peripheral hearing system. Though this was a case study, it was well done with extensive testing both neurologically and audiologically and added much to the concept of the insula and hearing and/or hearing processing at high levels of the auditory nervous system.
Subsequent to the intriguing 1995 article by Habib and colleagues, a number of important studies on the insula and its relationship to hearing began to appear. Much of this work was captured in the 2003 article by Bamiou and colleagues (Bamiou, Musiek, & Luxon, 2003). I will pull from this article some key findings in this review of the literature. It appears that the insula may be sensitive to the perception of environmental and non-verbal stimuli. This was shown with the help of functional imaging approaches. Additionally, the insula showed greater activation for targeted acoustic stimuli than novel stimuli, also demonstrated by functional imaging. The insula has also been implicated in regard to temporal processing not only in the Bamiou review but also in a key study in 2006 which showed deficits for individuals with strokes of the insula on tests of temporal resolution and sequencing (Bamiou, Musiek., Stow, Stevens, Cipolotti, Brown, & Luxon, (2006). There are also studies reported in the Bamiou et al. (2003) review that argued that musical rhythm and phonological processing may take place, at least in part, in the insula. I could go on with mention of accumulating articles about hearing and the insula but I feel the point has been made.
In summary, after years of being ignored, the insula is gaining its recognition as an auditory center in the brain. This is not to say that other processes do not occur in the insula, but that it does play a role in audition. Therefore, in classes and conversations about hearing it may now be pertinent for one to say “What about the insula?”.
References
- Sudakov, K., MacLean, P., Reeves, A., Marino, R., (1971) Unit study of exteroceptive inputs to claustrocortex in awake, sitting, squirrel monkeys. Comparative Brain Research, 118, 19-34
- F.B. Collavita, F.V. Szeligo, S.D. Zimmer, Temporal discrimination in cats with insular–temporal lesions, Brain Res. 79 (1974) 153– 156.
- Gollegly, K. and Musiek, F. (1993) Auditory dysfunction in a patient with subcortical and insular involvement, Seminars in Hearing,14, 245 -253.
- Habib, G. Daquin, L. Milandre, M.L. Royere, M. Rey, A. Lanteri, G. Salamon, R. Khalil, Mutism and auditory agnosia due to bilateral insular damage-role of the insula in human communication, Neuropsychologia 3 (1995) 327–339.
- B.T. Hyman, D. Tranel, Hemianesthesia and Aphasia. An anatomical and behavioural study, Arch. Neurol. 46 (1989) 816–819.
- Bamiou, D.-E., Musiek, F.E., Stow, I., Stevens, J., Cipolotti, L., Brown, M.M., & Luxon, L.M. (2006). Auditory Temporal Processing Deficits in Patients with Insular Stroke, Neurology, 67, 614-619.
- Bamiou, D., Musiek, F., Luxon, L., (2003) The insula ( Island of Reil) and it’s role in auditory processing: Literature review, Brain research reviews, 42, 143- 154.
