Introduction
In 2017, an article appeared in Pathways about the role the insula plays in hearing and more specifically auditory processing. At that time it was mentioned that there were number of articles, though scattered, that implicated an auditory role for the insula. Recently, an article on the insula caught my attention: “Inefficient Involvement of Insula in Sensorineural Hearing Loss” (Xu et al., 2019) that added some good but different knowledge to the concept of the insula and its role in hearing.
I thought the new research by Xu and colleagues was a valuable contribution and should be discussed for our readers. Prior to this, however, I also thought a brief review of research on the insula would be useful to those that are not familiar with this topic.
Background
The insula also known as the “Island of Reil” is located immediately medial to the temporal lobe. If one was to remove or “pull down” the temporal lobe the insula would be obvious. For many years, the functions of the insula were somewhat of a mystery though some did consider multiple, low level functions were possible. Later, multiple sensory/emotional processing was often eluded to in early research. One of the first insights of auditory function related to the insula was offered in the late 1950s by the noted neuroanatomist Clinton Woolsey. Woolsey showed definite tonotopic organization of the insula of the cat. Subsequent to Woolsey’s research a key study came out of NIH in animals that also implicated the insula as an auditory processor and if damaged some aspects of hearing would be compromised (Sudakov et al., 1971). A major break-through on insular function came a few years later in an animal study by Collavita and colleagues (1974). These researchers showed that, if the insula was ablated, auditory pattern recognition could no longer be accomplished. Therefore, complex auditory processing ability was lost. Over the next few decades, periodically case studies as well as other more extensive studies were published implicating the insula in auditory function/dysfunction. These studies were important but were scattered across various journals from labs with varied interests. The result was little uptake in clinical community of the role of the insula in hearing. In 2003 and 2006 two major publications by Bamiou and colleagues helped substantiate the insula as key player in higher auditory processing. The trend of this research was that if the insula was damaged, temporal processing suffered. This seemed to occur for most individuals who had insular compromise.
New Information
Given the background just mentioned, it is now of interest to discuss the recent article by Xu and colleagues (2019). In this study, functional and structural MRIs were conducted on a group of individuals with sensorineural hearing loss (SNHL) and a group control subjects. This was to determine if neural activity and connectivity was compromised in the SNHL group due to deprivation (reduced auditory input) to the insula. Also, to determine if connectivity to other parts of the insula and brain may be responsible for altered audio-visual integration and emotion functions. This could inform us much about the insula and it’s relationships to hearing. In this regard, we will take the liberty of using certain (but not all) findings in this study for interpretations within the context of fundamental and high level hearing processes in the insula.
One of the key findings in the Xu study was decrease functional connectivity (fc) within the insula that was linked to SNHL. This means that those with SNHL were likely providing less input to the insula. Also, that the insula is directly affected by peripheral alterations (i.e. hearing loss) and that subsequent connectivity in the insula to other areas are also decreased. It appears that decreased connectivity was noted in a number of other brain regions such as thalamus, putamen, pre and post central gyrus, mid cingulate, pre frontal areas and Rolandic operculum. The study also showed the lack connectivity may be correlated to cognitive and emotional functions.
This research is clearly an example of impaired sensory integration linked to SNHL and auditory input to the insula. Auditory input to the insula could well affect it’s “distribution” of information to other areas of the insula and the brain. It also argues for a high level of complex actions by the insula starting with the accurate integration of auditory input – without this crucial step in function, multiple sensory and other processes can be compromised. This in turn, may affect behavior in many ways.
Summary
Applying this new information, to earlier studies several comments seem appropriate. One is that, like in recent studies, it appears the insula can function at a highly complex level. This is opposed to the very early research that presumed it was a rather low level in its physiologic actions. Another is that the insula is directly influenced by and is compromised by deprivation of acoustic input (similar to other auditory areas of the brain). Also, what happens in the insula, may not stay in the insula – it has multiple connections to other areas which can be affected by the function of the insula. Finally, this is further evidence that indeed the insula is capable of playing an important role in complex auditory processing – such temporal resolution/sequencing (Bamiou et al., 2006).
References
Musiek, F., What about the insula, Pathways, April, 2017
Xu, Xiao-Min et al. “Inefficient Involvement of Insula in Sensorineural Hearing Loss.” Frontiers in neuroscience vol. 13 133. 20 Feb. 2019, doi:10.3389/fnins.2019.00133
Woolsey, C. Cortical Sensory Organization: Multiple Auditory Areas, 1982, Humana Press, Clifton, NJ
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
Collavita, F.V. Szeligo, S.D. Zimmer, Temporal discrimination in cats with insular–temporal lesions, Brain Res. 79 (1974) 153– 156.
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.
Bamiou, D., 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.
