Sensory Integration Education

This article on research into auditory modulation and efferent pathways was originally published in SensorNet Issue 55, January 2019:

Innovation in Identification: Auditory Modulation and Efferent Pathways

By Valeria Isaac, MSc, OTR

Valeria is an occupational therapist with 20 years’ of experience in the area of paediatrics. She originally graduated as an occupational therapist from the University of Chile, and completed the OT610 program from USC in California. She is currently the owner of Neurosens, an organisation dedicated to education and research in SI, in Chile. Valeria presented at this year’s ISIC in Hong Kong on her research into auditory modulation and efferent pathways. She provides us with a background to her research and updates on her progress to date.

The auditory system is a complex sensory system that has been studied mainly for its participation in speech and language development. As James E. Peck (1994) would say “Hearing is one of our most precious gifts. With it we acquire the marvelous power of speech.” This is true, but not just speech. Other areas of cognitive performance are also involved with functions within the auditory system, specifically influencing attention and organisation of behaviour.

Let's imagine a situation, where you are in a restaurant with friends sitting at a table. The place is completely full, everybody talking and laughing loudly, there is background music, the moving of chairs and sounds from plates, forks etc. However, despite the loud surrounding noise and auditory overload, you manage to orient your hearing and engage in a perfectly functional conversation with your friends, and enjoy your evening together.

How do you manage to listen to a conversation immersed in such a sound competing environment? How does your brain manage to organise and make sense out of all the auditory input its receiving, and orient attention to what is aurally relevant?

For many years, audition has been studied considering only its “afferent” pathways, viewed mainly as a sensory system specialised in detecting sound waves and transducing them to electrical signals that travel towards the central nervous system to be processed and interpreted. But the auditory system is not only afferent, there are also efferent projections originating from the auditory cortex descending towards the brainstem and even the cochlea. Inside the cochlea, the organ of Corti houses the auditory receptors called the “inner hair cells.” But beside these receptor cells, are the outer hair cells, which receive efferent projections from the brainstem through what is known as the olivocochlear nerve. These outer hair cells are motile cells, which have the ability to contract and elongate, boosting cochlear vibrations, and therefore performing as a cochlear amplifier. In other words, through the efferent auditory pathway, the auditory cortex can influence and modify incoming auditory input by modulating cochlear sensitivity.

Some of the studies to evidence these functions in auditory efferent pathways were conducted by May and McQuone (1995) using cats. They trained cats in an auditory discrimination task to be performed in quiet and in the presence of noise. Once a bilateral lesion of the olivocochlear fibers was conducted, the cats showed a significant deficit in performing the task only when tested in noise, but not in quiet. Similar studies with cats (Heinz et al. 1998) and also with monkeys (Dewson, 1968) showed the same results, bilateral lesions of the olivocochlear system would significantly impair their performance in auditory discrimination tasks in the presence of noisy backgrounds.

More recent studies have shown that lesions in efferent auditory fibers not only impair auditory discrimination in noise, but can also affect performance in other sensory discrimination tasks. Studies by Dr. Delano (Delano et al. 2007) using chinchillas show that during selective attention to a visual discrimination task, cochlear sensitivity is reduced, probably due to activation of olivocochlear fibers. In summary, the proposed functions of the efferent auditory pathways are mainly antimasking sounds in noisy environments (auditory discrimination) allowing our auditory brain to voluntarily amplify and sharpen those sounds particularly interested in perceiving, and modulation of cochlear sensitivity during cross-modal attention.

I attended this year´s ISIC in Hong Kong to present about this topic and show a little bit about the research I have been doing for the past year in the University of Chile. I am currently conducting a research project which aims to evaluate an olivocochlear reflex in children diagnosed with Attention Deficit and Hyperactivity Disorder (ADHD), because of their characteristic difficulty with selective attention especially in busy environments, such as the classroom. Children tend to receive the diagnosis of ADHD due to a variety of underlying reasons, and our proposal is that a possible dysfunction of the efferent auditory circuits at the level of the brainstem may be one of these physiological underlying factors for a subgroup of ADHD children.

Along with Alexis Leiva (my laboratory partner) we have just concluded evaluating the olivocochlear reflex, using evoked otoacoustic emissions, in a total of 62 children with ADHD, ages 5 to 12. All the children were previously assessed for normal hearing and speech development. We are now in the process of analysing the collected data, studying the presence and amplitudes of the cochlear efferent responses within a broad range of sound frequencies in each child. We have observed that the ADHD children that were found to have altered auditory efferent responses tend to have associated reports, by teacher and/or parent, of sensitivity and over-reactive behaviours to loud sounds and noisy environments. This data is still under statistical analysis, but we hope to have this work finally published in the near future.

So there is definitely more to auditory processing than just speech and language. A child can have normal hearing and speech development, and still might present with difficulties in auditory functions, which could impact cognitive performance, like attention and the ability to adapt and function within noisy environments.

This highlights the importance of further research in the auditory system, and its consideration as a possible underlying physiological factor for certain behaviours. This continues to broaden our perspectives when evaluating a child’s behaviour and performance, hoping that as we gain more knowledge in neurophysiology it will lead us towards better evaluation procedures and, therefore, more successful treatment opportunities for each child’s specific diagnostic needs.

I enjoyed participating in this year´s ISIC. It’s always a pleasure to interact with therapists from different parts of the world, and discover the wonderful work being done, it's very inspiring. I especially enjoyed the presentation by Annamarie Van Jaarsveld from South Africa, where she showed one of her projects about making playground equipment using recycled materials, and also the presentation about premature care from an SI perspective in the NICU by Mahek Uttamchandani. It’s touching to see how much love and dedication is put into all these projects and daily work with the children and families all around the world.

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