The tiny fish seen below gives insight into autism. The zebrafish is helping scientists understand how the human brain processes sound.
For many on the spectrum, sensory sensitivity is one of our most common sensory differences. Loud noise can cause us to experience sensory overload and anxiety.A review of behavioural and electrophysiological studies on auditory processing and speech perception in autism spectrum disorders (Haesen, Boets & Wagemans, 2011) Loud noises can even cause us pain. Whistling gives me the sensation of what I imagine an ice-pick stabbing into my ear might feel like. As such I have never been able to enjoy hearing my son whistle. This phenomenon where typical sounds cause pain is called hyperacusis, and it is present in about 40% of autistics.Hyperacusis | Audiology Information Series | American Speech–Language–Hearing Association Also, certain frequencies and multiple sounds can be overwhelming for us.
The DSM-5 finally included sensory differences as part of our diagnostic criteria, although you can be diagnosed without having sensory sensitivities. The majority of autistic people do experience sensory differences; the estimation is that about 60-96% of us meet the sensory sensitivity criterion.Toward an Interdisciplinary Understanding of Sensory Dysfunction in Autism Spectrum Disorder: An Integration of the Neural and Symptom Literatures (Schauder & Bennetto, 2016) Our sensory sensitivity is related to social differences and repetitive behaviour.Restricted, Repetitive Behaviors in Autism Spectrum Disorder and Obsessive–Compulsive Disorder: A Comparative Review (Jiujias, Kelley & Hall, 2017) If you think about it this makes sense—we avoid loud and vexatious places and when we can’t we stim to cope.
Researchers hypothesized that autistics habituate more slowly to sounds than non-autistics. In other words, they suspected that it would take us longer to get used to and be able to ignore sounds. It was a reasonable premise as scientists knew that the amygdala of autistics habituates slower to faces compared to non-autistic people.Habituation is altered in neuropsychiatric disorders—A comprehensive review with recommendations for experimental design and analysis (McDiarmid, Bernardos & Rankin, 2017) However, the research showed that we, in fact, do not habituate to sound more slowly. We did rate the auditory stimuli as more arousing (scientific gibberish for more stressful), though.Stop Making Noise! Auditory Sensitivity in Adults with an Autism Spectrum Disorder Diagnosis: Physiological Habituation and Subjective Detection Thresholds (Kuiper, Verhoeven & Geurts, 2019)
Autistic participants in a different study showed a significant reduction in magnitude and rate of simple loudness adaptation (the decrease in loudness that takes place when a continuous sound is presented alone for a period of time, typically a soft sound less than 30dB above the threshold) relative to age and ability-matched non-autistic adults.A striking reduction of simple loudness adaptation in autism (Lawson et al., 2015)
Zebrafish carry identical genetic mutations that cause autism. Scientists were able to find the neural pathways that lead to sound hypersensitivity in humans and zebrafish, and study the brain neuron by neuron!Altered brain-wide auditory networks in a zebrafish model of fragile X syndrome (Constantin et al., 2020)
When the researchers exposed the zebrafish to bursts of sound, the mutated zebrafish were able to hear softer sounds:Fish give insight on sound sensitivity in autism | ScienceDaily
The fish with mutations had more connections between different regions of their brain, and their responses to the sounds were more plentiful in the hindbrain and the thalamus.
So why were the thalamus and hindbrain involved? The thalamus works as a control center (much like an air traffic controller who direct aircraft through space), relaying sensory information from the body to parts of the brain. The hindbrain co-ordinates behavioural responses. In other words, the thalamus says “There is a loud noise coming from your left”, and the hindbrain makes us ‘run away’.
Super power, not disability
Researchers found what I think many of us on the spectrum already know. Our brain transmits MORE auditory information. It transmits more of everything. That is because we filter sounds differently. Compared to non-autistics, we have more local hyperconnectivity.
Society struggles to understand that normocentrocity is not the optimal standard of a human being. In a way, it is funny. No one that I know wants to be average, and research backs this up. Illusory superiority is a cognitive bias where a person overestimates their qualities as superior. Everyone thinks they are above 50%. Yet when it comes to research, the average becomes something desirable.
Our hearing is above average, and despite researchers calling it a disability, we now know that a link between musical genius or exceptional musical talent is superior hearing. Our super hearing gives us a marked advantage.A sound advantage: Increased auditory capacity in autism (Remington & Fairnie, 2017)
Some of our advantages include enhanced pitch discrimination (the ability to distinguish between different pitches and tones) and increased auditory perceptual capacity (the ability to perceive sounds). At the extreme, this advantage produces autistic savants who can write and play music like grandmasters with incredible talent and flourish. But, there is a price for every gift, and for us, we have a harder time ignoring sounds. We can experience normal, neutral sounds harsh.
In one study, participants would listen to animal sounds and decide if they heard a dog barking or a lion roaring. This is an experiment where people with better auditory perception, such as autistics, will have the advantage. As suspected, autistics ended up scoring much higher than controls.
In a second task, participants listened to a recording of people at a party. In the middle of the recording, a man repeated, “I’m a gorilla, I’m a gorilla…” 47% of autistics were affected by this compared to only 12% of non-autistics.
Each of us is unique, and it is these unique features that allow our entire race to exist and evolve. I think of my super-hearing as a gift for the majority of the time. The brain simply can not wire a person to hear more without causing them to hear more in all situations. Sometimes, that can make typical sounds overwhelming.