Sensory processing is the term that describes how a person senses the outside world or his/her own body. This happens through different senses: sight, smell, touch, taste, hearing, sense of the internal state of the body (interoception), sense of position or movement of our own body (proprioception), and sense of balance and spatial orientation (vestibular system).
Sensory organs (like the eye) transform signals from the outside world (like light) into signals that can be read by the brain (electrochemical signals: action potentials), which then inform the brain about the outside world (for example you see objects around you).
Do you know why some children with autism may have strong reactions of distress to loud noises or bright light? More than 96% of children with ASD report hyper- and hyposensitivities in different sensory domains, which means that their experience with different senses is enhanced or reduced.
ASD is highly variable in manifestations in different people, yet what does appear to be common to different people across the spectrum are alterations in behavioural responses to sensory information. Sensory alterations, in fact, have been frequently observed among people with autism. These sensory features have been organized in different behavioural patterns defined as hyper-responsiveness (i.e., increased, aversive or avoidant responses to stimuli), hypo-responsiveness (i.e., delayed or absent responses to stimuli), and sensory seeking behaviours (i.e., intense fascination or repeated engagement with specific sensory qualities of stimuli or sensory-based actions with the body).
Most research has focused, historically, on the social-communication and cognitive features of autism, however, interest in sensory function in autism has grown among researchers across diverse fields in more recent years. The reason of such interest lies in the potential cascading effects that alterations in the processing of sensory information and in the integration of multisensory information may have on higher-order social and cognitive functions.
Early sensory experiences are fundamental for child development as they help to shape construction and refinement of brain circuits. For this reason, alterations in vision and hearing can have strong effects on the development of complex cognitive functions, social behaviour, but also motor function.
Differences in the way we experience the external world can lead to differences in the way we respond and adapt to the external world. Differences in the way an infant experiences the external world can lead to differences in the things he learns from the external world.
Previous studies have shown an association of sensory features that manifest very early in development and persist throughout childhood to core features of autism. In particular, hypo-responsiveness and sensory seeking behaviours observed when kids are about 2 years old have been associated to later childhood outcomes such as social and language skills.
Thus, the investigation of sensory alterations in autism does not only add another piece to the puzzle, but may be critical to improve our understanding of the mechanisms underlying the emergence of autism as a whole. The association of sensory features with core symptoms as in the social domain may provide potential targets for therapeutics.
The main question here is whether differences in sensory processing are a primary or secondary feature of autism, for instance a result of learned behaviours. The interpretation of findings so far has been complicated by the high level of variability characteristic of the autism spectrum as well as the difficulty in designing tasks that can precisely probe the complex neural networks underlying sensory processing and multisensory integration.
I will go more into details of findings from different senses and then conclude with the theories trying to explain sensory processing in ASD.
Our primary sensory experience of the external world is visual. Since up to 80% of the information that the human brain receives from the external world is processed visually, far more of the human brain is dedicated to vision than to any other sense. Thus, vision is often considered to be the most important sense. Is this true? All senses and their interaction are important. What we can say is that vision underlies the development of critical mechanisms for social behaviour like gaze processing, which is an important prerequisite for imitation, joint attention (sharing attention to something interesting with someone else, by shifting gaze or pointing, or following someone else’s gaze), or even empathy.
Infants in their first year of life use vision to begin to reach out and grasp, which allows them to get to know and manipulate objects, and can thus be fundamental for motor and cognitive development. On the other hand, infants use vision to observe and recognise faces and different nuances of face expressions, which are key social cues for establishing a relationship with someone else, like the mother, and for learning to recognise someone else’s intentions.
Many people with ASD experience alterations with vision. In particular, many studies indicate that people with ASD might prefer intricate details of an image, as opposed to the image as a whole. That is what is called a preference for local compared to global processing of visual stimuli, a preference for details. And this is why they have been described as:
Depending on the task, this can be either a strength or a disadvantage. People with ASD tend, in fact, to be faster in detecting details even in very complex scenes, and they tend to be more attracted by details with salient contrast, colour and orientation, rather than specific parts of a scene that carry semantic information, like faces or actions.
In terms of auditory stimulation, people with ASD show difficulties in processing speech prosody while they have no difficulties or even particular talents in processing musical pitch. Speech prosody and music have many acoustic features in common, like similar fundamental frequency or temporal variations. The main difference between these two kinds of stimuli is in the social content.
Researchers have shown the brain of people with ASD processes speech or vocal sounds, but not musical or environmental sounds, differently compared to that of people without ASD. They explained this by a general bias of the autistic brain towards non‐vocal information resulting in enhanced processing of musical or non‐linguistic pitch, but also diminished appreciation of linguistically functional pitch (prosody).
VISION
Our primary sensory experience of the external world is visual. Since up to 80% of the information that the human brain receives from the external world is processed visually, far more of the human brain is dedicated to vision than to any other sense. Thus, vision is often considered to be the most important sense. Is this true? All senses and their interaction are important. What we can say is that vision underlies the development of critical mechanisms for social behaviour like gaze processing, which is an important prerequisite for imitation, joint attention (sharing attention to something interesting with someone else, by shifting gaze or pointing, or following someone else’s gaze), or even empathy.
Infants in their first year of life use vision to begin to reach out and grasp, which allows them to get to know and manipulate objects, and can thus be fundamental for motor and cognitive development. On the other hand, infants use vision to observe and recognise faces and different nuances of face expressions, which are key social cues for establishing a relationship with someone else, like the mother, and for learning to recognise someone else’s intentions.
Many people with ASD experience alterations with vision. In particular, many studies indicate that people with ASD might prefer intricate details of an image, as opposed to the image as a whole. That is what is called a preference for local compared to global processing of visual stimuli, a preference for details. And this is why they have been described as:
“seeing the tree, but not the forest”
Depending on the task, this can be either a strength or a disadvantage. People with ASD tend, in fact, to be faster in detecting details even in very complex scenes, and they tend to be more attracted by details with salient contrast, colour and orientation, rather than specific parts of a scene that carry semantic information, like faces or actions.
TOUCH and SENSORIMOTOR
Touch is the first sense to develop. It is the sense through which infants learn about the environment and themselves, as well as the primary way for the infant to communicate with the mother. As such, sensorimotor differences are among the first signs detectable of ASD in infancy, already in the first year of life.
There is the general idea that people with ASD do not like to be touched at all, and are likely to have strong reactions even to the slightest touch. The variability that is characteristic of ASD can actually be observed also in tactile sensation, and can explain why we observe in autism people having painful responses to touch, while others may not even notice a wound from a sharp object. Thus, while some people may pull away when someone tries to hug them, others may like to hug tightly.
Research on the neurophysiological responses to tactile stimulation in ASD has investigated the initial brain response to individual tactile stimulation (measuring neural sensitivity) and the effect of repeated stimulation (measuring neural repetition suppression: the usual decrease in response to the second stimulus with respect to the first stimulus as an effect of habituation to that stimulation). Studies show reduced habituation of brain responses , and so in behavioural response, to tactile stimulation. A lack of habituation can result in overwhelming stimulation in people with ASD, and explain the higher sensitivity as well as strong behavioural responses in these people. This is attributed to reduced levels of the neurotransmitter GABA in the somatosensory cortex, a neurotransmitter responsible for inhibition of neural activity in the brain.
Something once reported by the autistic scholar, Temple Grandin, regarding her hearing capabilities can help understand what we are talking about:
There is the general idea that people with ASD do not like to be touched at all, and are likely to have strong reactions even to the slightest touch. The variability that is characteristic of ASD can actually be observed also in tactile sensation, and can explain why we observe in autism people having painful responses to touch, while others may not even notice a wound from a sharp object. Thus, while some people may pull away when someone tries to hug them, others may like to hug tightly.
Research on the neurophysiological responses to tactile stimulation in ASD has investigated the initial brain response to individual tactile stimulation (measuring neural sensitivity) and the effect of repeated stimulation (measuring neural repetition suppression: the usual decrease in response to the second stimulus with respect to the first stimulus as an effect of habituation to that stimulation). Studies show reduced habituation of brain responses , and so in behavioural response, to tactile stimulation. A lack of habituation can result in overwhelming stimulation in people with ASD, and explain the higher sensitivity as well as strong behavioural responses in these people. This is attributed to reduced levels of the neurotransmitter GABA in the somatosensory cortex, a neurotransmitter responsible for inhibition of neural activity in the brain.
HEARING
Something once reported by the autistic scholar, Temple Grandin, regarding her hearing capabilities can help understand what we are talking about:
In terms of auditory stimulation, people with ASD show difficulties in processing speech prosody while they have no difficulties or even particular talents in processing musical pitch. Speech prosody and music have many acoustic features in common, like similar fundamental frequency or temporal variations. The main difference between these two kinds of stimuli is in the social content.
Researchers have shown the brain of people with ASD processes speech or vocal sounds, but not musical or environmental sounds, differently compared to that of people without ASD. They explained this by a general bias of the autistic brain towards non‐vocal information resulting in enhanced processing of musical or non‐linguistic pitch, but also diminished appreciation of linguistically functional pitch (prosody).
THEORIES OF AUTISM
Two perceptual theories of autism try to explain these strengths and weaknesses in processing sensory stimuli. The first is called the weak central coherence (WCC) theory, according to which individuals with autism have a weaker capability to process perceptual stimuli in a more global context. The second theory is called of enhanced perceptual functioning (EPF), according to which the neural networks underpinning perceptual processing are much more specialized in autism.
Another possible explanation is that of higher perceptual capacity in autism. Researchers in the autism field have shown that people with autism are able to pay attention to a higher amount of information at one time. This could explain their increased sensitivity and higher ability to notice small details but can also lead to more distraction or to the over-arousal that many people with autism report.
It is important to highlight the profound impact that these sensitivities can have on daily life of people with autism. Here is an example of how an autistic person described a sensory overload in an online forum:
Sensory symptoms have been indicated among the most unwanted symptoms by people with autism, and those aspects of their condition that they would really want us to address. Increased arousal or frequent distraction can be very distressing in various contexts and can consequently cause anxiety. This may lead, in turn, to withdrawal from sensory-rich environments, which are often the more social ones, especially nowadays. This can, then, lead to more social isolation and have a negative impact on the quality of life of people with autism.
Another possible explanation is that of higher perceptual capacity in autism. Researchers in the autism field have shown that people with autism are able to pay attention to a higher amount of information at one time. This could explain their increased sensitivity and higher ability to notice small details but can also lead to more distraction or to the over-arousal that many people with autism report.
It is important to highlight the profound impact that these sensitivities can have on daily life of people with autism. Here is an example of how an autistic person described a sensory overload in an online forum:
I can’t think. I can’t find the right word that I know I know. I feel like my mind is in a fog. Things seem spacey, unreal. I feel my head could spin and sometimes I do get dizzy from it. (autismforums.com, 2017)
Sensory symptoms have been indicated among the most unwanted symptoms by people with autism, and those aspects of their condition that they would really want us to address. Increased arousal or frequent distraction can be very distressing in various contexts and can consequently cause anxiety. This may lead, in turn, to withdrawal from sensory-rich environments, which are often the more social ones, especially nowadays. This can, then, lead to more social isolation and have a negative impact on the quality of life of people with autism.
Thus, it is highly important to understand these sensory mechanisms observed in autism and try to identify possible strategies to minimize unwanted effects, as those reported directly by people with ASD.
References
https://doi.org/10.1177/1362361320922640
Robertson, C., Baron-Cohen, S. Sensory perception in autism. Nat Rev Neurosci 18, 671–684 (2017). https://doi.org/10.1038/nrn.2017.112
Happé, F., & Frith, U. (2006). The weak coherence account: detail-focused cognitive style in autism spectrum disorders. Journal of autism and developmental disorders, 36(1), 5-25.
Mottron, L., Dawson, M., Soulières, I., Hubert, B., & Burack, J. (2006). Enhanced perceptual functioning in autism: An update, and eight principles of autistic perception.(2006). DOI: http://dx. doi. org/10.1007/s10803-005-0040-7.
Grandin T. Calming effects of deep touch pressure in patients with autistic disorder, college students, and animals. J Child Adolesc Psychopharmacol. 1992 Spring;2(1):63-72. doi: 10.1089/cap.1992.2.63. PMID: 19630623.
Järvinen-Pasley A, Wallace GL, Ramus F, Happé F, Heaton P. Enhanced perceptual processing of speech in autism. Dev Sci. 2008 Jan;11(1):109-21. doi: 10.1111/j.1467-7687.2007.00644.x. PMID: 18171373.
Piccardi ES, Begum Ali J, Jones EJH, Mason L, Charman T, Johnson MH, Gliga T; BASIS/STAARS Team. Behavioural and neural markers of tactile sensory processing in infants at elevated likelihood of autism spectrum disorder and/or attention deficit hyperactivity disorder. J Neurodev Disord. 2021 Jan 4;13(1):1. doi: 10.1186/s11689-020-09334-1. PMID: 33390154; PMCID: PMC7780639.
Puts NA, Wodka EL, Tommerdahl M, Mostofsky SH, Edden RA. Impaired tactile processing in children with autism spectrum disorder. J Neurophysiol. 2014 May;111(9):1803-11. doi: 10.1152/jn.00890.2013. Epub 2014 Feb 12. PMID: 24523518; PMCID: PMC4044368.
Simmons DR, Robertson AE, McKay LS, Toal E, McAleer P, Pollick FE. Vision in autism spectrum disorders. Vision Res. 2009 Nov;49(22):2705-39. doi: 10.1016/j.visres.2009.08.005. Epub 2009 Aug 12. PMID: 19682485.
Little, J.‐A. (2018), Vision in children with autism spectrum disorder: a critical review. Clin Exp Optom, 101: 504-513. https://doi.org/10.1111/cxo.12651
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