By guest blogger Helge Hasselmann
Besides problems with social interactions, it has been known for a while that many people with autism experience sensory abnormalities, such as hypersensitivity to sounds, light or touch. With sensory impairment now officially included in diagnostic manuals, researchers have been trying to see if there’s a link between the sensory and social symptoms. Such a link would make intuitive sense: For instance, it is easy to imagine that if someone experienced sensory stimuli more strongly, they would shun social interaction due to their complexity. More specifically, you would expect them to struggle with filtering out and making sense of social cues against the backdrop of sensory overload.
Past research has suggested that tactile hyper-responsiveness in particular may be relevant. The correct processing of tactile information plays an important role in differentiating yourself from others (so-called "self-other discrimination"), a crucial requirement for social cognition. In fact, touch may be unique among the senses because there is a clear difference in the tactile feedback received when you touch something compared to when you see someone else touch something. Now a study in Social Cognitive and Affective Neuroscience has used recordings of participants’ brain waves to provide more evidence that tactile sensations are processed differently in people with autism and that this may contribute to their social difficulties.
The researchers from Ghent recruited 19 men and women diagnosed with high functioning autism and 17 healthy control participants matched for gender, intelligence and age – both groups had an average age of around 32 years. While everyone filled out questionnaires assessing everyday problems with sensory processing, participants with autism received an additional questionnaire on the severity of their autism symptoms
To deliver tactile sensations, two electrodes were placed on the participants’ index and middle fingers of their right hand, which was covered with a dark cloth. Before the experiment began, 30 tactile stimulations (mild electric currents) were delivered to both fingers to familiarise participants with the device.
The actual experiment was relatively straightforward and involved the researchers recording the participants’ brain waves with EEG throughout. Each trial began with a picture displaying either a human or a wooden hand in a non-flexed position, resembling the position of the participants’ hidden right hand. Next, a picture was shown of either a tapping index or middle finger, which was accompanied by a single stimulator “tap” sent to the electrode on either the participant’s index or middle finger. This way, the tactile feedback the participants experienced was either congruent with the observed action (e.g. they saw a tapping index finger and received stimulation to their index finger) or incongruent (e.g. they saw a tapping index finger but received stimulation to their middle finger).
The researchers were especially interested in spikes of brain activity that are known to be relevant to processing tactile sensations, including the so-called N100 (a negative spike that occurs 100ms after the sensation) and a later positive spike (the P3) that has been linked with self-other discrimination in various contexts. For example, previous studies found an amplified P3 signal when participants heard their own name compared to another name or saw their own face compared to someone else’s. Crucially, this congruency effect is also seen when observed movements match experienced tactile sensations.
The researchers hypothesised that if social impairment is related to sensory processing, then the participants with autism would show a reduced congruency effect in P3, and the size of this irregularity would be negatively correlated with their social functioning. And this is exactly what they found.
Compared to healthy controls, participants in the autism group demonstrated abnormalities in their brain waves both at earlier stages of sensory processing (the N100) as well as later stages. In line with the hypothesis, congruent trials (tactile feedback compatible with the observed action of a human hand) led to amplified P3 signals in the healthy controls, but not in the autistic group. Furthermore, in the participants with autism, the greater their sensory and social impairments, the smaller the amplitude of the P3 component of their brain waves in response to congruent pictures.
These findings allow for two suggestions: First, individuals with autism struggle to differentiate between themselves and others (based on purely tactile information), which is similar to what other studies have found. Second, this difficulty was more pronounced in those individuals who also showed a higher degree of social and sensory impairment, suggesting these two domains are linked.
So does this mean that sensory hypersensitivity causes the social deficits that individuals with autism face? While this study cannot establish which is caused by which (or if they are both caused by a completely different factor), it does provide experimental evidence of a link between the two. To what extent this knowledge will improve treatment options for patients with autism remains to be studied by future investigations.
Post written by Helge Hasselmann. Helge studied psychology and clinical neurosciences. Since 2014, he is a PhD student in medical neurosciences at Charité University Hospital in Berlin, Germany, with a focus on understanding the role of the immune system in major depression.