Autism is a neurodevelopmental disorder that affects 1 in 54 children 1 and is characterized by persistent deficits in social communication and interaction, and the presence of restricted, repetitive patterns of behavior, interests, or activities (RRB) 2. Since the publication of the DSM-5, the RRB criterion of autism includes, for the first time, a focus on sensory differences. While not a diagnostic feature, there is also mounting evidence that perception, the interpretation of sensory stimuli, is better or enhanced among autistic individuals across most sensory modalities 3,4. Enhanced perception in autism includes superior visual discrimination 5, enhanced low-level auditory perception 6,7, lower thresholds for vibrotactile stimulation 8,9, and enhanced olfactory perception10.
Evidence is beginning to suggest that perception may be related to sensory differences in autism. In one example, McKernan et al. 11 found that sensory overresponsivity, defined by atypical responses to sensory stimulation that may include very rapid and intense responses, or prolonged and lasting responses 12, predicted enhancements in tactile discrimination in autistic individuals. However, whether this finding extends to different sensory modalities (e.g., audition) and measurement types (e.g., neurophysiology) is unknown. The relationship between sensory overresponsivity, or hyperreactivity, and enhanced perception is complicated, as sensory overresponsivity is generally endorsed by parents as reflecting behavioral challenges (e.g., children putting their hands over their ears in response to loud sounds), whereas enhancements in perception are often seen as a source of strength in autism 13–15. Understanding whether and how sensory behaviors relate to enhanced perception could provide connections linking these disparate areas of study and could have implications for intervention and early diagnoses as both facets are early emerging and predictive of autism 16–18.
One way to disentangle the complex relationship between perception and sensory features in autism is to look at brain signatures that remain as close as possible, temporally, to the sensory input being processed and map these onto sensory behaviors and autistic traits 3,19. Specifically, event-related potentials (ERPs), with their millisecond temporal resolution, allow for the examination of two early stages of neurophysiological processes that have been considered atypical in autism and reflective of enhanced perception: discrimination and habituation. Discrimination is the process by which the brain determines that two stimuli differ. It is elicited experimentally using ERPs by comparing the brain’s response to a novel stimulus embedded at low rates in the context of a common repetitive stimulus and is commonly measured by the Mismatch Negativity component (MMN) 20,21. In contrast, habituation reflects the attenuation of an organism’s response to a stimulus after repeated exposure and is commonly measured by comparing the neural response to two successive repetitive stimuli using clicked-evoked paradigms 22.
Discrimination and habituation in autism have been measured behaviorally using psychophysical procedures 6,23,24, and neurophysiologically using ERPs 21,25 and fMRIs 26,27 across different sensory modalities and stimulus characteristics (e.g., frequency, duration, intensity). When measured using behavioral or psychophysiological measures in the auditory domain, enhanced discrimination and attenuated habituation are observed in autistic individuals 6,19,25,28−31 but these findings have not been consistently replicated, likely as a result of differences in measurement, participants’ ages and cognitive abilities. Discrepancies may also reflect the heterogeneity of autism, highlighting the probable presence of individual differences 22,25,32−37. For example, using fMRI methodology, Green et al. 38 found that over the course of 15-seconds of simultaneously presented auditory (i.e., traffic noises) and tactile stimuli (i.e., scratchy wool), 9 to 17-year-old autistic participants with average IQ habituated less than age and IQ matched typically developing (TD) participants. Decreased habituation was evidenced by both enhanced activation in the amygdala and somatosensory cortices, as well as slow, inconsistent decreases in activity over time. This finding was particularly true for autistic participants and sensory overresponsivity, which was measured by the Sensory Over-Responsivity (SensOR) 39 scale and specific, modality independent, items from the short Sensory Profile 40. In contrast, the neural activation of the TD participants quickly and consistently decreased over time. These findings indicate that at least a subset of autistic individuals with higher levels of sensory overresponsivity may also demonstrate altered habituation and discrimination, though given the poor temporal resolution of fMRI, it is difficult to know when in the information processing chain these differences emerged.
Together these findings raise the possibility that early neural indicators of sensory processing might be important to our understanding of the relation between sensory perception and sensory behaviors in autism. However, to-date, ERP studies have generally measured discrimination using components (e.g., P3, MMN) that occur 200 to 300ms after the stimuli are presented and compare these to neurophysiological indices of habituation occurring much earlier (e.g., P50). In the present investigation, we examine brain signatures of discrimination and habituation as close as possible, temporally, to the sensory input being processed. To achieve this goal, we leverage a simple and common auditory oddball paradigm in which two trains of stimuli are interleaved, with one being presented 80% of the time (the standard) and the other presented 20% of the time (the oddball). Preliminary studies provide promising data for studying habituation in the context of the oddball paradigm by comparing responses to successive standards at the P1 19 or P150 25. Discrimination can be measured in the oddball paradigm by comparing standards and deviants via the MMN. In addition, difference between standards and deviants can also be seen earlier, at the P1, but to our knowledge, this early modulation, which occurs in the same timeframe as the measure of habituation, has never been examined beyond infancy 25.
Because components with earlier latencies are considered representative of more passive, automatic processing, and later components reflect more cognitive processing 41, examining the P1 in addition to the more traditional MMN provides an assessment of more automatic discrimination and could thus be useful in clarifying the links between sensory perception and sensory behaviors in autism 42. Accordingly, we focus on the early P1 ERP component for habituation, and both the early P1 and later MMN component for discrimination in the same group of autistic children and typically developing children. Further, we examine how these ERP components relate to parent-reported sensory behaviors as well as to autistic traits more broadly for both the TD and autistic participants to elucidate preliminary links between brain and behavior. Only one study to date correlated perception – measured by P1 habituation and MMN discrimination – with the Sensory Profile, specifically focusing on the sensory sensitivity quadrant 19. They found that TD children (but not autistic children) with larger MMNs had less parent-reported sensory sensitivity and that those who habituated less had more typical auditory sensory registration. However, the authors did not match on, nor covary for IQ in their study, and the autistic group had significantly lower IQ than the comparison group (see publications by Russo and Burack 43–45 for a thorough review of the importance of matching). This is problematic for determining whether differences are related to autism or to IQ, and is particularly relevant to studies of auditory processing in autism, as many studies have found that autistic participants with higher IQ often exhibit better discrimination 23,29,31.
Studying differences in neurological sensory processing at the group level, and analyzing individual differences within and across groups as they relate to behavioral indices of autism and sensory processing is key to uncovering broader developmental principles that tie sensation, perception, and autistic features together irrespective of diagnostic label 46. Here, we take a step towards this goal by correlating electrophysiological measurements of sensory perception with parent-reports of children’s autistic traits, measured by the Autism Spectrum Quotient (AQ), and sensory overresponsivity using a subset of relevant items from the Sensory Profile 11,38. Our aims are to (1) determine if there are differences in ERP markers of habituation and early and late discrimination between autistic and TD children that are matched on age and IQ, (2) determine how autistic traits and sensory overresponsivity relate to early and late neurophysiological indicators of discrimination and habituation, and (3) investigate the relationships between ERP components of discrimination and habituation.