Characterization and Measurement of Sensory Behaviors in Autism Spectrum Disorder: A Comprehensive Review

1. Introduction: Sensory Processing in Autism Spectrum Disorder

Atypical sensory-based behaviors are a ubiquitous feature of autism spectrum disorders (ASD) and have been incorporated into the diagnostic criteria in the DSM-5. Sensory processing abnormalities are a core component of autism, and considerable research has been conducted to understand the neural underpinnings of sensory processing in autism by examining neurophysiological responses to auditory, tactile, and visual stimuli in individuals with ASD [1]. Studies indicate that over 90% of children with Autism Spectrum Disorders demonstrate atypical sensory behaviors [2]. Indeed, sensory abnormalities are commonly recognized as diagnostic criteria in autism spectrum disorder (ASD), as reported in the last edition of the Diagnostic and Statistical Manual of Mental Disorder (DSM-V) [3].

This review examines how sensory seeking and avoidance behaviors associated with autism are characterized and measured in the scientific literature. It explores conceptual frameworks, assessment methods, neurobiological correlates, and relationships with other autism features, while identifying gaps and limitations in current research.

2. Conceptual Frameworks for Understanding Sensory Processing in ASD

2.1 Models of Sensory Processing

A seminal contribution to understanding sensory processing is Dunn's Model of Sensory Processing, which provides a theoretical framework for characterizing sensory behaviors. This model identifies four sensory processing patterns: sensory seeking, sensory avoiding, sensory sensitivity, and low registration. These patterns are based on a person's neurological thresholds and self-regulation strategies [4].

As with temperament, each person has some level of responsiveness within each sensory processing preference (i.e., a certain amount of seeking, avoiding, etc., not one or the other). Dunn suggests that one's sensory processing preferences simultaneously reflect nervous system needs and form the basis for the manifestation of temperament and personality [4].

2.2 Sensory Processing Patterns in ASD

Research has identified various patterns of sensory processing in ASD. Confirmatory factor analysis has empirically validated three sensory constructs of interest: hyperresponsiveness, hyporesponsiveness, and sensory seeking [5]. Sensory hypersensitivity (aversion to certain sounds, touch, etc., or increased ability to make sensory discriminations) and/or hyposensitivity (desire to stimulate the senses, or a reduced response to sensory stimuli) are now recognized diagnostic features of autism spectrum conditions (ASC) [6].

Hyper- or hyporeactivity to sensory input or unusual interest in sensory aspects of the environment is now included in the DSM-5 diagnostic criteria. However, there are children with sensory processing differences who do not meet an ASD diagnosis but do show atypical sensory behaviors to the same or greater degree as ASD children [2]. This highlights the complexity of sensory processing differences and the need for careful differentiation and assessment.

2.3 Sensory Phenotypes in ASD

Recent research has moved beyond simple categorizations to identify more nuanced sensory phenotypes within the autism spectrum. Atypical reactions to the sensory environment are often reported in autistic individuals, with a high degree of variability across the sensory modalities. These sensory differences have been shown to promote challenging behaviors and distress in autistic individuals and are predictive of other functions including motor, social, and cognitive abilities. Preliminary research suggests that specific sensory differences may cluster together within individuals creating discrete sensory phenotypes [7].

Using Short Sensory Profile data from 599 autistic children and adults between the ages of 1 and 21 years, researchers identified five sensory phenotypes: (1) sensory adaptive, (2) generalized sensory differences, (3) taste and smell sensitivity, (4) under-responsive and sensation seeking, and (5) movement difficulties with low energy [7]. This approach helps account for the heterogeneity observed in sensory processing across individuals with ASD.

Similarly, factor mixture modeling (FMM) has been applied to test multidimensional factor models of sensory processing in ASD, with the aim of identifying homogeneous sensory subgroups that differ intrinsically in their severity along continuous factor scores [8]. The 'three-subgroup/seven-factor' FMM was found to be superior, with identified subgroups differing in sensory severity from severe, moderate to low [8].

Cluster analysis of sensory functioning in adults with ASC has further identified distinct patterns: In an intermediate group, hyperreactivity, sensory interests, and sensory/motor issues were significantly elevated, but not hyporeactivity. In a high frequency subgroup, all aspects of sensory processing were significantly elevated, and co-occurrence of hyper- and hyporeactivity was evident [9]. These findings underscore the complex nature of sensory processing in ASD.

3. Assessment Methods and Measurement Tools

3.1 Questionnaire-Based Assessments

Parent and self-report questionnaires are the most commonly used tools for assessing sensory processing in individuals with ASD. A scoping review identified five categories of assessment approaches: Self- and Proxy-Report Questionnaires, Psychophysical Assessment, Direct Behavioral Observation, Qualitative Interview Techniques, and Neuroimaging/EEG. Sensory research to date has focused on individuals with high-functioning ASD, most commonly through the use of self-report questionnaires. The Adolescent and Adult Sensory Profile (AASP) is the most widely used assessment measure, however, a number of other assessment approaches may demonstrate strengths specific to the ASD population [10].

Given that the DSM-5 criteria for ASC now include sensory reactivity, there is a need for adult questionnaires investigating basic sensory functioning. The Sensory Perception Quotient (SPQ) was developed to assess basic sensory hyper- and hyposensitivity across all five modalities [11]. Adults with ASC report more sensitivity to sensory stimuli on the SPQ, and greater sensory sensitivity is associated with more autistic traits. The SPQ provides a new tool to measure individual differences on this dimension [11].

The Adult/Adolescent Sensory Profile (AASP), the Cardiff Anomalous Perceptions Scale (CAPS), and the Glasgow Sensory Questionnaire (GSQ) are questionnaire measures of abnormal sensory responsivity. Autism traits have been found to be significantly correlated with scores on all three sensory scales, with this relationship being linear across the whole range of autism quotient scores [12].

The SPQ-Short was developed to measure sensory symptoms based on perceptual rather than affective, regulative, or attention components. It has been validated in a large sample of adults with (n = 657) and without autism spectrum disorder (n = 585) [13]. The autism spectrum disorder group reported higher sensory sensitivities than the comparison group, and women with autism spectrum disorder reported higher sensitivities compared with men with autism spectrum disorder. The SPQ-Short correlates with all Autism Quotient (AQ)-Short subscales, except for the "imagination" subscale, and seems suitable to further explore the relationship between basic sensory sensitivities in autism spectrum disorder and their related symptoms such as over- and under-responsivity to sensory stimulation [13].

For children, the Sensory Profile-2 (SP-2) questionnaire in its school version, answered by teachers, has been used to evaluate the degree of response of children to sensory stimuli at school [14]. The Autism Spectrum Screening Questionnaire (ASSQ) and parental questionnaires have also been employed to understand sensory abnormalities. In an epidemiological population of 8-year-old children (n = 4397), the prevalence of sensory abnormalities was 8.3%, in an ASD sample (n = 28), 53.6%, and in a non-ASD sample (n = 4369), 8.0%, respectively [15].

3.2 Direct Observational Measures

While questionnaires offer valuable insights, direct observational measures provide more objective assessments of sensory behaviors. Researchers and practitioners typically use parent-report measures or informal clinical observations to understand the presence and nature of sensory interests, repetitions, and seeking behaviors (SIRS). The Sensory Processing Assessment for Young Children, an observational measure, has been used to characterize SIRS across groups of children with ASD, developmental delays, and typical development [16].

Group differences have been identified in frequency and intensity of overall SIRS, complexity of SIRS, and incidence of particular types of SIRS (i.e., posturing, sighting, proprioceptive seeking, spinning). Facial affect was also explored and found to be primarily neutral during engagement in SIRS across groups [16].

Some researchers have established cohorts based on sensory over-responsivity using both parent report (Short Sensory Profile (SSP)) and direct assessment (Sensory Processing-Three Dimensions: Assessment (SP-3D:A)) measures. Using direct assessment, 31% of children with neurodevelopmental disorders had auditory over-responsivity (AOR) and 27% had tactile over-responsivity (TOR) [17]. The inter-test agreement between SSP and SP-3D:A for AOR was 65% and TOR was 50%. Both questionnaire and direct observation measures should be used in clinical and research settings, as the SSP parent report and SP-3D:A direct observation ratings overlapped only moderately for sensory-related behaviors [17].

3.3 Physiological and Neurobiological Measures

Physiological measures offer insights into the neurobiological mechanisms underlying sensory behaviors in ASD. Psychophysiology studies have verified sensory processing patterns; persons with strong preferences in each pattern have unique patterns of habituation and responsivity in skin conductance [4].

Diffusion tensor imaging (DTI) fiber tractography has been used to evaluate the structural connectivity of specific white matter tracts in individuals with ASD, defining white matter tracts using probabilistic streamline tractography and assessing the strength of tract connectivity using mean fractional anisotropy. Both individuals with Sensory Processing Disorders (SPD) and ASD demonstrate decreased connectivity relative to controls in parieto-occipital tracts involved in sensory perception and multisensory integration [2].

Individuals on the autism spectrum are often reported as being hyper- and/or hyporeactive to sensory input. These sensory symptoms were one of the key observations that led to the development of the altered excitation-inhibition (E-I) model of autism. Higher levels of Glx (glutamate + glutamine) in the primary sensorimotor cortex were associated with more parent-reported difficulties of sensory hyper- and hyporeactivity, as well as reduced feed-forward inhibition during tactile perception in children with ASD [18].

Magnetic resonance spectroscopy (1H-MRS) has been employed to examine whether GABA levels in brain regions critical to higher-order motor and/or multiple sensory functions were associated with abnormal sensory responses in ASD. Researchers have evaluated atypical sensory processing with clinically-validated assessment tools and measured GABA levels in four regions: one each in the primary visual cortex, the left sensorimotor cortex, the left supplementary motor area (SMA), and the left ventral premotor cortex (vPMC) [19]. A correlation analysis revealed a negative association between left vPMC GABA and the severity of sensory hyper-responsiveness across all participants, and the independent ASD group. These findings suggest that reduced inhibitory neurotransmission (reduced GABA) in a higher-order motor area, which modulates motor commands and integrates multiple sensory modalities, may underlie sensory hyper-responsiveness in ASD [19].

4. Prevalence and Demographic Factors in Sensory Processing

4.1 Prevalence of Sensory Abnormalities

Analysis of a large sample comprising 25,627 four- or eight-year-old autistic children identified through the multistate Autism and Developmental Disabilities Monitoring Network (2006–2014) found that the majority (74%; 95% confidence interval: 73.5%–74.5%) of the children studied had documented sensory features [20].

Across different samples, tactile and auditory hypersensitivity were found to predict an ASD diagnosis. In a large epidemiological study, the prevalence of sensory abnormalities was 53.6% in an ASD sample, compared to 8.0% in a non-ASD sample [15].

4.2 Sex Differences in Sensory Processing

Sex differences in sensory processing have been identified in several studies. Adult females with ASC showed more lifetime sensory symptoms (p = 0.036), fewer current socio-communication difficulties (p = 0.001), and more self-reported autistic traits (p = 0.012) than males [21].

Women with autism spectrum disorder report higher sensory sensitivities compared with men with autism spectrum disorder. Gender did not have an effect in the group without autism spectrum disorder. The Dutch SPQ-Short demonstrated that adults with autism spectrum disorder were more sensitive compared with adults without autism spectrum disorder [13].

In a multivariable model examining sensory features, children who were male and those whose mothers had more years of education had higher odds of documented sensory features. Children from several racial and ethnic minority groups had lower odds of documented sensory features than White, non-Hispanic children [20]. These findings suggest potential disparities in the identification of sensory features that should be considered in clinical practice.

4.3 Age and Developmental Considerations

Despite growing evidence that sensory symptoms persist into adolescence and adulthood, there is a lack of norms for older age groups, and pediatric assessments may not target appropriate functional outcomes or environments [10]. This highlights a significant gap in the assessment of sensory processing across the lifespan.

Investigations of sensory processing in infants at elevated likelihood of ASD and/or ADHD have included both behavioral and neural markers of tactile sensory processing. These studies have examined whether infant markers are associated with later ASD or ADHD traits [22]. Reduced neural repetition suppression to tactile input at 10 months significantly predicted ASD (but not ADHD) traits at 24 months across the entire sample. Elevated tactile sensory seeking at 10 months moderated the relationship between early reduced neural repetition suppression and later ASD traits, suggesting it may act as a protective factor, mitigating the relationship between early tactile neural repetition suppression and later ASD traits [22].

5. Relationship Between Sensory Processing and Other Autism Features

5.1 Sensory Processing and Repetitive Behaviors

Examination of the association between sensory features and restricted, repetitive behaviors in children with autism and those with developmental delays has revealed the co-occurrence of these behaviors in both clinical groups. Specifically, high levels of hyperresponsive behaviors predicted high levels of repetitive behaviors, and the relationship between these variables remained the same controlling for mental age. Sensory seeking was associated with ritualistic/sameness behaviors [5].

The relationship between sensory profile and repetitive behaviors in autism spectrum disorder has long been known, with direct correlations between Sensory Short Profile (SSP) and Repetitive Behaviour Scale-Revised (RBS-R) total scores. Among the subscales, the strongest correlations involved "Visual/Auditory Sensitivity", related to "Stereotyped Behaviour" and "Sameness Behaviour". "Under-Responsive/Seeks Sensation" was related to "Stereotyped Behaviour" [23].

Repetitive and Restricted Behaviors (RRB), one of the core symptom categories for Autism Spectrum Disorders (ASD), comprises heterogeneous groups of behaviors with two or more factors (subcategories) within the RRB domain. Using principal component analysis of item scores obtained from Autism Diagnostic Interview-Revised (ADI-R), two distinct subcategories within Restricted and Repetitive Behaviors were identified: Repetitive Sensory Motor (RSM) and Insistence on Sameness (IS) [24].

Previous research indicates that 'insistence on sameness' (IS) and 'repetitive sensory-motor actions' (RSMA) are two factors within the ASD 'repetitive and stereotyped behavior' domain [25]. It has been replicated that RSMA is more strongly associated than IS with measures of ASD severity [25].

5.2 Sensory Processing and Social Functioning

Sensory processing issues have been frequently reported in individuals with Autism Spectrum Disorders (ASD), and within the group of children with ASD, higher intensities of sensory issues were associated with more prominent social difficulties and lower adaptive functioning [26].

Qualitative studies with autistic adults have revealed that they experience a range of visual hypersensitivities, including to light, motion, patterns, and particular colors, which contributed to distraction and were frequently part of a wider multisensory issue. Such experiences had significant negative impacts on personal wellbeing and daily life with participants describing fatigue, stress, and hindrances on day-to-day activities (e.g., travel and social activities) [27].

Altered processing of touch is a stronger mediator of social symptoms' severity than altered processing of other senses like vision or audition. Researchers have proposed that sensory difficulties may be linked to social problems in ASD through insufficient self-other distinction centered on touch [28]. Individuals with ASD showed a diminished congruency effect for human hands only in the P3-complex, suggesting difficulties with signaling observed action-based touch of others that does not match own touch experiences. Crucially, this effect reliably correlated with self-reported social and sensory everyday difficulties in ASD, potentially denoting a novel theoretical link between sensory and social impairments in the autism spectrum [28].

5.3 Sensory Processing and Cognitive/Adaptive Functioning

Age, adaptive behavior, and traits associated with autism, attention-deficit and hyperactivity disorder, and obsessive and compulsive disorder have been found to differ significantly across sensory phenotypes. Accounting for the potential confounding effects of age and IQ, participants in different sensory subgroups had different levels of social-communicative symptoms, restricted and repetitive behaviors, adaptive functioning skills, and symptoms of inattention and anxiety [7].

Identified sensory subgroups have been further differentiated by the severity of core and co-occurring symptoms, and level of adaptive functioning, providing novel evidence on the associated clinical correlates of sensory subgroups [8].

Present diagnostic criteria for autism spectrum conditions (ASC) include social communication and interaction difficulties, repetitive behavior and movement, and atypical sensory responsivity. Research has explored whether sensory responsivity and motor coordination differences can account for the severity of autistic behaviors in children with ASC. Children with ASC scored significantly lower on receptive language, coordination, sensory responsivity and a sensorimotor subscale, Modulation of Activity (MoA) compared to a typically developing group. In the ASC group, MoA significantly predicted ASC severity across all ASC measures; receptive language and sensory responsivity significantly predicted parental reported autism measures; and coordination significantly predicted examiner observed reported scores [29].

6. Neural and Biological Mechanisms

6.1 Brain Structure and Function

Neurophysiological responses to auditory, tactile, and visual stimuli in autistic individuals have been examined using a variety of neuroimaging techniques, including electroencephalography (EEG), magnetoencephalography (MEG), and functional MRI. Research has explored the impact of covert and overt attention on sensory processing [1].

Studies using diffusion tensor imaging (DTI) fiber tractography have evaluated the structural connectivity of specific white matter tracts in boys with ASD and boys with Sensory Processing Disorders (SPD), relative to typically developing children. Both the SPD and ASD cohorts demonstrate decreased connectivity relative to controls in parieto-occipital tracts involved in sensory perception and multisensory integration. However, the ASD group alone shows impaired connectivity, relative to controls, in temporal tracts thought to subserve social-emotional processing [2].

Sensory dysfunction is a core symptom of autism spectrum disorder (ASD), and researchers have measured the covariation of volumes of brain structures (i.e., structural magnetic resonance imaging) that may be involved in abnormal sensory processing, in order to infer connectivity of these brain regions. Specifically, the structural covariation of sensory-related cerebral cortical structures, in addition to the cerebellum and amygdala, has been quantified by computing partial correlations between the structural volumes of these structures [30]. Results showed decreased structural covariation between sensory-related cortical structures, especially between the left and right cerebral hemispheres, in participants with ASD. In contrast, these same participants presented with increased structural covariation of structures in the right cerebral hemisphere. Additionally, sensory-related cerebral structures exhibited decreased structural covariation with functionally identified cerebellar networks. Also, the left amygdala showed significantly increased structural covariation with cerebral structures related to visual processing [30].

6.2 Neurochemical Mechanisms

The altered excitation-inhibition (E-I) model of autism posits that an increased ratio of excitatory to inhibitory signaling may explain certain phenotypical expressions of autism spectrum disorders (ASD). While there has been strong support for the altered E-I model of autism, much of the evidence has come from animal models. With regard to in-vivo human studies, evidence for altered E-I balance in ASD come from studies adopting magnetic resonance spectroscopy (MRS). Spectral-edited MRS can be used to provide measures of the levels of GABA+ (GABA + macromolecules) and Glx (glutamate + glutamine) in specific brain regions as proxy markers of inhibition and excitation respectively. Region-specific elevations of Glx in the primary sensorimotor cortex (SM1) have been found in ASD [18].

Individuals with autism spectrum disorder (ASD) often exhibit abnormal processing of sensory inputs from multiple modalities and higher-order cognitive/behavioral response to those inputs. Several lines of evidence suggest that altered γ-aminobutyric acid (GABA), the main inhibitory neurotransmitter in the brain, is a central characteristic of the neurophysiology of ASD. The relationship between GABA in particular brain regions and atypical sensory processing in ASD has been investigated [19].

6.3 Genetic Factors

Genetic factors have been implicated in sensory processing differences in ASD. Contactin genes CNTN5 and CNTN6, which code for neuronal cell adhesion molecules that promote neurite outgrowth in sensory-motor neuronal pathways, have been linked to ASD. Both deletions and private coding sequence variants of CNTN6 were enriched in individuals with ASD compared to controls [31]. Clinical investigations of patients carrying CNTN5 or CNTN6 variants showed that they were hypersensitive to sounds (a condition called hyperacusis) and displayed changes in wave latency within the auditory pathway. These results reinforce the hypothesis of abnormal neuronal connectivity in the pathophysiology of ASD and shed new light on the genes that increase risk for abnormal sensory perception in ASD [31].

Genome-wide association studies (GWAS) have been carried out for Restricted and Repetitive Behaviors (RRB) subcategories, including Repetitive Sensory Motor (RSM) and Insistence on Sameness (IS). Analysis indicates that RRB in ASD can be represented by these two distinct subcategories. Subsequent univariate and multivariate genome-wide association studies of these RRB subcategories enabled the detection of associated SNPs at 8p21.2-8p21.1. Genomic features of this region and pathway analysis suggest that common variants in 8p21.2-8p21.1 may contribute to RRB, particularly IS [24].

7. Implications for Intervention and Clinical Practice

With additional characterization, neurophysiologic profiles of sensory processing in ASD may serve as valuable biomarkers for diagnosis and monitoring of therapeutic interventions for autism and reveal potential strategies and target brain regions for therapeutic interventions [1].

Research using the Adolescent/Adult Sensory Profile (AASP) found that participants with ASC had mean scores outside normal parameters. Participants reported difficulties in at least one sensory domain, with hearing affecting them the most. Content analysis revealed sensory sensitivity to affect the participant's learning and that sensory experiences were largely negative. Results suggest that schools need to create sensory profiles for each individual with ASC [32].

Abnormal sensory-based behaviors are a defining feature of autism spectrum disorders (ASD). Dr. A. Jean Ayres was the first occupational therapist to conceptualize Sensory Integration (SI) theories and therapies to address these deficits, based on neurological knowledge of the 1970's. Since then, advancements in neuroimaging techniques make it possible to better understand the brain areas that may underlie sensory processing deficits in ASD. Many aspects of Ayres' hypotheses about the nature of the disorder were found to be highly consistent with current literature on sensory processing in children with ASD, but there are some discrepancies across various methodological techniques and ASD development. With additional characterization, neurophysiological profiles of sensory processing in ASD may serve as valuable biomarkers for diagnosis and monitoring of therapeutic interventions, such as SI therapy [33].

Research has identified cut-off scores for auditory over-responsivity (AOR) and tactile over-responsivity (TOR) using the SSP parent report and SP-3D:A direct observation. A combination of questionnaire and direct observation measures should be used in clinical and research settings, as the SSP parent report and SP-3D:A direct observation ratings overlapped only moderately for sensory-related behaviors [17].

8. Limitations and Future Directions

Despite significant advances in understanding sensory behaviors in ASD, several limitations exist in the current literature:

1. Methodological considerations: Sensory research to date has focused on individuals with high-functioning ASD, and many assessment measures are not standardized or psychometrically validated with an autism population [10]. Most results are based on parent-report measures of sensory processing, adaptive behavior, and other traits, which may limit the generalizability of the findings. Autistic individuals with an intellectual disability are often underrepresented in research samples [7].

2. Sample size limitations: The relatively small sample size and the inclusion of a broad range of autistic individuals (e.g., with low cognitive and/or verbal abilities) may limit the power to detect subtle group differences and associations in many studies. Replications are needed to verify results [34].

3. Lifespan considerations: Despite growing evidence that sensory symptoms persist into adolescence and adulthood, there is a lack of norms for older age groups, and pediatric assessments may not target appropriate functional outcomes or environments [10].

4. Disparities in identification: Potential disparities in the identification of sensory features have been noted, particularly for children from racial and ethnic minority groups. These disparities should be examined in future research and considered clinically to avoid reduced access to supports for sensory features and related functional problems [20].

Future research directions should include:

1. Further examination of sensory subgroups to provide a platform to interrogate the neurobiological and genetic correlates of altered sensory processing in ASD [8].

2. Integration of sensory symptoms and neuroscience perspectives, which are largely developing independently from each other, to move toward an interdisciplinary approach. This will inevitably aid in a better understanding of the underlying biological basis of these symptoms and help realize the translational value through its application to early identification and treatment [35].

3. Development of sensory phenotypes that may aid in the development of effective treatments and interventions for sensory difficulties, given the large degree of heterogeneity in sensory difficulties seen in the autistic population [7].

4. Improvement of understanding of sensory experiences on a personal and public level, as well as development of potential support. Autistic adults have expressed significant concern about their visual experiences, and there is a need to improve understanding of visual experiences on a personal and public level as well as for developing potential support [27].

Conclusion

This review has comprehensively examined how sensory behaviors associated with autism are characterized and measured in the scientific literature. The field has evolved from basic descriptions of sensory abnormalities to sophisticated conceptual frameworks, assessment methodologies, and neurobiological investigations. Clear evidence supports the prevalence and significance of sensory processing differences in ASD, their relationship with other autism features, and their impact on daily functioning and quality of life.

Future research should aim to address current limitations by developing more standardized assessment tools, including more diverse populations, adopting lifespan perspectives, and integrating multidisciplinary approaches to better understand and support individuals with ASD who experience sensory processing challenges.