IntroductionTo successfullyinteract with our environment, we need the ability to perceive sensoryinformation from the world and translate those into appropriate actions.However, for successful translation, we first need to organize and interpretthe sensory stimuli to be able to form an appropriate response to theenvironmental demands, such as a movement.
For optimal functioning, the sensorysystem receives information by more than just one sense. Perception is guidedby the five sensory systems, vision, audition, olfaction, gustation, andsomatosensation +add proprioceptive and vestibularbalance and motor. The information we get from those multiple sensorysources can be complementary or redundant; successful sensory integrationenables precise perception. Humans are not born with multisensory integration.This ability matures during childhood (Burr & Gori, 2012).
While some earlystages, such as head orientation towards an auditory stimulus occurs in earlyinfancy (Neil, Chee?Ruiter, Scheier,Lewkowicz, & Shimojo, 2006), other sensory integration processes don’tmature until late childhood (Barutchu, Crewther, & Crewther, 2009; Gori,Del Viva, Sandini, & Burr, 2008). SensoryProcessing DisorderSensory processingdisorder (SPD) was first described by A. J. Ayres (1972) after systematicobservations while working with children with learning disabilities. Based onAyres, SPD is characterized by significant difficulties to organize or regulatesensory information by the nervous system. While there are naturally occurringvariations in sensitivity and reactivity to sensory stimuli, the inadequateprocessing of multisensory input as described in SPD manifests in substantialproblems in performance that prohibit optimal functioning and create dissonancewith the environment (Ayres & Robbins, 2005). In contrast to a healthy,well-organized neural system that can process multisensory information andtranslate them into appropriate actions, it is assumed that SPD is rooted in animmature sensory system that is inefficient in neuronal signaling andorganization.
The resulting abnormal perception or experience of theenvironment or processing in the brain may lead to difficulties / adverse effects/deficits in development, socio-emotional regulation, and academic performance(Ben-Sasson, Carter, & Briggs-Gowan, 2009).While there is agrowing body of research indicating that sensory dysfunction may qualify as anindependent disorder, so far none of the major diagnostic classificationsystems (e.g., DSM-5, ICD-10) is acknowledging SPD as such (AmericanPsychiatric Association, 2013). Though there is consensus that difficulties insensory processing occur in children, opponents of the disorder claim that theobserved symptoms are non-specific could be explained as comorbid phenomenon ofother neurodevelopmental disorder and lack evidence to qualify for anindependent diagnosis (Zimmer et al., 2012). So far, only the DiagnosticClassification of Mental Health and Developmental Disorders of Infancy andEarly Childhood (DC: 0-3) has included SPD in their manual (Zero to Three,2005). Amid this controversy, alarge-scale study including 706 typically developing kindergarten childrenfound that an estimated 5.
3 – 13.7% of children without other developmentaldelays meet criteria for SPD based on parent reports (Ahn, Miller, Milberger,& McIntosh, 2004). Additionally, sensory dysfunction is often a comorbidsymptom in children with developmental disorders, such as autism spectrumdisorder (ASD), as well as children affected by regulatory disorders, such asattention-deficit/hyperactivity disorder (ADHD). These groups show asignificantly higher prevalence of sensory dysfunction compared to typicallydeveloping children, reported as high as 40 – 80% (Ahn et al., 2004; Kientz& Dunn, 1997).
Comorbidity. The occurrence of “sensory issues” in autismspectrum disorders is common and has been extensively described in theliterature (see Ben-Sasson, Hen, Fluss, Cermak, Engel-Yeger, & Gal, 2009for meta-analysis). Tomchek and Dunn (2007) reported in a study including 562children between the age of 3-6 years that children with ASD show a significantdifferent sensory pattern compared to matched typically developing children.The groups differed most significant in subcategories of hypo-responsivenessand sensation-seeking. Similarly, another study found a unique pattern ofhypo-responsiveness to sensory stimuli in children with autism as young as5-months, compared to healthy controls as well as children with otherdevelopmental disorders (DD), (Baranek, David, Poe, Stone, & Watson,2006). On the other hand,hyper-responsiveness manifested quite similar in ASD and DD children and wasdistinguishable to the neurotypical controls. Additionally, the observedpattern of over-responsiveness was related to estimated developmental age inthose two groups (ASD and DD). Since 2013, sensory impairments are included inthe Repetitive and Restricted Behaviors Impairment category to diagnose ASD inthe DSM-5 (American Psychiatric Association, 2013).
But related sensorydisabilities are not just comorbid with developmental disorders; atypicalprocessing is as well reported in individuals with schizophrenia, especially inthe auditory and visual domain (Javitt, 2009). High sensitivity to sensoryprocessing was found to be correlated to avoidance behaviors in social anxietydisorder (Hofmann & Bitran, 2007). The Role of Sensitive Periods. Atypical sensoryprocessing is more common in children born preterm (Anday, Cohen, Kelley, , 1989; Kessenich, 2003; Wiener, Long, DeGangi, & Battaile, 1996) aswell as previously institutionalized children (Cermak & Daunhauer, 1997;Wilbarger, Gunnar, Schneider, & Pollak, 2010). The increased prevalencesuggests that maturation of the underdeveloped nervous system in preterminfants may play a role. Likewise, early adverse experiences, such as caregiverdeprivation or lack of optimal stimulation during early development ininstitutionalized children, seem to be involved in the inefficient organizationof sensory information.
The importance of sensitive periods for the maturationof sensory pathways has been shown by Hubel and Wiesel (1970). In theirlandmark studies with cats, they demonstrated that monocular and binocularclosure during sensitive periods leads to a significant decrease in neuronalconnections of the deprived eye with limited to no recovery. The effect wasonly observed over a specific developmental timeframe; further, after maturationof the visual system, the experiment caused no detectable effects. But not justearly deprivation of sensory experiences impacts developmental trajectories.Earlier stimulation of sensory systems that usually develop later can influencethe maturation of others. For example, surgical eye-opening in rat pups onpostnatal day (PN) 7, eight days before this occurs naturally, compromises thedevelopment of the earlier developing olfactory system (Turkewitz & Kenny,1985). Moreover, artificial early visual stimulation starting on PN7 altersbehavioral changes rat pups usually show about the same time as eye-openingoccurs naturally (Kenny & Turkewitz, 1986).
This indicates that not onlydeprivation during sensitive periods can have a significant impact on thematuration of sensory processing. The natural limitation of sensory inputduring development can be linked to the optimal organization and maturation ofother sensory systems. On the other hand, premature stimulation of one systemcan impair the neurodevelopment of another. Thus, not just deprivation andstimulation, but their timing and the possible hierarchical order of sensorydevelopment seem to impact species-typical organization and maturation ofsensory processing.
The role of earlyexperiences for sensory integration in human development has been shown, forexample, with the McGurk effect (McGurk & Power, 1980). The effect occurswhen conflicting sensory input is given. For example, a subject listens to aspoken phoneme, while another phoneme is visually pronounced by a speaker.
Themajority of participants reports hearing a third phoneme, a mix between theauditory and the visually processed one. This illusion, caused by multisensoryfusion can be observed in typically developing children as well as in deaf-bornchildren who received cochlear implants during early infancy. However, childrenwith congenital deafness whose hearing was restored after 30 months of ageshowed a significant visual dominance in the paradigm, indicating that thesensory integration as observed in the McGurk effect is not innate but developsbased on experience during early childhood (Schorr, Fox, van Wassenhove, , 2005).