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To successfully
interact with our environment, we need the ability to perceive sensory
information from the world and translate those into appropriate actions.
However, for successful translation, we first need to organize and interpret
the sensory stimuli to be able to form an appropriate response to the
environmental demands, such as a movement. For optimal functioning, the sensory
system receives information by more than just one sense. Perception is guided
by the five sensory systems, vision, audition, olfaction, gustation, and
somatosensation +add proprioceptive and vestibular
balance and motor. The information we get from those multiple sensory
sources can be complementary or redundant; successful sensory integration
enables precise perception. Humans are not born with multisensory integration.
This ability matures during childhood (Burr & Gori, 2012). While some early
stages, such as head orientation towards an auditory stimulus occurs in early
infancy (Neil, Chee?Ruiter, Scheier,
Lewkowicz, & Shimojo, 2006), other sensory integration processes don’t
mature until late childhood (Barutchu, Crewther, & Crewther, 2009; Gori,
Del Viva, Sandini, & Burr, 2008).

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Processing Disorder

Sensory processing
disorder (SPD) was first described by A. J. Ayres (1972) after systematic
observations while working with children with learning disabilities. Based on
Ayres, SPD is characterized by significant difficulties to organize or regulate
sensory information by the nervous system. While there are naturally occurring
variations in sensitivity and reactivity to sensory stimuli, the inadequate
processing of multisensory input as described in SPD manifests in substantial
problems in performance that prohibit optimal functioning and create dissonance
with the environment (Ayres & Robbins, 2005). In contrast to a healthy,
well-organized neural system that can process multisensory information and
translate them into appropriate actions, it is assumed that SPD is rooted in an
immature sensory system that is inefficient in neuronal signaling and
organization. The resulting abnormal perception or experience of the
environment 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 a
growing body of research indicating that sensory dysfunction may qualify as an
independent disorder, so far none of the major diagnostic classification
systems (e.g., DSM-5, ICD-10) is acknowledging SPD as such (American
Psychiatric Association, 2013). Though there is consensus that difficulties in
sensory processing occur in children, opponents of the disorder claim that the
observed symptoms are non-specific could be explained as comorbid phenomenon of
other neurodevelopmental disorder and lack evidence to qualify for an
independent diagnosis (Zimmer et al., 2012). So far, only the Diagnostic
Classification of Mental Health and Developmental Disorders of Infancy and
Early Childhood (DC: 0-3) has included SPD in their manual (Zero to Three,
2005).  Amid this controversy, a
large-scale study including 706 typically developing kindergarten children
found that an estimated 5.3 – 13.7% of children without other developmental
delays meet criteria for SPD based on parent reports (Ahn, Miller, Milberger,
& McIntosh, 2004). Additionally, sensory dysfunction is often a comorbid
symptom in children with developmental disorders, such as autism spectrum
disorder (ASD), as well as children affected by regulatory disorders, such as
attention-deficit/hyperactivity disorder (ADHD). These groups show a
significantly higher prevalence of sensory dysfunction compared to typically
developing children, reported as high as 40 – 80% (Ahn et al., 2004; Kientz
& Dunn, 1997).

Comorbidity. The occurrence of “sensory issues” in autism
spectrum disorders is common and has been extensively described in the
literature (see Ben-Sasson, Hen, Fluss, Cermak, Engel-Yeger, & Gal, 2009
for meta-analysis). Tomchek and Dunn (2007) reported in a study including 562
children between the age of 3-6 years that children with ASD show a significant
different sensory pattern compared to matched typically developing children.
The groups differed most significant in subcategories of hypo-responsiveness
and sensation-seeking. Similarly, another study found a unique pattern of
hypo-responsiveness to sensory stimuli in children with autism as young as
5-months, compared to healthy controls as well as children with other
developmental disorders (DD), (Baranek, David, Poe, Stone, & Watson,
2006).  On the other hand,
hyper-responsiveness manifested quite similar in ASD and DD children and was
distinguishable to the neurotypical controls. Additionally, the observed
pattern of over-responsiveness was related to estimated developmental age in
those two groups (ASD and DD). Since 2013, sensory impairments are included in
the Repetitive and Restricted Behaviors Impairment category to diagnose ASD in
the DSM-5 (American Psychiatric Association, 2013). But related sensory
disabilities are not just comorbid with developmental disorders; atypical
processing is as well reported in individuals with schizophrenia, especially in
the auditory and visual domain (Javitt, 2009). High sensitivity to sensory
processing was found to be correlated to avoidance behaviors in social anxiety
disorder (Hofmann & Bitran, 2007).

The Role of Sensitive Periods. Atypical sensory
processing is more common in children born preterm (Anday, Cohen, Kelley, &
Hoffman, 1989; Kessenich, 2003; Wiener, Long, DeGangi, & Battaile, 1996) as
well as previously institutionalized children (Cermak & Daunhauer, 1997;
Wilbarger, Gunnar, Schneider, & Pollak, 2010). The increased prevalence
suggests that maturation of the underdeveloped nervous system in preterm
infants may play a role. Likewise, early adverse experiences, such as caregiver
deprivation or lack of optimal stimulation during early development in
institutionalized children, seem to be involved in the inefficient organization
of sensory information. The importance of sensitive periods for the maturation
of sensory pathways has been shown by Hubel and Wiesel (1970). In their
landmark studies with cats, they demonstrated that monocular and binocular
closure during sensitive periods leads to a significant decrease in neuronal
connections of the deprived eye with limited to no recovery. The effect was
only observed over a specific developmental timeframe; further, after maturation
of the visual system, the experiment caused no detectable effects. But not just
early deprivation of sensory experiences impacts developmental trajectories.
Earlier stimulation of sensory systems that usually develop later can influence
the maturation of others. For example, surgical eye-opening in rat pups on
postnatal day (PN) 7, eight days before this occurs naturally, compromises the
development of the earlier developing olfactory system (Turkewitz & Kenny,
1985). Moreover, artificial early visual stimulation starting on PN7 alters
behavioral changes rat pups usually show about the same time as eye-opening
occurs naturally (Kenny & Turkewitz, 1986). This indicates that not only
deprivation during sensitive periods can have a significant impact on the
maturation of sensory processing. The natural limitation of sensory input
during development can be linked to the optimal organization and maturation of
other sensory systems. On the other hand, premature stimulation of one system
can impair the neurodevelopment of another. Thus, not just deprivation and
stimulation, but their timing and the possible hierarchical order of sensory
development seem to impact species-typical organization and maturation of
sensory processing.

The role of early
experiences for sensory integration in human development has been shown, for
example, with the McGurk effect (McGurk & Power, 1980). The effect occurs
when conflicting sensory input is given. For example, a subject listens to a
spoken phoneme, while another phoneme is visually pronounced by a speaker. The
majority of participants reports hearing a third phoneme, a mix between the
auditory and the visually processed one. This illusion, caused by multisensory
fusion can be observed in typically developing children as well as in deaf-born
children who received cochlear implants during early infancy. However, children
with congenital deafness whose hearing was restored after 30 months of age
showed a significant visual dominance in the paradigm, indicating that the
sensory integration as observed in the McGurk effect is not innate but develops
based on experience during early childhood (Schorr, Fox, van Wassenhove, &
Knudsen, 2005). 

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