The Neurological Basis of Sensory Integration
Sensory Integration Therapy began with Ayres theory of sensory integration where she outlined the neurological foundation based on five principles of brain function. Sensory integration theory “describes how the nervous system translates sensory information into action and posits that adequate sensory integration is an important foundation for adaptive behavior” (Lane et al., 2019). Emphasis is on the active and dynamic sensory—motor processes that contribute to physical movement and facilitate development and interaction with and in the environment (both physical and social) (Lane et al., 2019). Hence, integration of the many senses is foundational to function (Lane et al., 2019). Theory led to neurological research which has demonstrated support for Ayres theory of sensory integration and developed our understanding of how sensory and motor systems function together to manifest behaviour (Lane et al., 2019). The Vestibular System
The vestibular system detects head movements, linear movements and gravity providing the brain with information about the head’s static position relative to gravity as well as the direction and velocity of head movements (Lane et al., 2019). This sensory information travels to several brain regions functioning to regulate of arousal, control both static and dynamic posture, as well as to maintain balance and equilibrium, bilateral coordination and a stable visual field (Lane et al., 2019). Together, these functions converge onto behaviour through the vestibular systems contribution to spatial perception that allows the body to navigate efficiently though space (Lane et al., 2019).
Vestibular information travelling to the reticular formation in the brainstem activating the arousal system in an autonomic response (Lane et al., 2019). Movement that is rapid or has unpredictable acceleration (like spinning or a rollercoaster) activates this parasympathetic pathway to increase alertness (Lane et al., 2019). Opposite movement that is slow and rhythmic (like rocking or swinging), activates the sympathetic pathway to decrease arousal (Lane et al., 2019). In addition to its role in the brainstem, vestibular information also travels from the brainstem to the cerebellum ensuring postural and head control is refined and efficient (Lane et al., 2019). In turn, vestibular information travels from the brain through vestibulospinal tracts activating muscles to maintain head control and posture during both whether the body is moving or not (Lane et al., 2019). Effective postural control relies on the sum of these connections of which are the foundation for developing more complex motor skills (Lane et al., 2019).
Vestibular information also helps coordinate eye and head movements by sending information along the medial longitudinal fasciculus to the cranial nerves that control extraocular muscles enabling rapid and precise adjustment of eye position (Lane et al., 2019). This allows for a stable perception of the visual field as a person moves and when they shift their gaze during movement (Lane et al., 2019). As such, the vestibular system plays a role in anticipatory motor plans and actions through these connections with vision and proprioception (Lane et al., 2019). The vestibular system also contributes to bilateral motor coordination since it is a bilateral system affecting muscle activation throughout the body (Lane et al., 2019). When there is a deficit in vestibular responses there is decreased bilateral integration, decreased coordination of head and eye movements and poor equilibrium reactions (Lane et al., 2019).
Ayres hypothesized inefficient vestibular processing would negatively affect the capacity of arousal regulation required to self-regulate emotion and behaviour as well as the higher-level cognitive functions required for learning (Lane et al., 2019). For many years, it was assumed that vestibular processing could not contribute to high-level thinking based on the belief that vestibular connections did not reach the cerebral cortex (Lane et al., 2019). However, neuroscience has identified several cortical areas that receive vestibular information (anterior parietal lobe, temporo-parietal junction, posterior parietal and medial superior temporal cortices, hippocampal and parahippocampal cortices, and cingulate gyrus and retrosplenial cortex) (Lane et al., 2019). Research has also identified the vestibular system is important for spatial memory, object recognition and numerical cognition.
The Somatosensory System
Touch and proprioception are the two systems that make up somatosensation (Lane et al., 2019). Tactile information is sent to the primary sensory cortex (S1) and the secondary somatosensory cortex (S2), here this information manifests tactile discrimination, object manipulation, and grasp thus playing a role in motor planning by linking past and present sensations (Lane et al., 2019). Somatosensation is integrated with other sensory systems as Ayres proposed and occurs on multiple levels. The posterior parietal cortex integrates visual and motor information (Lane et al., 2019). Somatosensory—vestibular—visual information is integrated in the vestibular nuclei, the thalamus and the cortex. Multisensory integration is the foundation of spatial orientation, postural stability and detection of self-motion (Lane et al., 2019). Tactile information sent to the insula is involved in interoception and homeostatic regulation, these projections are sent to the orbitofrontal cortex contributing to affect (Lane et al., 2019).
Touch has widespread influence on central nervous system processes and performs many functions from simple reflexes like withdrawal from pain or to more complex like the associated stress reducing properties of massage (Lane et al., 2019). Integration is evident even at early ages, newborns can discriminate between touch from another person and that of an object (Lane et al., 2019). The somatosensory system is also implicated in praxis impacting feedforward processing and the prediction of movement somatosensory (Lane et al., 2019).
Praxis: Sensation Informing Action
Praxis refers to the ability to plan and execute skilled movements or actions through ideation, motor planning and execution encompassing both cognitive processes and motor skills (Lane et al., 2019). Praxis allows us to operate effectively in the physical environment to carry out activities in a purposeful and organized manner through coordination of our thoughts and physical actions (Lane et al., 2019). As such, there are connections between sensation, motor areas and praxis (Lane et al., 2019). Ideation refers to one’s conceptualization of actions prior to execution (Lane et al., 2019). Conceptualization of using an object for a planned motor action occurs in the anterior intraparietal sulcus (AIP), here visual perception provides object-specific perceptual information which is then sent to F5 in the ventral premotor cortex (Lane et al., 2019).
Sensory Modulation
Sensory modulation is the way in which the nervous system regulates sensory information from the environment by filtering and adjusting the intensity of such information allowing for appropriate responses to stimuli by maintaining arousal, attention, activity level and emotion regulation (Lane et al., 2019). Essentially, the brain modifies (increases or decreases) incoming information then, using that information the response is also modified (Lane et al., 2019). Think of dodging a ball in gym class, you must know where your body is in space, perceive the ball is coming towards you, it’s going to hit you. All forms of sensory information are taken in and processed then a plan of action is formed, and the brain sends the body signals to coordinate muscle movement to execute that plan. The way the brain interprets sensory information directly relates to behaviour because the actions people do rely on that information to create a picture of the environment to operate within it.
The consequences of dysfunction in sensory modulation is the improper increasing or decreasing of sensory information causing individuals responses to also be exaggerated (Lane et al., 2019). Hyperreactive responses to stimuli caused by heightened sensitivity, conversely, hypo-reactive responses are the result of a failure to register stimuli (Lane et al., 2019). For example, heightened sensitivity to auditory, olfactory, visual, and gustatory sensations are typical for those with ASD causing distress (Lane et al., 2019). On the other side of the spectrum, sensory registration difficulties prevent the brain from noticing what is happening in the environment resulting in failure to orient to the environment appropriately (Lane et al., 2019). Think of not hearing someone call your name when you are in the same room or not being able to feel light touch. Little research has been done regarding hypo-reactivity, however it has been demonstrated the auditory system is affected in individuals with ASD where 50-75% have slowed reactions to auditory stimuli (Lane et al., 2019). Hypo-reactivity is also associated with poor social functioning and academic performance as well as changes in the subcortical auditory system (Lane et al., 2019).
Hyperactive individuals have elevated sympathetic activity and slower habituation to stimuli and display stronger fight or flight responses to typically non-aversive sensory experiences (Lane et al., 2019). These individuals also have inadequate parasympathetic activation, thus these autonomic systems are unbalanced (Lane et al., 2019). Due to inefficient sensory gating, hyperreactive individuals have deficits in their ability to filter out unnecessary stimuli which allows information to be overprocessed (Lane et al., 2019). Brain imaging supports the idea that these individuals have impairments in their multisensory integrations since they have reduced white matter in the parietal and occipital tracts where auditory, tactile and visual information is integrated (Lane et al., 2019). These tracts are reduced in both individuals with ASD and to a larger extent hyperreactive individuals (Lane et al., 2019). In addition, children with ASD have differences in the regions responsible for social emotional processing, the amygdala and hippocampus (Lane et al., 2019). Similarly, those with ADHD are affected by decreased white matter in the prefrontal tracts mediating motor, cognitive and behavioural functions (Lane et al., 2019).
Atypical connectivity is also found in the salience network of children with ASD and hyperreactivity (Lane et al., 2019). The salience network comprises many regions (insular regions, anterior cingulate cortex, dorsolateral prefrontal cortex, temporal poles, and amygdala) providing us with the ability to determine what sensory input is salient or important and needs attention (Lane et al., 2019). These children not only have greater connectivity within this region but also between this region and the primary sensory cortex (Lane et al., 2019). However, visual association areas have reduced connectivity (Lane et al., 2019). Overall, the varying atypical connectivity supports the idea that hyperreactivity is the result of sensory modulation dysfunction where attention to social cues is limited and attention to basic sensory input is exaggerated (Lane et al., 2019). Overall, these findings suggest that behaviour is manifested via the neural mechanisms of sensory modulation and/or sensory integration, whereby dysfunction in an area can cause adverse reactions or behaviours (Lane et al., 2019).
Supplemental Videos on Sensory Modulation:
https://www.youtube.com/watch?v=NPKUEBhW284&ab_channel=ReachingFamilies
https://www.youtube.com/watch?v=eH1oKRVGDrg&ab_channel=GarforthEducation
Sensory Integration Therapy’s Influence on Neuroplasticity
When taking into consideration sensory integration as a whole, it includes information processing, integration and modulation of various sensory systems we can understand how that information is translated into action or behaviour. Dysfunction in these processes leads to maladaptive responses to environmental stimuli and disordered behaviour (Lane et al., 2019). Sensory Integration Therapy (SIT) is based on the principle of neuroplasticity where experience influences the nervous system to change which in turn influences brain function and behaviour (Lane et al., 2019). Through the continuing process of creating and organizing neuronal connections, experience-dependent learning can improve synaptic efficacy when persistently stimulated. Simply put, “neurons that fire together, wire together” (Lane et al., 2019). Many studies demonstrate how learning and environmental enrichment can cause the development of new connections in the brain throughout the lifespan (Lane et al., 2019). For example, lasting functional changes within the brain are initiated via learning and novel experiences by increasing gray matter density, glial volume, angiogenesis and neurogenesis (Lane et al., 2019). Essentially, neuroplasticity allows behaviour and function in everyday activities to be improved through participation in the individualized sensorimotor activities utilized in SIT which change and enhance connections among the nervous system’s components being targeted (Lane et al., 2019). Studies utilizing SIT techniques found children with ASD who participated in the intervention compared to a control group had greater improvements in functional skills, socialization, independence in self-care and participation in daily activities (Lane et al., 2019). Overall, SIT is an evidence-based intervention where improvements in functional skills are related to neuroplasticity (Lane et al., 2019). Future studies will uncover if in fact these neuroplastic functional improvements are accompanied by corresponding alterations in the brain's capacity to process multisensory information (Lane et al., 2019).
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