Original model
Ayress theoretical framework for what she called Sensory Integration Dysfunction was developed after six factor analytic studies of populations of children with learning disabilities, perceptual motor disabilities and normal developing children. Ayres created the following nosology based on the patterns that appeared on her factor analysis:

Dyspraxia: poor motor planning (more related to the vestibular system and proprioception)
Poor bilateral integration: inadequate use of both sides of the body simultaneously
Tactile defensiveness: negative reaction to tactile stimuli
Visual perceptual deficits: poor form and space perception and visual motor functions
Somatodyspraxia: poor motor planning (related to poor information coming from the tactile and proprioceptive systems)
Auditory-language problemsBoth visual perceptual and auditory language deficits were thought to possess a strong cognitive component and a weak relationship to underlying sensory processing deficits, so they are not considered central deficits in many models of sensory processing.In 1998, Mulligan found a similar pattern of deficits in a confirmatory factor analytic study. Quadrant model
Dunns nosology uses two criteria: response type (passive vs. active) and sensory threshold to the stimuli (low or high) creating four subtypes or quadrants:
High neurological thresholdsLow registration: high threshold with passive response. Individuals who do not pick up on sensations and therefore partake in passive behavior. Sensation seeking: high threshold and active response. Those who actively seek out a rich sensory filled environment.Low neurological thresholdSensitivity to stimuli: low threshold with passive response. Individuals who become distracted and uncomfortable when exposed to sensation but do not actively limit or avoid exposure to the sensation. Sensation avoiding: low threshold and active response.
The following link is a fact sheet regarding the role of Occupational Therapy in treating sensory processing disorder: https://www.aota.org/-/media/corporate/files/aboutot/professionals/whatisot/cy/fact-sheets/factsheet_sensoryintegration.pdf
   Although Ayres initially developed her assessment tools and intervention methods to support children with sensory integration and processing challenges, the theory is relevant beyond childhood.Sensory integration therapy is driven by four main principles:
Just right challenge (the child must be able to successfully meet the challenges that are presented through playful activities)
Adaptive response (the child adapts their behavior with new and useful strategies in response to the challenges presented)
Active engagement (the child will want to participate because the activities are fun)
Child directed (the childs preferences are used to initiate therapeutic experiences within the session)Serious questions have been raised as to the effectiveness of this therapy particularly in medical journals where the requirements for a treatment to be effective is much higher and developed than its occupational therapy counter parts which often advocate the effectiveness of the treatment.