Sensory integration occurs in the central nervous system (it is generally thought to occur in the mid-brain and brainstem specifically) and refers to the ability to incorporate information taken in through the various senses (smell, vision, taste, touch, hearing and movement) and make meaningful sense of that information.
So sensory integration dysfunction, therefore, refers to difficulties in processing this information from the senses and responding appropriately to it. Sensory integration dysfunction often expresses itself in either an over- or under-reaction to certain stimuli, and may cause a child to experience both developmental and behavioral problems. Sensory integration dysfunction may be associated with premature birth, brain injury, learning disorders, or autism.
In order to complete tasks, children must be able to screen out or ignore certain irrelevant sensory stimuli such as background noise or visual information. Children with sensory integration dysfunction often lack the ability to do this and may appear constantly distracted and/or unfocused. Conversely, some children may respond in the opposite manner, they may fail to respond to direct and essential stimuli, such as their name being called.
Sensory integration disorders can be difficult to detect, especially in young children. We expect young children to be curious about all sorts of stimuli, to have short attention spans, and to be easily distracted. But there are some signs that may help to indicate when a dysfunction is present. These include: extreme disorganization or purposelessness of activities, a lack of variety in playtime activities, a failure to explore new environments or move around, excessive clumsiness, difficulty returning to a calm state after an upset, and seeking excessive sensory stimuli.
Sensory Integration Dysfunction in Young Children by Linda C. Stevens