We typically think of five senses: vision, hearing, smell, taste and touch. 
These senses give us information about the world around us.  

There are also 3 more internal senses which give us feedback about our bodies: 
 
 Proprioception (Position) - sensory receptors are located in the joints and tell us about the position of different parts of our body.  
 Vestibular (Balance) - sensory receptors are located in the inner ear and tell us about how our bodies are moving in space and in relation to gravity.  
 Interoception helps us know when we are thirsty, hungry, hot, cold, need to use the bathroom and more.  

For most of us, sensory integration occurs without conscious thought or effort. Since our brain uses information about sights, sounds, touch, movement, sound and smell in an organized way, we assign meaning to our sensory experiences and behave accordingly. For some children, sensory integration does not develop smoothly; their brain is unable to accurately process information coming from their senses.   

For children who have difficulties with sensory integration and who can’t rely on their senses for an accurate picture of their environment, it becomes very difficult for them to respond in an “appropriate way”.  

Messages can be over amplified, thereby making the person hypersensitive to that input. A "normal" amount of input becomes overwhelming and upsetting. For example, a clock ticking in the classroom can sound as loud as a hammer banging.   

Messages to the brain can also be diminished, requiring exaggerated stimulation to elicit a response, as normal levels of stimulation elicit limited or no response. This child may need rough and tumble play to get a sense of their own body.   

Often, children can be hypersensitive to some sensory input, while hyposensitive to other sensory input.  The intensity can range from mild to severe. Since these children (or adults) cannot rely on their sensory systems to give them an accurate picture of the world, they don't know how to behave in response. Children may appear clumsy, hyperactive, oppositional, easily distracted, obsessive-compulsive, or anxious, when in fact they are just reacting to and compensating for their unreliable and unpredictable view of their environment.