More often than not, children with autism exhibit unconventional reactions to sensory stimulation. Some children show a hypersensitivity to stimuli (e.g. can hear lights buzzing, cannot tolerate touch, are fascinated with spinning objects, must smell everything, etc.) while others display a hyposensitivity to stimuli (e.g. demonstrate high pain tolerance, act as if deaf, etc). A child with autism may be fascinated with a piece of lint, or may spend hours rocking or watching objects twirl. In general, these types of reactions are providing some sort of sensory stimulation for the child.
It is believed that these sensory difficulties stem from neurological dysfunction in the brain. We are bombarded with thousands of sensations daily. Our ability to integrate these sensations by attending to the important ones and filtering out the non-essential input helps us to function efficiently. Without smooth functioning of this system, the individual is unable to accurately interpret his/her environment and respond and adapt.