One theory on causes of autism is atypical brain development. The thought is that with autism there's accelerated growth at a different time than a majority of the population. The consequences, in terms of early development, include differences within the cortex. The cortex is the part of the brain which is responsible for higher brain functions, including sensation, voluntary muscle movement, thought, reasoning, and memory. The result may be that autistic people perceive things differently and may have less ability to block sensory input.