Inner speech has been linked to higher-order cognitive processes including 'theory of mind', self-awareness and executive functioning all of which are impaired in people with autism who tend to report using visual rather than verbal thinking. A study of 45 children looked at whether they used visual or verbal methods in their short-term memory and compared autistic children with children with learning disabilities. The children were given a recall task involving some objects that looked alike and others which sounded alike. The idea was that those children who used verbal methods would get most confused with objects that sounded alike while those children who used visual methods would get most confused over objects that looked alike. The researchers found that mental age was more important than whether the children were autistic or not. Those children with a mental age over 7 used inner speech while those with a mental age of less than 7 used visual stimuli.
Williams, David, Happe, Francesca and Jarrold, Christopher - Intact inner speech use in autism spectrum disorder: evidence from a short-term memory task Journal of Child Psychology and Psychiatry January 2008, 49(1), 51-58