A popular hypothesis about autism is that it arises because of reduced brain connectivity. This hypothesis can explain some of the symptoms of autism, such as the delayed onset of language and communication skills and an impoverished theory of mind. The thought behind this hypothesis is that reduced brain connectivity leads to less efficient processing and poorer storage of incoming information.
One major problem for this hypothesis has been to explain why 10 percent of autists have savant abilities—extraordinary cognitive abilities that require improved brain connectivity at least in local areas. Another problem is to explain the higher incidence of synesthesia, an extraordinary form of sensory blending, among people with autism. In his 1989 book on synesthesia, one of the first dedicated to the phenomenon, American neurologist Richard Cytowic predicted that 15 percent of people with autism experience synesthesia, which would be significant compared to the 4.4 percent of the general population. In a recent population study, psychologist Simon Baron-Cohen and his colleagues from Cambridge University found that the incidence of synesthesia is even higher among autistic individuals: 18.9 percent of autistic individuals have a form of synesthesia.*
Several recent studies have shown that synesthesia in non-autistic individuals involves hyperconnected brain regions. It is generally believed that the pruning of the brain that normally takes place during child development is incomplete in certain predisposed individuals, leading to unusual types of binding of sensory features, such as the binding of sound and color or numbers and geometry. Though it is possible that synesthesia is triggered by different mechanisms in autistic and non-autistic individuals, there is a strong possibility that a shared mechanism underlies many of the cases. So, the high incidence of synesthetes among autistic individuals is surprising if the autistic brain, in fact, exhibits fewer neural connections than the ordinary brain.
One possible theory about autism that would be compatible with both hyper- and the hypoconnectivity is that autism is the result of reduced connectivity in the left hemisphere and increased connectivity in the right hemisphere. Since language and communication skills are to a large extent left-hemisphere skills, that would explain the delayed language and communication skills in autism. The increased connections in the right hemisphere, on the other hand, could explain the increased incidents of synesthesia and savant skills.
* Their numbers for non-autistic individuals are also higher: 7.2 percent of non-autistic individuals were found to have a form of synesthesia.
