If, for whatever reason, it's not possible to filter stimuli at early stages, one has to sort them out after they've been evaluated. We believe that in autism during conditions of rapidly changing attentional demands, early selection fails to function appropriately, leaving a later, less efficient process to take up the slack. In order to get a large number of stimuli into this later process, the autistic brain enters a state of hyper-arousal.

You can think of arousal as a sort of master volume control for the senses, whereas selective attention is more like a stereo equaliser, with separate controls for separate sensory channels. With the equaliser broken, the autistic brain can choose either to turn down the volume and let nothing in, or to turn up the volume and let everything in.

Unfortunately, when everything is let in, later processing is easily overloaded.

`Physiological Studies of Attention in Autism: Implications for Autistic Cognition and Behaviour', Matthew Belmonte, 26 January 2002