• title
• what is attention?
• early selection
• late selection
• outline
• rationale
• paradigm
• stimuli
• electrode placements
• electrodes
• behavioural results
• signal processing
• normal EEG results
• autistic EEG results (short SOA)
• autistic EEG results (long SOA)
• EEG findings
• questions
• fMRI methods
• regions of interest
• strategy
• fMRI analysis #1
• fMRI analysis #2
• fMRI results: normal subjects, extrastriate
• fMRI results: normal subjects, intraparietal
• fMRI results: autism, extrastriate
• fMRI results: autism, intraparietal
• fMRI findings
• roles of intraparietal sulcus
• new technologies
• So what?
• autistic behaviour as epiphenomenon
• Cartesian Theatre: projectionist wanted
• further work
• acknowledgements

In behavioural experiments, people with autism show a selective impairment in shifting attention rapidly between different sensory channels. The ability quickly and effortlessly to shift attention between external events and people with whom those events are being shared is key to the development of joint social attention in infants. A deficit in this capacity can curtail the ability to integrate complex sensory experiences into coherent percepts. Although behavioural studies help to characterise attentional abnormalities in autism, understanding what causes these abnormalities requires study of the underlying physiology. Using quantitative electroencephalography and functional magnetic resonance imaging (fMRI), we've shown that slowed shifting of attention in autism is a result of fundamental differences in the autistic brain's computational approach to attention. In the normal brain, attention to a location in one or the other visual hemifield activates visual cortex in the contralateral hemisphere. As attention shifts across the midline from one hemifield to the other, so does this pattern of regional brain activation. In autism, in contrast, such shifts of attention are associated with undifferentiated activity spread across both hemispheres. These physiological results suggest that the autistic brain cannot filter incoming stimuli at early stages of perceptual processing, and instead must separate relevant from irrelevant stimuli by some later occurring, less efficient means. Unable to increase response selectively to individual sensory channels, the autistic brain when called on to process closely spaced stimuli from different channels responds by heightening the level of generalised arousal -- rather like being forced to resort to the master volume control on a stereo system, when what one really needs is a finely tuned equaliser. Our most recent fMRI findings in normal subjects have confirmed results on brain regions active during early selection of relevant visual stimuli, and have newly identified a region associated with late suppression of irrelevant stimuli. In autism, the former region shows no attentional effect, while in the latter the effect is greater than normal, suggesting that the computational load associated with attention is being shifted from early to late processing. These initial studies have been carried out with high-functioning adults. In future work involving children and siblings, and examining anterior as well as posterior cortical locations, we hope to discover how this abnormal neurophysiology of attention develops. This work was supported in part by a grant from the National Alliance for Autism Research.

Copyright © 2002 by Matthew Belmonte