Psychophysiological perspectives on autism

Abstract

Autism is a severe developmental neuropsychiatric disorder, with an onset in the first three years of life. It essentially affects aspects of behaviour which are generally regarded as 'human'. Core characteristics of autism are abnormalities in language, communication and soical interaction, narrowed interests and stereotyped behaviour. Although theories of different nature exists on what the underlying deficit for these abnormalities might be, a common aspect of all theories may be a defect in attention. Event-related potentials (ERPs) are, due to their very high temporal resolution, very well suited for the study of attentional processing in the brain. ERP studies of the P3, a large positivity occurring around 300 milliseconds after a stimulus is presented, have pointed out that this component is much smaller in autistic individuals than in controls, especially over posterior parts of the brain. However, none of these studies have made a detailed study of attentional processing before 300 milliseconds. In this thesis, visual and auditory selective attention tasks are described in groups of high functioning autistic children and adolescents. It was found that autistic children show profoundly smaller P3 amplitudes than controls, but that these abnormalities were not preceded by abnormalities in selective attention. However, autistic children already showed abnormalities in visual processing around 100 milliseconds after stimulus presentation, as seen in much smaller P1 amplitudes. In autistic adolescents such amplitude reductions were not observed, but this group did show abnormal selective attention. These abnormalities in selective attention were interpreted as compensatory mechanisms, normalizing P3. Whether abnormal P3 was a reflection of deficiencies in attentional capacity was studied with a probe task. In this task, autistic individuals did not show the same trade-off between task- and probe-stimuli as was seen in controls. When probe stimuli became increasingly irrelevant, controls invested less capacity in processing these stimuli as evident in smaller P3 amplitudes to those stimuli. Autistic patients did not show such a decrease in P3 amplitude. Therefore, autistic patients seem to have a deficiency in the allocation of processing capacity. In order to gain more insight in the underlying neural sources of the scalp recorded ERPs, the data from the visual selective attention task were subjected to high-resolution source localization techniques based on individual head models derived from Magnetic Resonance Images (MRI). It was found that autistic children showed a more superior location of the sources for P1, which is discussed in light of functional and structural MRI findings in autism. The use of the technique described in this thesis is new in the field of autism and provides valuable new insights in the disorder