A neuropsychological investigation of adults disabled by autism: Consequences of autism and mild mental retardation

This investigation examined three prominent theories concerning the etiology of autism. Study 1 used several neuropsychological tests to evaluate the left hemisphere dysfunction hypothesis in autism. Tests of cerebral lateralization included a handedness assessment, dichotic listening tests, a stereognosis test, and face perception tests. Study 2 used a problem-solving test, a design fluency test, and tests of word fluency to evaluate abilities associated with frontal lobe functions. Study 3 used a variety of memory tests to assess memory abilities in autism. Memory tests included verbal and visual paired associates, recall of a previously-drawn complex figure, recall of digit series and of visual-spatial sequences, and recognition memory for geometric and nonsense figures. Twenty subjects with autism and twenty control group subjects were matched on age and verbal intelligence. Subjects were divided into two groups based on verbal intelligence, namely, a high functioning group with borderline to average cognitive functioning and a low functioning group with mild retardation. Two groups of subjects were used to examine the impact of autism alone and of autism and mental retardation together on neuropsychological functioning. Mann-Whitney U tests and Fisher exact probabilities were used to analyze data for differences between subjects with autism and matched control group subjects. Study 1 uncovered evidence for reduced cerebral lateralization for language only in subjects with autism who also had mild mental retardation. This result suggests that these subjects sustained bilateral cerebral dysfunction. There was no evidence of a left hemisphere dysfunction in high functioning subjects with autism. Study 2 revealed significant impairments on tests associated with frontal lobe functions for subjects with autism. Finally, Study 3 revealed no memory impairments associated with autism, relative to control group subjects. This result argues against an autism-amnesia parallel or temporal lobe dysfunction. Results are discussed in light of current theorization on the etiology of autism which emphasizes multiple overlapping dysfunctions and cortical-subcortical interconnections. Directions for future research are suggested.