The mismatch negativity and the P3a components of the auditory event-related potentials in autistic low-functioning subjects
Introduction
Autism was first described by Kanner (1943) and is now considered to be an etiologically heterogeneous and biologically determined developmental disorder characterized by severe disturbances in reciprocal social relations, impaired development of language and communication skills and by limited repertoire of behavioral patterns with a restricted ability of abstraction (American Psychiatric Association, 1994). In recent years, the number of subjects being diagnosed with autism and associated syndromes has shown a significant increase (Powell et al., 2000, Chakrabarti and Fombonne, 2001, Kaye et al., 2001); for this reason, research on autism is assuming increasing importance.
The causes of autism have not yet been fully identified; however, it is well known that it can be caused by a variety of pathological events affecting brain development and which can occur before, during or after birth. Genetic factors might be of primary importance (Bailey et al., 1995) but the exact location of the eventually relevant genes has not yet been identified. In the absence of specific biological markers, autism is defined purely in terms of the behavioral patterns displayed, and its diagnosis is based on internationally accepted classification systems, such as the DSM-IV (American Psychiatric Association, 1994) and the ICD-10 (World Health Organization, 1992).
Among the numerous deficits in autistic children, abnormalities in central sensory input processing have been described by different authors (Verbaten et al., 1991, Kemner et al., 1994); for this reason, a number of studies involving the measurement of event-related potentials (ERPs) in autistic children were carried out and results mostly regarding the analysis of the P300 were reported (Courchesne et al., 1989, Ciesielski et al., 1990, Lincoln et al., 1993, Lincoln et al., 1995). Almost all these studies showed a decrease in the auditory P300 amplitude, and the results were interpreted as the difficulty of autistic children in modifying their expectancies to contextually relevant sequences of information.
In order to understand better the psychophysiological basis of auditory processing abnormalities in autism, we decided to study two additional automatic components of the auditory ERP: the mismatch negativity (MMN)—a component of the ERP which is recorded when, during repetitive auditory stimulation, rare changes are introduced (Näätänen, 2000)—and the novelty-related P3 (or P3a) which is recorded as a response to unexpected novel events occurring in a sequence of repetitive stimuli (Escera et al., 2000, Friedman et al., 2001).
Section snippets
Subjects and methods
Ten male subjects, mean age 12.3 years (SD 4.95, range 6–19), affected by autism and mental retardation were admitted to this study. The diagnosis of autism was made according to the DSM-IV (American Psychiatric Association, 1994) criteria for autistic disorder and a score on the Childhood Autism Rating Scale (Schopler et al., 1980) between 30 and 50. All patients were also mentally retarded and were drug-free from at least 2 weeks before the study began. All subjects were evaluated from a
N1
Table 1 shows the comparison between N1 latency and amplitude recorded from 3 scalp locations (Fz, Cz and Pz), in normal controls and autistic patients, and evoked by the standard stimuli. In this table, it is possible to see that no statistically significant difference was found in the amplitude of auditory evoked potentials recorded after the presentation of standard stimuli between the two groups. On the contrary, the latency of the N1 component was significantly shorter in the autistic
Discussion
As already said in Section 1, abnormalities in central sensory input processing have been described by different authors in autistic children (Verbaten et al., 1991, Kemner et al., 1994); for this reason, a number of studies involving the measurement of ERPs in autistic children were carried out and results mostly regarding the analysis of the P300 (or P3b) were reported (Courchesne et al., 1989, Ciesielski et al., 1990, Lincoln et al., 1993, Lincoln et al., 1995). Almost all of these studies
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