The mismatch negativity and the P3a components of the auditory event-related potentials in autistic low-functioning subjects.

OBJECTIVE

In order to understand better the psychophysiological basis of auditory processing abnormalities in autism, we decided to study two automatic components of the auditory event-related potentials (ERPs): the mismatch negativity (MMN)--a component of the ERP which is recorded when, during repetitive auditory stimulation, rare changes are introduced--and the novelty-related P3a which is recorded as a response to unexpected novel events occurring in a sequence of repetitive stimuli.

METHODS

Ten male subjects, mean age 12.3 years (SD 4.95), affected by autism and mental retardation were admitted to this study. All patients were also mentally retarded. Ten normal male subjects, mean age 12.2 years (SD 3.94), were used as controls. Auditory evoked potentials were recorded from 19 scalp electrodes (10-20 system), and stimuli were presented in sequences consisting of 2000 tones (70 dB, ISI=800 ms). Three types of stimuli were presented: (1) standard stimuli (1000 Hz tones, 80% of total stimuli), (2) deviant stimuli (1300 Hz tones, 10% of total stimuli), and (3) novel stimuli (complex and non-monotonal, 10% of total stimuli). To quantify the MMN, the evoked response to the standard tones was subtracted from the corresponding deviant stimulus response and its amplitude and latency at peak were measured over Fz, Cz and Pz; similarly, the P3a component of the ERP was obtained by subtracting the response to the standard tone from that to the novel stimuli and its amplitude and latency at peak were measured over Fz, Cz and Pz. Also, the amplitude and latency at peak for the N1 component of the auditory evoked potential obtained with the standard stimuli were measured over Fz, Cz and Pz. The correlation between age and MMN and P3a amplitude was also analyzed.

RESULTS

N1 showed significantly shorter latencies in the autistic groups. MMN elicited by deviant stimuli, but not that elicited by novel stimuli, was found to be significantly larger in autistic children than in normal controls. P3a showed higher amplitude in autistic subjects than in normal controls during childhood; the opposite was observed during young adulthood.

DISCUSSION

Our findings indicate that significant changes in ERPs can also be seen in non-cooperative individuals with autism and mental retardation, which might be different from the changes already reported for high-functioning autistic subjects and deserve further insight. These changes show developmental modifications that should be taken into consideration when analyzing data from autistic subjects.