Background:The N400 is an electrophysiological response to a stimulus that is incongruent with expectations derived from its context. The N400 effect, an index of differences between the way the brain processes congruity and incongruity, is commonly seen in both the auditory and visual modalities among typically developing persons. However, individuals with autism do not show an N400 in either modality. This finding may be consistent with the notion that individuals with autism may rely on more perceptually based processes in solving cognitive tasks.

Objectives:To test the notion of enhanced perceptual functioning within a multisensory context using a semantic matching task that was designed for use with Event-Related Potentials (ERP's).

Methods:Fourteen participants with ASD and 14 typically developing participants matched on IQ, gender and handedness, decided whether an auditory (dog bark) and a visual stimulus (a picture of a dog or a cat) matched or did not match. The stimuli could either be presented simultaneously or sequentially with an SOA of 650msecs. The measures of interest were RTs as well as the onset of a divergence between the processing of congruent and incongruent stimuli as measured by ERP.

Results:The TD persons displayed the expected N400 effect for all conditions, whereas for the persons with ASD the difference between the processing of congruence and incongruence occured much earlier in the simultaneous condition between 120-300msecs. In the delay condition, a typical N400 effect was noted for persons with ASD.

Conclusions: The results suggest that individuals with ASD do show a congruence effect, but that the timing of the distinction between congruence and incongruence occurs earlier in ASD, in time frames consistent with exogenous rather than endogenous processing. These findings are discussed in terms of enhanced perceptual functioning among persons with autism spectrum disorders.