Cortical Processing of Mental Addition in Visual and Auditory Modalities in Adults with High-Functioning Autism

Background: Individuals with high-functioning autism (HFA) or Asperger's syndrome have been shown to have average mathematical skills in general, and some may exhibit exceptional mathematical giftedness with a unique pattern of brain activation. As individuals with HFA show limitations in verbal social communication, it is unclear whether presenting mental addition problems in visual vs. auditory modalities would make a difference in the speed and accuracy outcomes. At the cortical level, very little is known about the temporal aspects of mathematical processing in HFA in comparison with normal age-matched controls.

Objectives: The present event-related potential (ERP) study aimed to explore possible differences in cortical mechanisms of mental calculation in visual and auditory modalities by testing HFA adults and control subjects with a mental addition task.

Methods: Ten adults with HFA and ten age-matched controls (age range: 19-38) participated in the study. All subjects were native speakers of English with normal hearing and right-handed. The mental addition task used two-digit numbers. There were two presentation conditions. Each trial in the visual condition started with a tone beep prompt. Then the two Arabic numerals with the “+” sign in between were presented together on screen, followed by an answer in Arabic numerals on the computer screen. Subjects were required to do mental addition and press button to indicate whether the given answer on screen was "correct" or "incorrect." The auditory condition replaced the question part (not the answer part) with auditory verbal stimuli. There were 120 trials for each condition. The auditory stimuli were presented at 60 dB sensation level. The EEG data were recorded with a 64-channel EEG system in an acoustically-treated booth. The sampling frequency was 512 Hz, and the passband was 0.016 - 200 Hz. Trials with peak amplitudes exceeding the range of +/- 50μV were rejected. The data were bandpass filtered offline at 0.5~40 Hz and the epoch window was -100 ~ 1000 ms for ERP averaging.

Results: Behavioral data showed no significant difference in response accuracy between visual and auditory modalities for mathematical calculation in either subject group. While the control group also showed no significant difference in reaction time between the two modalities, the HFA individuals took longer time in the auditory modality than the visual modality. Both behavioral and ERP data consistently showed strong effects of congruency in the two subject groups. It took longer reaction time for the subjects to respond to the incongruent answers than the congruent answers. The incongruency effect appeared similar between the two subject groups with a negative ERP component between 300 and 400 ms followed by a positive component between 400 and 600 ms in the visual modality. In the auditory modality, however, HFA adults showed delayed neural responses in both early and late components.

Conclusions: Both behavioral and ERP results indicate that stimulus presentation modality affects the speed and cortical responses for mathematical processing. Future work will examine this phenomenon from a developmental perspective to further investigate the relationship between language development and mathematical processing.