Objectives: Here, using Visual Evoked Potentials (VEPs), we aim to further explore how well spatial information is transmitted over a wide range of spatial frequencies, including those at the limit of visibility (visual acuity).
Methods: We recorded VEPs in ASD children between 5 and 17 years old and aged matched controls. The test stimulus was a vertical sine-grating presented at 80% contrast that underwent pattern reversal at a rate of 7.5 Hz. The grating was swept in spatial frequency from 2 to 30 cpd over 10 sec. trials. Responses were measured in the frequency domain at the 2nd and 4th harmonics of the 7.5 Hz stimulus frequency (15 and 30 Hz).
Results: VEP response between 18 and 30 cpd and the corresponding grating acuity threshold did not differ between control and ASD participants, consistent with behavioral reports of a lack of superior spatial resolution in ASD. However, the second harmonic response at 2 to 18 cpd was significantly depressed in ASD relative to controls. On the other end, the second harmonic responses at 2 cpd and at all spatial frequencies for the fourth harmonic were normal. Thus, the ASD response is abnormal over a very limited and specific set of medium spatial frequencies for a particular temporal frequency of the response.
Conclusions: Although the present data cannot determine the anatomical substrate for the altered pattern reversal VEP response, they suggest that a highly specific substrate within early, possibly pre-cortical parts of the visual pathway must be involved. Substantial portions of the spatial frequency spectrum and the entire fourth harmonic response were completely normal, ruling out attention or other task demand differences between groups as an explanation.