We recorded VEPs to the alternation of checkerboard, square-wave grating and sinusoidal grating patterns to evaluate the contribution of the fundamental spatial frequency and higher harmonic components in 12 normal subjects. Their fundamental spatial frequencies were equated and ranged from 0.5 to 8.0 c/deg. Both the transient VEP (T-VEP) and steady-state VEP (S-VEP) were obtained. The latency and amplitude of P100 of T-VEPs were measured. S-VEPs were Fourier analyzed, and phase and amplitude of the second harmonic response were measured. The mean P100 latency and the mean phase had a U-shaped function with a peak at a fundamental spatial frequency of 2.0 c/deg irrespective of the stimulus patterns, while the mean P100 amplitude and the mean amplitude of S-VEPs did not show such spatial selectivity. At low and medium spatial frequencies, differences in P100 latency, phase and amplitude between pattern types were more pronounced. However, this difference became insignificant at high spatial frequencies. These results indicate that VEP responses are predominantly determined by the fundamental spatial frequency, and that the contribution of higher harmonics to VEP responses is not negligible. Our results are consistent with the concept that the human visual system may use spatial frequency-domain information.
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