Effects of meridional variation on steady-state visual evoked potentials

Two experiments were carried out to assess the influence of meridional variations on the visually evoked potentials (VEPs) elicited by pattern alternation. The first experiment involved a comparison of the oblique effect obtained with grating and checkerboard stimuli. Greater amplitudes resulted using stimuli which contained vertically, as opposed to obliquely, oriented fundamental Fourier components. The second experiment revealed that the peak delay * of the VEP was markedly shorter when subjects wore cylindrical lenses oriented such that they emphasized a fundamental Fourier component of the checkerboard stimulus. These results underline the importance of describing complex patterned stimuli in terms of Fourier analysis when evaluating meridional aspects of visual function. * [As Regan (1966. 1972. 1978) poits out. time to any peak in a steady-state VEP can be a function of numerous variables: (a) transmission time (b) nonlinearity of the amplitude-frequency plot and or (c) differences in the spatial orientation of cortical generators (Halliday and Michaels. 1970: Jeffreys and Axford. 1972). Apparent latency is defined as (δφ/(36OδF) where φ = phase. F = frequency and (δφ/(δ F) is the slope of the phase vs frequency plot. Thus. the calculation of apparent latency requires measurement of phase changes over a range of different temporal frequencies. The present data were obtained at only a single temporal frequency and it is therefore impossible to assess the individual influence of factors listed above. We have therefore used the term peak delay as opposed to more rigorously defined terms, like apparent latency or phase lag.]