Temporal and spatial frequencies interact in the contrast transfer function of the pattern electroretinogram

The contrast transfer function (CTF) of the pattern electroretinogram (PERG) depends on temporal frequency. For transient stimulation it is fully linear; at faster stimulation rates it becomes strongly non-linear with an accelerated shape. In this study we investigated a range of stimulus parameters with the aim of studying the influence of temporal and spatial frequencies, as well as contrast levels, on the CTF; effects were quantified via an "index of linearity" IL. Both reversal rate and check size influenced linearity (p<.001), examples: At a constant check size of 0.8 degrees, 7.7 rps: IL=1.0; 0.8 degrees/24 rps: IL=0.5; at a constant reversal rate of 19 rps, IL was 0.5 for 0.8 degrees, but rose to 0.8 both for 0.2 degrees and 18 degrees. The reason for this complex response surface remains a puzzle, it cannot be explained by varying parvo/magnocellular contributions, and its possible influences on recordings in patients merit further studies.