Non-linearities in the focal ERG evoked by pattern and uniform-field stimulation. Their variation in retinal and optic nerve dysfunction.

The amplitude of the second harmonic of the focal electroretinogram (ERG) in response to either modulation of the luminance of the uniform-field or the spatial contrast of a patterned field (pattern-reversal ERG) was measured in a group of normal subjects as well as in patients with two different types of unilateral dysfunctions, namely optic atrophy or temporary retinal ischemia. Such patients had a reduced visual acuity in their affected eyes but normal full-field flicker (20 Hz) ERG. In normal eyes, for the same stimulation area and modulation depth, the second harmonic of the uniform-field response is smaller (mean value 62%) than that of the optimal pattern (around 1.5 cycles/degree). The results on patients show that the second harmonic of the pattern response, but not that of the uniform-field response, is reduced in cases of optic atrophy secondary to trauma or optic neuritis. This result suggests generators different, at least in part, for the second harmonic evoked by modulation of either luminance or spatial contrast. By contrast, both responses are reduced in cases of temporary retinal ischemia. These findings are discussed in light of the recent literature on the origin of the pattern ERG. The possible clinical applications of the technique are outlined.