TEMPORAL ANALYSIS OF THE TOPOGRAPHIC ERG - CHROMATIC VERSUS ACHROMATIC STIMULATION

The topographic electroretinogram evoked by multi-focal exchange of bl ack and white or red and green stimuli was analysed into linear and no n-linear Wiener kernels. The first-order (temporally linear) response showed a biphasic waveform which inverted as the luminance ratio of th e exchanged colours passed through unity (established both psychophysi cally and photometrically). A short latency non-linearity which was de pendant on luminance contrast was observed in both chromatic and achro matic ERG. However, in the chromatic second-order response, a long-lat ency non-linearity, foveally prominent, with a distinct skew in power towards the nasal retina, appeared around the isoluminant point, betwe en the points of silent substitution for the L and M-cone types. Model ling of the second-order responses showed that over a wide range of lu minance ratios, the chromatic ERG is well described by a linear combin ation of the achromatic (contrast-dependent) component and the respons e at isoluminance. The difference in second-order response between col oured and black and white stimulation, at the same luminance contrast, showed that the long-latency non-linearity is recorded when the red a nd green cone types are operating out of phase and peaks in amplitude at a green/red luminance ratio of 0.8. This interpretation was confirm ed by the lack of the long-latency non-linearity in colour-anomalous s ubjects (whether deficient in the L or the M-cone type). A marked simi larity exists between the properties of the long-latency nonlinearity and the frequency-doubled response generated in the ganglion cells of the magnocellular pathway. (C) 1998 Elsevier Science Ltd. All rights r eserved.