The pattern ERG in response to colored stimuli

The mouse pattern electroretinogram

Mouse models of optic nerve disease such as glaucoma, optic neuritis, ischemic optic neuropathy, and mitochondrial optic neuropathy are being developed at increasing rate to investigate specific pathophysiological mechanisms and the effect of neuroprotective treatments. The use of these models may be greatly enhanced by the availability of non-invasive methods able to monitor retinal ganglion cell (RGC) function longitudinally such as the Pattern Electroretinogram (PERG). While the use of the PERG as a tool to probe inner retina function in mammals is known since 25 years, relatively less information is available for the mouse. Here, the PERG technique and the main applications in the mouse are reviewed.

Temporal analysis of the topographic ERG: chromatic versus achromatic stimulation

The topographic electroretinogram evoked by multi-focal exchange of black and white or red and green stimuli was analysed into linear and non-linear Wiener kernels. The first-order (temporally linear) response showed a biphasic waveform which inverted as the luminance ratio of the exchanged colours passed through unity (established both psychophysically and photometrically). A short latency non-linearity which was dependant on luminance contrast was observed in both chromatic and achromatic ERG. However, in the chromatic second-order response, a long-latency non-linearity, foveally prominent, with a distinct skew in power towards the nasal retina, appeared around the isoluminant point, between the points of silent substitution for the L and M-cone types. Modelling of the second-order responses showed that over a wide range of luminance ratios, the chromatic ERG is well described by a linear combination of the achromatic (contrast-dependent) component and the response at isoluminance. The difference in second-order response between coloured and black and white stimulation, at the same luminance contrast, showed that the long-latency non-linearity is recorded when the red and green cone types are operating out of phase and peaks in amplitude at a green/red luminance ratio of 0.8. This interpretation was confirmed by the lack of the long-latency non-linearity in colour-anomalous subjects (whether deficient in the L or the M-cone type). A marked similarity exists between the properties of the long-latency non-linearity and the frequency-doubled response generated in the ganglion cells of the magnocellular pathway.

Utility of the color pattern-electroretinogram (PERG) in glaucoma

Pattern-onset electroretinograms (PERGs) with red-green color contrast (CC) and green-"black" luminance contrast (LC) stripe patterns (0.3 c/deg) were recorded in a group of 80 control subjects and in a group of 42 patients having glaucomas of varying etiology and severity. The PERG data were correlated with the results of static perimetry and optic disc morphometry. In the glaucoma group the PERG was reduced significantly and by relatively similar amounts with both CC and LC stimuli. A significant correlation of the PERG reduction with visual field loss was found only with the CC, not with the LC PERG. Correlations between PERG amplitudes and neuroretinal rim areas of the optic disk were similar for the LC and for the CC stimulus. The rather poor percentage of correct classification of controls and patients based on the PERG or the optic disc morphometry alone can be improved by two-dimensional discriminant analysis using both CC PERG and papillometry data.

Differences between pattern onset and pattern reversal retinal responses

The pattern-evoked electroretinogram was recorded to pattern onset-offset and pattern reversal stimuli in two color-normal subjects with either luminance contrast of black-red (600 nm) and black-green (526 nm) square-wave stripe patterns or color contrast red-green patterns. The size of the onset response shows a spatial tuning with luminance contrast patterns and only a simple low-pass filter function with color contrast patterns. The peak latency of the response to luminance contrast increases with increasing spatial frequency but stays constant with color contrast patterns. The size of the reversal responses, however, shows only a low-pass filter function under both contrast conditions. The peak latency to luminance contrast shows a slight increase and to color contrast it remains constant with increasing spatial frequency. The differences noted under the various stimulus conditions must take into account the possible effects of different luminance modulation depths of onset and reversal stimuli, the modulation transfer function of the eye, and the activity of luminance-antagonistic and color-antagonistic receptive fields.