V 198Selective cone function assessed by a multifocal silent-substitution ERG technique
H. Jaegle, L. T. Sharpe
Introduction: The so-called silent substitution technique for selectively stimulating each of the three - L(red), M(green) and S(blue) - cone classes has been available for several years. By this method, two test colors are chosen in such a way so that, when switching between them, two of the cone classes are unstimulated (are silenced), and only the third is actively modulated. The combination of this technique with the multifocal electroretinography provides a new method for assessing the function of the three cone classes independently.
Methods: The VERIS system (Ver. 3.0.1) was used in combination with "silent substitution" techniques, based on the Stockman & Sharpe (1999) cone fundamentals, to isolate responses from individual cone classes in the human mFERG. The stimuli were generated on a Sony Trinitron F500 monitor, which was spectroradiometrically (Instrument Systems CAS 140) and photometrically (Minolta CA-100) calibrated. Corneal ERG responses were recorded with a DTL fiber electrode. Maximal Michelson contrasts of 47%, 47% and 92% were achieved for the L-, M- and S-cone stimuli, respectively. 103 locations were stimulated in the visual field of view of 90° x 90° at a viewing distance of 16 cm.
Results: In normals, the latencies for the L- and M-cone driven responses in the central fovea were c. 30 ms with the M-cone preceding the L-cone driven response by c. 3 ms. The S-cone driven response latency in the center was c. 36 ms with a late component at c. 65 ms (the two are correlated). The amplitudes and latencies of the cone driven signals are linearly dependent upon stimulus intensity and cone contrast. All types of cone-driven latency decreased with retinal eccentricity.
Discussion: Using a silent substitution technique in combination with the standard multifocal ERG, we are able to measure the topography of the L-, M- and S-cone driven signals. In normals, the topography of all three cone classes are very similar. The response signal amplitudes of the L- and M-cone driven signals, however, depend directly on the individuals L to M cone ratio in the retinal mosaic.
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