Journal of Vision - The multifocal visual evoked potential and cone-isolating stimuli: Implications for L- to M-cone ratios and normalization, by Hood, Yu, Zhang, Albrecht, Jägle, & Sharpe

Donald C. Hood	Department of Psychology, Columbia University, New York, NY, USA
Alice L. Yu	Department of Psychology, Columbia University, New York, NY, USA Division of Experimental Ophthalmology, University Eye Hospital, University of Tübingen, Tübingen, Germany
Xian Zhang	Department of Psychology, Columbia University, New York, NY, USA
Johannes Albrecht	Division of Experimental Ophthalmology, University Eye Hospital, University of Tübingen, Tübingen, Germany
Herbert Jägle	Division of Experimental Ophthalmology, University Eye Hospital, University of Tübingen, Tübingen, Germany
Lindsay T. Sharpe	Department of Psychology, University of Newcastle, Newcastle-upon-Tyne, UK

Multifocal visual evoked potentials (mfVEP) were recorded with a pattern-reversing display that modulated only the long wavelength-sensitive (L) cones or only the middle wavelength-sensitive (M) cones. Outside the central 5.8° (radius), the ratio of the amplitudes of the mfVEP responses to L- and M-cone modulation varied across the six subjects, ranging from 1.1 to 1.7. The responses from the central 1° (radius) showed a substantially lower ratio, ranging from 0.8 to 1.1 (average of 0.9). The variation among individuals outside the central fovea is probably due to differences in the ratio of the L/M cone input to both magno- and parvocellular pathways. The substantially lower ratios for the central responses is consistent with an L/M cone ratio closer to 1.0 in the central 1° and/or an adjustment in the gain of the L- versus M-cone contributions to the central parvocellular pathways. Taking into consideration evidence from other techniques, we believe it is unlikely that most individuals have a L/M cone ratio of 1.0 in the fovea. Instead, it appears that there is a change in gain before the mfVEP is generated in area 17.