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B. D. Gomes, M. G. Lima, G. S. Souza, A. R. Rodrigues, C. A. Saito, M. da Silva Filho, L. C. L. Silveira; Color Discrimination Ellipses of Trichromats Measured With Visual Evoked Potential (VEP): Effect of Temporal Configuration and Different Harmonic Measurement. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3309. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To measure colour discrimination thresholds with VEP under three presentation modes and to compare these data with psychophysical measurements using the same stimuli as well as the Mollon-Reffin test.
Four normal trichromats (23.8 ± 3.1 years old) were monocularly tested. Stimuli consisted of sinusoidal isoluminant chromatic gratings made from chromaticity pairs located along four different colour directions centered on one reference point (CIE 1976: u’ = 0.225; v’ = 0.415). Heterochromatic Flicker Photometry (HFP) protocol was used to obtain the isoluminance condition for every subject and for all chromaticity pairs. Spatial frequency was 2 cycles/deg. Presentation modes: transient, onset (300 ms) / offset (700 ms) periods; steady-state, onset (50 ms) / offset (50 ms) periods; and steady-state, 5 Hz phase reversion. For transient presentation, the biphasic negative-positive (N-P) amplitude of the VEP was related to the chromatic difference. For steady-state presentations, a Fast Fourier Transform (FFT) procedure was used to obtain the amplitude of the VEP 10 Hz and 20 Hz harmonics, which was then related to the chromatic difference. Color thresholds were found by extrapolating the chromatic differences to the null amplitude value.
For all subjects and all stimulation methods, the ellipses showed small sizes, low ellipticities, and were vertically oriented. There was no statistical difference between ellipses obtained with VEP (transient and 10 Hz steady-state) and both psychophysical methods, either gratings or Mollon-Reffin stimuli (One-way ANOVA; p > 0.05). The VEP ellipses obtained with the transient presentation were more related in size with those obtained with 10 Hz steady-state reversal than 10 Hz steady-state onset / offset presentation. In steady-state condition, 20 Hz harmonic amplitude varied non-linearly in all color directions except those that strongly activate the koniocellular pathway.
Transient onset/offset and steady-state reversal VEPs can be reliably used in objective studies of color discrimination in normal trichromats. The 20 Hz non-linearity in some color directions is in agreement with the properties of the M cell pathway as revealed by single unit recordings.
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