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Webvision: Color Perception Page 10 of 18 (DeValois et al, 1966). 3. Electrical recording of ganglion cells from primate retina showing opponent colour processes (Gouras, 1968; de Monasterio and Gouras, 1975, Zrenner and Gouras, 1981). Stage Theory: This has led to the modern model of normal colour vision which incorporates both the trichromatic theory and the opponent colour theory into two stages (figure 16). The first stage can be considered as the receptor stage which consists of the three photopigments (blue, green and red cones). The second is the neural processing stage where the colour opponency occurs. The second stage is at a post-receptoral level, and occurs as early as the horizontal cell level. Figaro 1.6, Model for nerm�I human cotou; yisiau _ I( 5 I Jnen.imine Colour Vision Defleiencles. Colour vision deficiencies (CVD) can be congenital or acquired. Congenital CVD means that the CVD is present at birth and is inherited while acquired CVD occurs secondary to eye disease. Congenital CVD comprised of —8 % of males and —0.5% of females. I CVD are classified into three groups. These are monochromasy, dichromasy and anomalous, trh�o'People with normal colour vision are called trichromats. Monochromats are typically totally colour blind and may have one cone pathway in addition to the rod pathway. Dichromats have a cone photopigment missing, therefore, they only have two cone channels. Anomalous trichromats have all three cone photopigments, however, one cone photopigment is anomalous having a shifted peak sensitivity The types and prevalence of CVD is listed in Table 1. Table f Prevalence of congenital colour deficiencies Types of colour vision deficiency males females Overall _8% —0.5% EAnomalous trichromasy protanomaly10/1< / FONT> IL— 0.01 % IVI �t kttp://www.webvision.med.utah.edii allColor.html 6/18/01