Detection and resolution of visual stimuli by turtle photoreceptors

Abstract

1. Hyperpolarizing responses up to 30 mV in amplitude were recorded from cones and from certain cells believed to be rods in the isolated retina of the swamp turtle, Pseudemys scripta elegans. 2. The responses evoked by weak flashes of light reach their maximum in 100–140 msec in red-sensitive cones, 140–180 msec in green-sensitive cones, and 300–600 msec in the rod-like cells (20° C). 3. The cone response evoked by weak flashes of light is linearly related to light intensity and obeys the superposition principle in that the response to a very weak step of light is the integral of the response to a very weak flash. 4. On the basis of their spectral sensitivities cones can be divided into three distinct classes, namely red-sensitive cones whose relative quantum sensitivity is maximal at 630 nm, green-sensitive cones with a maximal sensitivity at 550 nm and blue-sensitive cones with a maximum at 460 nm. 5. The difference between the spectral sensitivity of rods with a maximum at about 520 nm and green-sensitive cones (λ_max = 550 nm) is consistent with the view that both receptors contain a 518₂ retinal pigment as reported by Liebman & Granda, but that light is filtered by an orange oil droplet in green-sensitive cones. 6. The spectral sensitivities of both red- and green-sensitive cones agree well amongst themselves at long wave-lengths but differ markedly in the extent of the reduction at short wave-lengths. This variation is attributed to differences in the extent to which light is filtered through the coloured oil droplets. 7. There is a significant positive correlation between the absolute sensitivity of red- and green-sensitive cones and the reduction in sensitivity at short wave-lengths. This would be explained if a greater fraction of the light passes through the oil droplet in the most sensitive cells. 8. The absolute flash sensitivities of the most sensitive receptors were about 250 μV photon⁻¹ μm² in red- and green-sensitive cones, 120 μV photon⁻¹ μm² in blue-sensitive cones, and 1300 μV photon⁻¹ μm² in rods. 9. If the effective collecting area (which includes factors for absorption etc.) is taken as 10 μm² in a red-sensitive cone the peak hyperpolarization produced by 1 photon would average 25 μV. 10. Provided that small spots of light are used, individual receptors obey the ‘univariance principle’ and the response produced by light of strength I′, and wave-length λ₁ can be matched by a light of strength kI′ and wave-length λ₂, where k is the same for all values of I′. 11. A small proportion of cones behave like isolated units in that they have very sharp sensitivity-profiles and obey the univariance principle with respect to the position as well as to the wave-length of light. 12. The majority of red and green cones have more diffuse sensitivity-profiles, sometimes with bumps on the descending limb, and behave as though cones with the same spectral sensitivity were electrically coupled to one another. 13. The relation between the area of illumination and flash sensitivity agreed approximately with that calculated from the spatial profile.