FIELD SENSITIVITY ACTION SPECTRA OF CONE PHOTORECEPTORS IN THE TURTLERETINA

1. The Stiles two-colour increment threshold technique was applied to turtle cone photoreceptors in order to derive their field sensitivity action spectra. 2. Photoresponses of cone photoreceptors were recorded intracellularly. Flash sensitivities were calculated from small ampli tude (< 1 mV) responses. The desensitizing effects of backgrounds of d ifferent wavelengths were measured and the background irradiance neede d to desensitize the cone by a factor of 10 (1 log unit) was defined a s threshold. The reciprocals of these thresholds were used to construc t the field sensitivity action spectrum. 3. The field sensitivity acti on spectra of long-wavelength-sensitive (L) and medium-wavelength sens itive (M) cones depended upon the wavelength of the test flash used to measure them. This excludes the possibility that turtle cones can fun ction as single-colour mechanisms in the Stiles sense. 4. In fourteen L-cones, the average wavelength of peak sensitivity of the field sensi tivity action spectrum was 613.7 +/- 7.7 nm for the 500 nm test and 63 5.6 +/- 9.6 nm for the 700 nm test. For six M-cones, these values were 558.5 +/- 6.8 and 628.8 +/- 10.6 nm for the 500 and 700 nm tests, res pectively. 5. Two physiological mechanisms are suggested as contributi ng to the dependency of the field sensitivity action spectrum upon tes t wavelength. One is based upon the transmissivity properties of the c oloured oil droplets, while the other hypothesizes excitatory interact ions between cones of different spectral type. 6. Computer simulations of the field sensitivity action spectra indicate that both mechanisms are needed in order to account for the dependency of the field sensit ivity action spectrum upon the wavelength of the test flash.