Field sensitivity action spectra of cone photoreceptors in the turtle retina.

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 amplitude (< 1 mV) responses. The desensitizing effects of backgrounds of different wavelengths were measured and the background irradiance needed to desensitize the cone by a factor of 10 (1 log unit) was defined as threshold. The reciprocals of these thresholds were used to construct the field sensitivity action spectrum. 3. The field sensitivity action spectra of long-wavelength-sensitive (L) and medium-wavelength-sensitive (M) cones depended upon the wavelength of the test flash used to measure them. This excludes the possibility that turtle cones can function as single-colour mechanisms in the Stiles sense. 4. In fourteen L-cones, the average wavelength of peak sensitivity of the field sensitivity action spectrum was 613.7 +/- 7.7 nm for the 500 nm test and 635.6 +/- 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, respectively. 5. Two physiological mechanisms are suggested as contributing to the dependency of the field sensitivity action spectrum upon test wavelength. One is based upon the transmissivity properties of the coloured oil droplets, while the other hypothesizes excitatory interactions 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 sensitivity action spectrum upon the wavelength of the test flash.

Neural interactions between cone photoreceptors and horizontal cells in the turtle (Mauremys caspica) retina.

Horizontal cells and cone photoreceptors in the vertebrate retina are interconnected by a complex network of synapses leading to the generation of color-coded responses in chromaticity horizontal cells. A simple cascade model of excitatory feedforward and inhibitory feedback synapses had been suggested to underlie these observations. In this study, the photoresponses of cones and horizontal cells were recorded intracellularly from the turtle eyecup. Three different approaches were adopted in order to test quantitatively the cascade model. Comparing linearity functions between these neurons indicated multiple excitatory inputs to each type of horizontal cells. The depolarizing photoresponses of R/G C-type horizontal cells were considerably faster than those of L-type horizontal cells but slower than those recorded from L-cones. This observation disagrees with the basic assumption of the cascade model that assign the depolarizing photoresponses of R/G C-type horizontal cells to a negative feedback pathway from L-type horizontal cells onto M-cones. Finally, the action spectra of each of the three types of horizontal cells could not be solely accounted for by input from one spectral type of cones. Only by assuming excitatory and inhibitory inputs from all spectral types of cones, the action spectra of all types of horizontal cells could be reconstructed. These findings suggest that the negative feedback pathways from horizontal cells onto cones in the turtle retina cannot solely account for the chromatic properties of the horizontal cells and support a direct inhibitory inputs from cones to turtle horizontal cells.

Relationships between the electroretinogram a-wave, b-wave and oscillatory potentials and their application to clinical diagnosis.

The electroretinogram is the electrical response of the retina to a light stimulus. The amplitude and temporal pattern of its components, the a-wave, the b-wave and the oscillatory potentials, depend on the functional integrity of the retina, on the intensity of test flash reaching the retina and on the ambient illumination. The latter contributions to the normal variability in the electroretinogram can be circumvented by constructing the relationships between the different electroretinogram waves. The electroretinogram responses were recorded from 18 dark-adapted subjects with normal vision. The slope of the a-wave and the amplitude of the b-waves were measured in the time domain. The oscillatory potentials were isolated by a digital filter and were transformed to the frequency domain for quantitative measurement. The relationship between each pair of variables could be fitted by linear segments. Our findings suggest that this mode of electroretinogram analysis can be useful in localizing the site of action of retinal disorders and that the relationship between the a-wave slope and the power density of the oscillatory potentials is a useful index for identifying disorders of the inner retina.