Effects of calcium ions on L-type horizontal cells in the isolated turtle retina.

A technique by which the retina can be isolated from the turtle eye is described. Scanning electron microscopy revealed morphological variability between preparations and also between regions of the same one. Large areas were often totally free of any pigment epithelial cells, yet contained a high proportion of photoreceptors with complete outer segments. However, adjacent regions may contain photoreceptors without outer segments or with fragmented ones. The physiological properties of the horizontal cells also demonstrated large variability between different preparations. In all cases, lowering calcium concentration from 2 mM to 0.1-0.5 mM depolarized the horizontal cells and augmented the amplitude of the maximum photoresponses. However, these effects were accompanied by changes in the photoresponse kinetics and by a reduction in the horizontal cell sensitivity to light. Moreover, prolonged exposure to low calcium induced permanent damage to the retina as was indicated by the reduction in the response amplitude after superfusion with 2 mM calcium solution had been resumed. The toxic effects of low calcium were most apparent when superfusion with 0.1-1.0 microM calcium concentration was performed. These solutions induced complex time-dependent effects on the resting potential of horizontal cells and on the amplitude and kinetics of the photoresponses. We conclude from these observations that the normal concentration of extracellular calcium in the turtle retina is in the 2 mM range.

Effect of tactile stimulation pulse characteristics on sensation threshold and power consumption.

The psychophysical responses of human subjects to vibratory tactile stimulation of the skin were investigated experimentally. The parameters of the waveform important to the minimization of power consumed by the tactile array of electromechanical vibrators and the maximization of the skin sensitivity to the stimulus were explored to develop optimum stimulation. Parameters investigated included the amplitude, frequency, and duty cycle of the current waveform used to drive the vibrators as well as the number of pulses per stimulating burst and the recovery time between bursts. Graphical techniques were used to determine the optimal combination of the parameters which gave a stimulus that excited the skin to above tactile threshold while maintaining at a relative minimum the power required for the stimulus. The optimal stimulation waveform contains a burst of 10 rectangular pulses of 4% duty cycle separated by a period of nonstimulation of 2 s. Such a waveform can elicit a sensitivity of 29.4 mA-1 consuming only 55 microW of power.

Application of an invariant spectral form to the visual pigments of crustaceans: implications regarding the binding of the chromophore.

Visual pigment absorption spectra were measured in single photoreceptors of a stomatopod, a crayfish, a hermit crab, and five species of brachyuran crab. All fitted a Mansfield (1985) invariant form for visual pigment, the form also fitted by vertebrate retinal-based visual pigments. This is consistent with a theoretical model based on the structure of visual pigment molecules (Greenberg et al., 1975; Honig et al., 1976) which predicts that spectral bandwidth decreases as lambda max increases. The conformation to the invariant form implies that for any given chromophore bandwidth times lambda max is a constant.

A new method for determining peak absorbance of dense pigment samples and its application to the cone oil droplets of Emydoidea blandingii.

A new method measures the spectral absorption of small volume samples having peak optical densities (Dp) too high (up to 50 and higher) to be measured by standard direct or indirect methods. This method specifically corrects for the effect on the microspectrophotometer (MSP) of wavelength-independent light which bypasses the sample. The method involves; (1) measuring by MSP the long wavelength cutoff of the dense samples and the absorption spectrum of thinned samples of the same pigment, (2) calculating Dp from these. The method's application is illustrated for the colored oil droplets in the retinal cones of the turtle, Emydoidea blandingii.

Pigment types, densities and concentrations in cone oil droplets of Emydoidea blandingii.

The peak optical densities (Dp) of colored oil droplets in the retinal cones of the turtle, Emydoidea blandingii, were found to range from 1.2 to 8.8 for yellow droplets, 3.2 to 11.4 for orange, and 8.8 to 58 for red. The yellow droplets' spectra matched that of zeaxanthin, the red and orange matched astaxanthin. One type of clear droplet absorbed negligibly in the visible spectrum. A second type had a Dp approximately 0.9 somewhere between 370 and 400 nm. The accessory cone had no oil droplet, but its ellipsoid contained a pigment with the absorption spectrum of zeaxanthin and Dp approximately 1.1.

Blue-sensitive cones in the primate retina: microspectrophotometry of the visual pigment.

Direct absorbance and bleaching absorbance-difference spectra were obtained using a photon-counting microspectrophotometer from the outer segments of ten blue-sensitive cones of macaque monkeys. The peak wavelength (lambda max) of the direct measurements was 426 +/- 3.4 nm, whereas the lambda max of the bleaching difference was 434 +/- 6.6 nm. We consider these values to be upper and lower bounds since both measurements may be shifted in opposite directions by wavelength-dependent effects. Therefore, the true peak sensitivity must be close to 430 nm.

Receptive field properties of the photopic luminosity horizontal cell of carp retina.

The receptive field of the LEHC of the carp's retina is different when tested with red versus green stimuli. The sensitivities to 706 nm vs 519 nm flashes were compared for various size spots centered on the receptive field. Full summation (area times intensity equaling a constant at threshold) and greater than full summation were found to occur up to larger diameters of spots with red illumination than with green. A further test was made of the effect of constant background green (502 nm) illumination on the sensitivity to red vs green flashes. At all background intensities the sensitivity to red vs green flashes. At all background intensities the sensitivity to green flashes was reduced, but over an optimal range of background intensities the sensitivity to red flashes was increased. These findings are explained in terms of a previously proposed model of cone-LEHC connections.