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At least 1,657 records · Page 92Linked to original sources
PubMed1993

Circadian motile activity of erythrophores in the red abdominal skin of tetra fishes and its possible significance in chromatic adaptation.

The red abdominal skin of the neon tetra Paracheirodon innesi and the cardinal tetra P. axelrodi was found to blanch at night or in the dark. Melatonin added to the bathing medium caused blanching of the red skin. Microscopic observations of the erythrophores indicated that the erythrosomes aggregated in response to norepinephrine, melanin-concentrating hormone (MCH), and melatonin. Of these compounds, melatonin was the most effective. By contrast, many erythrophores were refractory to MCH. Alpha-melanophore-stimulating hormone, isoproterenol, adenosine, and ATP each caused dispersal of the pigment to some extent. Isoproterenol dispersed the pigment only when an alpha-adrenergic blocker, tolazoline, was present. It appears that the change in color of the abdominal skin is primarily due to increased secretion during the night of the pineal hormone melatonin, while other hormonal and nervous factors may modify the distribution of the pigment in the erythrophores.

Adaptation, Physiological↗
PubMed1983

Cortical magnification factor and contrast sensitivity to luminance-modulated chromatic gratings.

Contrast sensitivity to luminance-modulated blue, green, red, and neutral gratings was measured at different spatial frequencies and eccentricities within 0-86 deg in the temporal visual field. Contrast sensitivity to gratings of constant area and spatial frequency was independent of wavelength composition but decreased with increasing eccentricity. When the gratings were scaled by the magnification factor of the human striate cortex to produce cortically similar stimulus conditions at different eccentricities (M-scaling), contrast sensitivities became independent of visual field location irrespective of grating colour. Using colour naming we found, in accordance with previous studies, that hue changed and saturation decreased when the eccentricity of a constant-size grating field increased. In contrast, the hue and saturation of M-scaled grating fields were independent of eccentricity. The results suggest that the effects of eccentricity on photopic colour vision can largely be counteracted by M-scaling which adjusts the spatial aspects of stimuli with respect to the decrease in ganglion cell density and increase in receptive field size towards the periphery of the visual field.

Color Perception↗
PubMed1967

The effect of chromatic aberration on visual acuity.

1. Differences of threshold contrast are predicted from optical theory for a grating acuity target in monochromatic and white light. The greatest differences, up to 65%, are predicted for gratings of lower contrast and pitch than those normally used in measurements of visual acuity.2. Using three subjects, we measured contrast thresholds with 1.5 and 2.5 mm diameter artificial pupils for natural and paralysed accommodation, using a tungsten lamp and wave-lengths of 546 and 578 mm.3. Excellent agreement is obtained between predicted and measured differences.4. Results confirm that observed acuity and sensitivity differences between white and monochromatic lights are largely optical in origin, but involve at least two independent colour mechanisms as spectral weighting functions. Stiles's pi(4) and pi(5) sensitivities afford a much better fit to observed differences than the C.I.E. visibility curve.

Accommodation, Ocular↗