Characterisation of dark adaptation in human cone pathways: an application of the equivalent background hypothesis
It is well accepted that in rod photoreceptors the photoproducts generated by a bleach cause desensitisation during dark adaptation. We examine whether this notion holds for cones. A model of cone dark adaptation is developed based on the equivalent background concept. The underlying theory of the m...
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Veröffentlicht in: | The Journal of physiology 2000-11, Vol.528 (3), p.591-608 |
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Sprache: | eng |
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Zusammenfassung: | It is well accepted that in rod photoreceptors the photoproducts generated by a bleach cause desensitisation during dark adaptation.
We examine whether this notion holds for cones.
A model of cone dark adaptation is developed based on the equivalent background concept. The underlying theory of the model
relies on a series of assumptions that link psychophysically determined detection thresholds to cone phototransduction. Correction
of thresholds for the reduced quantum-catching ability of the cones (due to the depletion of photopigment caused by a bleaching
light) is an important aspect of the model.
Foveal detection thresholds were measured for a small test flash presented on a large steady background field or presented
alone after adapting to the background field. Test and background fields were monochromatic, with wavelengths closely matched
to promote detection by the luminance mechanism.
The model provided a good description of the data collected under these conditions. Parameters of the model were similar for
all wavelengths and each observer, as were the derived equivalent background relationships. Analysis of previously published
data for Stilesâ Ï 5 mechanism gave analogous results.
The model is made up of two components. The early (fast) component is likely to be due to the direct action of the cone equivalent
of inactivated Rh* on the G-protein cascade and/or the reverse reaction of the cone equivalent of inactivated Rh* to Rh*.
The later (slow) component may be due to the direct action of cone opsin on the G-protein cascade. |
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ISSN: | 0022-3751 1469-7793 |
DOI: | 10.1111/j.1469-7793.2000.00591.x |