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O. A. Mahroo, K. N. Khan, C. Ganesan, S. Shen, Y. Mustafa, T. D. Lamb; Suppression of Human Cone Circulating Current by Steady Backgrounds Assessed at Early Times Using the Electroretinogram. Invest. Ophthalmol. Vis. Sci. 2008;49(13):5820.
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© ARVO (1962-2015); The Authors (2016-present)
It has been recently shown that human cones, in contrast to rods, appear to preserve half their circulating current in the presence of steady backgrounds that bleach 90% of their photopigment. This avoidance of saturation may result from two mechanisms: (1) at late times, bleaching photoproducts do not appear to affect the phototransduction cascade in cones, and (2) the background elicits fewer photoisomerisations at steady state due to the reduction in quantal catch. It follows, then, that at earlier times following onset of such illumination, before significant bleaching has occurred, cone current will be more suppressed. We tested this hypothesis.
Two subjects, with dilated pupils, were exposed to steady white backgrounds lasting 2 s, whose intensities ranged from 10 to 1200 photopic cd m-2, corresponding to between ~200 and 24 000 photopic Td. The electroretinogram a-wave responses to white flashes (17 photopic cd m-2 s), delivered 1.5 s after onset of these backgrounds, were compared at a range of early post-flash times (5-11 ms) to the dark-adapted response, to assess suppression of current. Responses to the switching on and off of the background itself were also explored to look at early current suppression and recovery. A blue background (~1500 scotopic Td) was presented continually to eliminate rod responses.
Responses were reduced significantly by the brighter backgrounds, by up to 80%, more so than at steady state bleaching levels. The estimated reduction of current with increasing background intensity was similar in form to that recorded in previous studies directly from monkey cones. The response to background onset was consistent with an expression expected to describe responses of single photoreceptors. The response to background extinction was similar in form to that seen following more prolonged exposures.
At early times ~1.5 s following the onset of bright steady backgrounds, human cone circulating current is reduced substantially, and subsequently recovers as pigment is bleached. Also, the early electroretinogram response to background onset is consistent with a measure of the shut-off of photoreceptor circulating current.
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