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J.G. Robson, S.M. Saszik, H. Maeda, L.J. Frishman; Transmission of Single–Photon Signals to Mouse Ganglion Cells Studied Using the Electroretinogram . Invest. Ophthalmol. Vis. Sci. 2005;46(13):2235.
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Purpose: to find out if there is a threshold non–linearity that could act as a photon–coincidence detector in the rod pathway of fully dark–adapted mice by determining the relation between the amplitude of a ganglion–cell component of the ganzfeld electroretinogram (ERG) and the energy of very weak stimuli. Methods: ERGs were recorded in normal mice, and in connexin36 knockout (Cx36KO) animals (all anesthetized with ketamine/xylazine), both before and after ganglion cells had degenerated following optic nerve crush. Results: In normal mice a negative STR (scotopic threshold response) could only be recorded with stimuli giving more than about 1 photoisomerization per 1000 rods (10–3 R*/rod; ∼10–5 sc Td s). Above this level the negative wave grew supralinearly (i.e. as though there were a threshold) before saturating at a maximum of about 40 µV. The early positive wave of the STR was too small to be certain whether or not it displayed similar "threshold" behavior. However, in Cx36KO mice, the absence of functional gap junctions between AII amacrine cells and depolarizing cone bipolar cells results in loss of response from ON ganglion cells and the response of OFF ganglion cells is more completely revealed in the ERG. In these animals an initially positive biphasic ERG could reliably be elicited with much weaker stimuli (10–4 R*/rod) and the amplitude could more easily be measured. The amplitude of the early positive wave increased most rapidly, and nearly in proportion to stimulus energy, when the energy was lowest. As the positive wave grew into the b–wave, the rate at which its amplitude increased became steadily slower until the response saturated completely. There was no sign of any supralinear increase in amplitude of either the initial positive wave or the later negative one. Following degeneration of the ganglion cells, the sensitive component of the b–wave (about 40 µV maximum amplitude) and the later negative wave were both absent, indicating that they had been generated by ganglion cells. Conclusions: Our failure to find any sign of a threshold non–linearity in the relation between the amplitude of the response of OFF ganglion cells and stimulus energy rules out the possibility of there being a threshold mechanism that could give rise to photon–coincidence detection in the more distal parts of the rod pathway (rod–bipolar and amacrine cells) or in OFF ganglion cells. The absence of a negative STR in normal eyes in response to stimuli below 10–3 R*/rod results from mutual cancellation in the ERG of weak signals of opposite polarity rather than from operation of a threshold mechanism.
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