Properties of retinal ganglion cell receptive fields at the lower limit of visual sensitivity

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The goal of this study was to investigate the properties of the retinal ganglion cell receptive field at low light levels. There has been considerable interest in whether the surround of a ganglion cell receptive field disappears and the center expands in size under scotopic conditions. The previous data from our laboratory had shown that, while antagonism between center and surround is reduced for ON-center Y-cell receptive fields within the scotopic range, the surround remains very much present. In addition, expansion of the receptive field center is quite modest (only 30% greater radius). We have now extended this work to focus on a range of scotopic light levels not explored by us previously. This is the range from where each rod would be expected to capture a photon once every 10 seconds down to a light level where each rod would be expected to capture a photon once every 5 minutes. A more significant expansion has occurred by the lowest light level we studied with the radius of the center summing area increasing to 250% its photopic dimension for ON-center X cells (170% greater radius for ON-Y cells). Over the same range, the responsivity of the center falls dramatically, as one might expect were the responses of ON-center cells to reflect simple summation of photons captured by rods within the center. Although the presence of a surround mechanism was less evident in the range studied than under photopic or higher scotopic light levels, our data suggest that a receptive field surround persists even to the lowest scotopic levels studied. The delay between center and surround signals increases progressively from photopic levels, with the result that center and surround signals are more nearly in phase at these low scotopic levels than they are under photopic conditions, where they are antagonistic. As a result of this phase "synchrony" both mechanisms are mobilized by the X- and Y-cells to sum photons, helping to preserve the cell's responsivity and maximize the signal to noise ratio

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  • 05/22/2018
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