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R. C. Renteria, C. L. Koehler, N. P. Akimov; Contrast Sensitivity of Developing Retinal Ganglion Cells in the Postnatal Mouse Retina. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1873.
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Contrast sensitivity, the ability to distinguish shades of gray in a scene, can be measured for a mouse's vision by monitoring the head-turning reflex optokinetic response to seeing a drifting grating. Sensitivity varies with grating spatial frequency and is relatively low at both high and low frequencies and is higher at middle frequencies. A mouse’s eyes open about two weeks after birth (postnatal day (P)14), and the optokinetic response measured at P16 has significantly and profoundly lower contrast sensitivity across a broad spectrum of spatial frequencies compared to the adult level, which is reached over the ensuing two weeks (by P28). We hypothesized that, if this immaturity in visual behavior were due to an immaturity in the retina, P16 retinal ganglion cells (RGCs) would require higher contrast stimuli to generate light responses than would P28+ RGCs.
RGC spiking responses were recorded extracellularly from P16-17 and P30+ retinas in vitro using a multielectrode array recording system while spot stimuli were presented at multiple locations across the array, repeated at 10 different contrasts. The spot intensity was switched every 0.5 sec from higher to lower intensity versus the background for 7 sec. Spot locations centered over the RGC receptive fields were determined by mapping using a Gaussian white-noise checkerboard stimulus. For each RGC, we determined the first contrast that caused a response, which was defined to require at least 1 spike in 3 of the last 4 spot cycles in a set period after an intensity change and a peak rate over twice the background firing rate for that cell.
We found that both ages had many cells that could respond to low contrasts. Both ages also had cells that required higher contrasts to stimulate responses. Nonetheless, younger retinas had a significantly higher proportion of cells that required higher contrasts, i.e., RGCs that had a higher contrast threshold. The young RGCs also had significantly different firing properties, including longer latencies and lower peak frequency on average.
These differences in threshold do not appear to be large enough to account for the profoundly lower contrast sensitivity of optokinetic reflexes in the younger mice, unless a small subset of RGCs drives the response.
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