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J. Liu, R. G. Smith; Contrast Performance of Ganglion Cells Is Greater When Voltage Clamped. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3622.
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The quality of the signal transmitted by a retinal ganglion cell is important because it sets the minimum detectable stimulus. The level of noise sets the quality of the signal and thus its information content. To explore how efficiently information is transferred, we measured contrast discrimination thresholds from light responses of guinea pig retinal ganglion cells using a single-interval 2-alternative forced-choice paradigm with an ideal observer.
In a whole-mount retinal preparation maintained at 37ºC, we voltage- and current-clamped On- and Off-center ganglion cells, presenting dark or bright spots with optimized diameter and randomly selected contrast. The stimuli were repeated and the responses to pairs of contrasts analyzed to find the contrast increment threshold and the number of gray levels.
We measured cell performance from graded potential, spike rate, and spike latency in current clamp mode or from current during voltage clamp. We found the performance of the graded signals of both On and Off ganglion cells under voltage-clamp (-50mV, -70mV, -90mV) was greater than that under current-clamp (0 pA), yielding 15.6 ± 1.1 gray levels (mean ± SD, n = 4 ) vs 10.9 ± 3.6 (n = 4). The spike performance was usually 2-3 fold worse than the graded signal performance, consistent with previous work (Dhingra and Smith, 2005), yielding 4.9 ± 1.6 (n = 4) for spike rate and 3.5 ± 1.1 (n = 4) for spike latency, respectively. There was no significant difference in the performance between On and Off ganglion cells.
One possibility is that the voltage clamp removes noise from stochastically-gated sodium channels known to exist on the soma and dendrites. Another possibility is that when the clamp voltage is close to the inhibitory reversal potential, noise from inhibition is removed. These results suggest that the activity of sodium channels and the inhibitory input to ganglion cells may both affect their performance.
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