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J. Liu, R. G. Smith; Precision and Signal Codes in the Alpha Ganglion Cell. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3853. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
The vertebrate retina may encode a visual stimulus in several aspects of its graded potential and spike train, reflecting different mechanisms of its presynaptic circuit. To test this hypothesis, we compared the information carried by different aspects of the ganglion cell signal.
In a whole-mount guinea pig retina maintained at 37°C, we current-clamped On (n=9) and Off (n=10) alpha ganglion cells and presented bright (for On cells) or dark (for Off cells) spots (2 Hz, 100 ms) at randomly selected contrasts (background 4.2 log photons/µm2/s). We presented responses to pairs of contrasts to an ideal observer which calculated the number of distinct gray levels (a measure of information about the stimulus) encoded in graded potential, binned spike rate and time to spike.
We confirmed that the gray levels coded in ganglion cell PSPs (~9) exceeded those in their spikes (~5). The gray levels encoded by the graded potential of both cell types were similar during the initial transient response in both PSPs (~7) and spikes (~4), but Off cells had a larger hyperpolarizing transient at stimulus offset which encoded ~3 gray levels. For a simple spike rate code, each gray level would require an extra spike, but we found that both On and Off cells transmit ~1-2 gray levels per spike, suggesting a more complex code. Indeed, measuring the gray levels encoded in the spike timing, we found the first spike for both cell types carried 2-3 gray levels.
Our results suggest different ways that alpha ganglion cells encode a visual stimulus. Their graded potential response encodes 2-fold more gray levels than their spike train, and information is encoded in both biphasic peaks. Most of the information is transmitted during the initial response transient, and the spike generator encodes mainly the transient information available in the graded potential. The time-to-spike measure confirms that spike timing is important in encoding contrast information. However, the Off-cell's hyperpolarizing transient encodes several gray levels, which would be carried by the spike train as a reduction from a high dark maintained rate. This offset transient may originate in synaptic input from the On-bipolar system through AII amacrine cells.
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