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. Author manuscript; available in PMC: 2012 Jan 1.
Published in final edited form as: Vis Neurosci. 2010 Nov 4;28(1):61–68. doi: 10.1017/S0952523810000301

Figure 4. RBPs respond to single photon absorptions and noise in RBPs may result from thermal isomerizations of rhodopsin.

Figure 4

A) Examples of fits for 4 RBPs to cumulative Poisson distributions for the requirements of 1, 2 or 3 photons (N). Despite the steep nonlinearity at the rod-RBP synapse, RBPs respond to single photons.

B) RBPs in darkness exhibit low amplitude as well as larger discrete noise events. A 56 s portion of a 180 s recording of a RBP in darkness is shown. The current was segmented into 2 s sweeps for display purposes. Bandwidth = DC to 5 Hz. There are several discrete events that resemble the single photon responses depicted in Figure 3E and F. Events that exceed the threshold in C are marked with arrows.

C) Ninety two-second sweeps from the cell in A were match filtered to isolate events resembling the time course of the single photon response and overlaid. The dashed line indicates placement of the event detection threshold, which was set at the mean plus 3 times the standard deviation of the entire record. The histogram to the right shows the amplitudes of all downward deflections in the current. Most of the low amplitude noise appears normally distributed, but not the events above threshold, which we consider the result of thermal isomerizations.

D) Intervals between events detected by threshold crossings were binned and fit to an exponential to find the mean interval. The plotted histogram was generated from data compiled from 8 RBP cells, providing an estimate of 3.8 s between SPR-like events in RBPs.