Spike Generator Limits Efficiency of Information Transfer in a Retinal Ganglion Cell

Abstract

The quality of the signal a retinal ganglion cell transmits to the brain is important for preception because it sets the minimum detectable stimulus. The ganglion cell converts graded potentials into a spike train with a selective filter but in the process adds noise. To explore how efficiently information is transferred to spikes, we measured contrast detection threshold and increment threshold from graded potential and spike responses of brisk-transient ganglion cells. Intracellular responses to a spot flashed over the receptive field center of the cell were recorded in an intact mammalian retina maintained in vitro at 37°C. Thresholds were measured in a single-interval forced-choice procedure with an ideal observer. The graded potential gave a detection threshold of 1.5% contrast, whereas spikes gave 3.8%. The graded potential also gave increment thresholds approximately twofold lower and carried ϳ60% more gray levels. Increment threshold "dipped" below the detection threshold at a low contrast (Ͻ5%) but increased rapidly at higher contrasts. The magnitude of the "dipper" for both graded potential and spikes could be predicted from a threshold nonlinearity in the responses. Depolarization of the cell by current injection reduced the detection threshold for spikes but also reduced the range of contrasts they can transmit. This suggests that contrast sensitivity and dynamic range are related in an essential trade-off.

Retina is structured to process an excess of darkness in natural scenes

“…To evaluate the fraction of OFF elements in the optimal mosaic, we recalled that the signal-to-noise ratios (SNRs) of ON and OFF receptive fields and redundancies within their arrays are similar (10,27). Thus, it was helpful to define parameters that characterized both the average of the number of signaling levels in ON and OFF channels,…”

“…Assuming similar intrinsic SNRs and redundancies in the ON and OFF pathways (μ ≈ 0; γ ≈ 0), ∼10 signaling levels (l ∼10) (27) and p OFF -p ON ∼ 0.1 (Fig. 2) then gives…”

“…For all filter radii, negative contrasts (dark regions) were more numerous. To quantify this asymmetry, we measured the proportion of negative contrasts, setting response threshold to match measurements from ganglion cells (25)(26)(27). As filter width increased, the contrast distributions flattened, causing the number of subthreshold responses to decrease.…”

Detection Sensitivity and Temporal Resolution of Visual Signals near Absolute Threshold in the Salamander Retina

“…Thus, to explain the discrepancies between RGC sensitivity and the rod limit, thresholding must occur after the pooling of rod signals. Candidate mechanisms include rectification at the bipolar-RGC synapse and in RGC spike generation (Dhingra and Smith, 2004).…”

“…Understanding the relationship between rod noise and visual sensitivity requires consideration of all three rod noise sources, especially because continuous noise may play a more important role in some species than others (Baylor et al, 1980(Baylor et al, , 1984. Second, limitations in downstream processing of rod signals, such as channel noise, synaptic transfer, and spike generation, could limit the fidelity of photon sensing (Dhingra and Smith, 2004). Third, previous studies focused only on the ability to detect a dim light.…”