Abstract
Purpose: :
An Off ganglion cell receives excitation from the Off bipolar cell and cross inhibition from the On pathway via the AII amacrine cell. The convergence of excitation and inhibition on the Off ganglion cell is hypothesized to improve its visual performance.
Methods: :
We tested the visual performance of the Off alpha ganglion cell in an in vitro preparation of the guinea pig retina. The visual task was to detect a low contrast full-field flash (100 ms). The reliability of detection was accessed by ideal observer analysis (Fisher LDA, criterion accuracy: 90% correct).
Results: :
We measured the performance of the spike train recorded extracellularly by loose patch. Blocking cross inhibition from the On pathway with L-AP4 increased spontaneous spiking, and increased threshold contrast from 3.3 ± 0.4 to 7.0 ± 1.4%, confirming that inhibition improves visual performance. To test the idea that inhibition improves performance by hyperpolarizing the membrane potential, we recorded spikes in whole cell mode and found that blocking inhibition depolarized the cell and increased threshold (from 4.1 ± 0.5 to 7.0 ± 1.5%). Injecting negative current to repolarize the membrane reduced spontaneous spiking yet failed to reestablish threshold contrast to control values (6.9 ± 2.3 vs. 4.1 ± 0.5% in control), suggesting that inhibition improves performance not by hyperpolarizing the cell but by adding information to the membrane potential. To test this idea, we measured the performance of the membrane potential recorded in the whole-cell mode with Na and K currents blocked by including QX-314 and Cs in the pipette solution. Blocking cross inhibition depolarized the membrane potential and increased contrast threshold (from 2.7 ± 0.9 to 9.9 ± 3.1%), as it did for the spikes.
Conclusions: :
Because blocking inhibition decreases the visual performance of the membrane potential, we infer that inhibition transmits information from the On to the Off pathway that contributes to the contrast detection task.
Keywords: detection • ganglion cells • amacrine cells