A functional analysis of the excitatory synaptic inputs to ON-OFF ganglion cells in the aquatic tiger salamander retina
The majority of ganglion cells in the salamander retina receive excitatory synaptic inputs from the ON and the OFF pathway, but little is known about the functional role of these cells. Previous studies suggest that they might be involved in the processing of complex visual features, however attempts to categorize cells into distinct groups of complex feature detectors have failed.
This work focuses on the functional analysis of the excitatory synaptic inputs to ON-OFF ganglion cells recorded in the retinal slice preparation. Responses to stimulation with contrast steps and random luminance sequences were analyzed regarding their temporal dynamics and contrast processing. We found that cells lie along a continuum of varying relative contributions from the ON and the OFF pathway and show asymmetries regarding contrast processing and temporal dynamics in the OFF and ON inputs.
We further investigated the use of a covariance based analysis to separate the ON and OFF components in current responses to white noise stimulation. Computer simulations were used to evaluate the performance of the analysis under different conditions. The analysis performs well in predicting the responses of simulated ON-OFF cells, however, caution needs to be applied in the physiological interpretation of the components of the estimated model, in particular if the assumption of contrast rectification is violated.
Finally, we show how ON-OFF cells can account for complex temporal response patterns. We show how a simple ON-OFF model can produce spiking responses that seem to code for violations in periodic temporal patterns. The model is consistent with the results of patch clamp recordings of the excitatory current responses to periodic flash sequences. The model further demonstrates how relatively small changes in the static nonlinearities within the ON and the OFF pathway can produce a variety of seemingly complex response patterns in the spike output.
In conclusion, ganglion cells in the salamander retina form a diverse group of cells varying along an ON-OFF continuum. The temporal asynchrony between the two pathways as well as variations in contrast rectification can account for complex response patterns observed in those cells and might play an important role in visual coding.