Maintained activity of cells in the three shrew's optic tract.

The maintained activity of different types of ganglion cells in the tree shrew's retina has been investigated in darkness and different adaptation luminances, with particular reference to on-centre sustained and transient cells. The firing rate of on-centre sustained cells rises with increasing luminance up to 5.10(-2) W/m2 (human photopic equivalent 5.10(4) cd/m2) whereas, for on-centre transient cells no simple relationship between luminance and activity could be found. The ratio of mean and standard deviation (regularity) of the interval length of sustained cells increases with light intensity, while in most transient cells a constant ratio is observed. Various kinds of interval histograms are observed for sustained units: exponential, gamma, bimodal and other types. Transient cells tend to fire in bursts with correspondent biomodal interval histograms. The first order serial correlation coefficient is positive for the majority of sustained cells and negative for most transient cells. It is argued that the final statistical properties of the maintained activity of retinal ganglion cells are mainly determined by the retinal circuitry between photoreceptors and ganglion cells.