Principal neuronal organization in the frog optic tectum revealed by a current source density analysis.

In the frog optic tectum, the spatiotemporal pattern of neuronal activity evoked by electrical stimulation of the optic tract was examined by means of a current source density (CSD) analysis. The CSD depth profile was highly reproducible in different experiments. In all seven CSD profiles, three current sinks A, B, and D were observed in the retinorecipient layers. Four out of the seven profiles show additional two sinks C and E below the retinorecipient layers. Very small and short lasting sinks related to afferent fiber activities precede sinks A and B by about 1 ms, which could be accounted for by monosynaptic delay, in the corresponding depth region. The earliest prominent sink A at the bottom of the retinorecipient layers reflects only excitatory monosynaptic activities derived from R3 and/or R4 retinal ganglion cells. The second prominent sink B in the superficial retinorecipient layer is composed partly of excitatory monosynaptic activity from medium-sized myelinated optic fibers. It may involve excitatory monosynaptic activity from unmyelinated optic fibers and further polysynaptic activity. The fourth prominent sink D in the intermediate retinorecipient layer partially reflects excitatory monosynaptic activity derived from unmyelinated optic fibers. It may also involve further polysynaptic activity. In contrast with these three sinks, the third prominent sink C and fifth sink E exclusively reflect intratectal polysynaptic activity that has not been reported in any previous CSD studies in the frog optic tectum. These sinks almost overlap spatially in the tectal layer. We also measured the intratectal resistance changes and computed inhomogeneous CSD depth profiles to show that the results from homogeneous CSD computation assuming constant conductivity are valid for our present study. Finally, we compared the present results with previously reported CSD studies on the frog optic tectum and discuss consistencies and discrepancies among these experiments.