Intracellular and current source density analyses of somatosensory input to the optic tectum of the frog

This is the first report of current source density (CSD) and intracellular analyses of non-optic processing in the frog optic tectum. Sciatic nerve stimulation was used to test for somatosensory input to the optic tectum. To demonstrate the distribution of somatosensory input, field potentials were recorded from the whole surface of both tecta. Two components were observed. An early component was found in the whole area, but a late component was detected only in medial and caudal regions of the contralateral tectum. The effect of different stimulus intensity suggested that the optic tectum receives mainly the tactile sensation with fast conducting, low threshold level afferents from the sciatic nerve. The result of CSD analysis suggests that somatosensory afferents terminate on the tectal neurons with vertically expanding dendrites at the medial site of the contralateral optic tectum where the late component was found. Intracellular recordings demonstrated postsynaptic potentials in the middle and deeper layers, which is consistent with results from mammalian superior colliculus in earlier studies. Additional stimulation of the optic tract demonstrated that some somatosensory neurons had bimodal responses. The responses of those in the middle layers appeared to participate in avoidance behavior, based upon previous CSD analysis of the tectum using optic tract stimulation. All somatosensory responses elicited in these neurons were IPSPs. The findings imply that the somatosensory input to the optic tectum gives a suppressive effect on avoidance behavior. A somatosensory effect on prey-catching behavior could not be found in the present small number of intracellular data.