Unitary EPSPs evoked by visual stimuli and spatial distribution of activated synapses in tectal neurons of the frog

Using intracellular recording techniques, we recorded the responses of tectal neurons of the frog to moving visual configurations. Most of the responses sampled in this experiment originated from class 3 retinal ganglion cells. We classified EPSPs according to their shapes into two quantitative criterions, rise time and peak amplitude. Rise time histograms for observed EPSPs had a single peak. This implies the localization of activated synapses, which is consistent with the morphological findings in frog's optic tectum. Amplitude histograms had several peaks which are regularly spaced by 0.3 to 0.6 mV. Through such examination, we successfully identified unitary EPSPs evoked by class 3 retinal fibers. We also observed unitary IPSPs in the same manner. These results were mainly obtained from responses elicited by manual movement of black cardboard. In the experiment where a computer controlled stimulus (a moving light slit) was used, the relationship between the position of the stimulus and rise time of EPSPs was examined.