Quantitative studies on ipsilateral type 2 retinotectotectal (IRTT) units in frogs: homologies with R3 ganglion cells

Contralateral and ipsilateral responses from deep layers of the rostral neuropil of the optic tectum of Rana esculenta were recorded extracellularly and quantitatively analyzed. Effects of the velocity and diameter of the stimulus on the neuronal response (measured as the mean firing frequency, R) were mainly tested in this work. 1. Among the population of changing contrast or event ganglion cells, R3-like units (with a weak response to background off-on stimulation) were defined in addition to typical R3 ganglion cells. 2. A power function relating R and the stimulus velocity (v) was established in all units (R = k v alpha), with alpha = 0.80-1.07 and k = 2.1-9.5 for R3 units, alpha = 0.55-0.77 and k = 5.8-15.2 for R3-like units, and alpha = 0.80-1.16 and k = 1.3-5.1 for ipsilateral I2 units. 3. The area function was expressed by a logarithmic function. In all classes the maximal response was obtained with 4.4 degrees-7.5 degrees targets, independent of the test velocity. 4. Both the velocity and the diameter of the stimulus influenced the value of the dynamic receptive field diameter. 5. Finally, results show that qualitative and quantitative properties of I2 units are similar to those of R3 ganglion cells.