Dynamic neuronal activity across the frog retinotectal map in response to a moving object revealed by simultaneous multiunit recording

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It has been well known that a retinotopic map is represented in the frog optic tectum. This suggests that spatial distribution of spike activity in the tectum is dynamically changed in response to movement of retinal image. In this study, by using multielectrode arrays, we examined spatiotemporal pattern of physiologically characterized retinal ganglion cell's and tectal neuron's activity to a black square expanding in looming fashion and with constant velocity.The two types of computer generated stimuli with various optical parameters were presented to right eye of the frog, Ranacatesbeiana. Microelectrode arrays, arranged in a geometric 5 x 5 grid were inserted into the left optic tectum in MS222 anesthetized animals, and neuronal activity across retinotectal map was measured.This experiment revealed neuronal activity expanding across the optic tectum in the manner particular to time course of expansion of retinal image. It showed that, as for looming stimulation, temporal pattern of presynaptic activity is quite different between the center of expansion and peripheral sites. The former showed sustained activity in early stage of expansion, while the latter showed transient activity during rapid expansion. On the other hand, as for expansion with constant velocity, the neuronal activity was transient in both sites. The similar spatiotemporal pattern of activity were observed in both R1/2 and R3 retinal ganglion cells. These results suggest that early sustained activity in the center of expansion and subsequent transient activity in peripheral region are essential for elicitation of frog escape behavior to looming stimulation. Furthermore, in the deep tectal layers, putative looming sensitive neurons were found, which have extremely wide receptive field and show strict tuning to collision course.