A quantitative analysis of ipsilateral tectal unit responses to moving stimuli in frogs

Ipsilateral retino-tecto-tectal (IRTT) units were recorded extracellularly in the rostral optic tectum of the frog (Rana esculenta). The activity of 79 superficial units (II type) was quantified in response to black disks of various sizes, moved vertically at various angular velocities and against a white background. The contrast ¦C¦ was constant during the experiments. Neuronal activity was analysed by two methods, yielding identical results:

(1)	I1 units responded transiently to moving and movement gated stationary stimuli; these units did not seem to be directionally sensitive nor responsive to changes in background illumination. Fifty-three % of units had a low spontaneous activity.
(2)	A power function relating mean firing frequency (¯R) and angular velocity (v) was established in the majority (78%) of units. The exponent and the constantk were 0.44–0.8 and 8.9–20, respectively.
(3)	The relationship between¯R and stimulus diameter (D) was best expressed by a logarithmic function. The maximum response occurred forD= 2–4. The optimal stimulus diameter was found to be independent of stimulus velocity.
(4)	When stimulated repetitively under steady conditions, I1 units showed about 10% fluctuations in mean response, which seemed to increase with stimulus diameter.

The results show that qualitatively and quantitatively, the properties of I1 units are very similar to R1–R2 (sustained) ganglion cells.