The effect of stimulus velocity on the response of movement sensitive neurons of the frog's retina

Summary 1. By means of metal-filled micropipettes the action potentials of 4 different classes of optic nerve fibers were recorded in Rana esculenta. The relationship between the angular velocity of the stimuli and the neuronal response was determined.2. If an object smaller than the excitatory receptive field (ERF) was moved through the receptive field of the different classes of retinal units the response depended on the angular velocity, contrast and size of the stimulus. The response was measured as the average impulses frequency (R) during the traverse of the ERF. Between R and the angular velocity (v) the equation R=k·v^c [impulses · sec^–1] was found. The exponent c was 0.5 for class 1 neurons, 0.7 for class 2 neurons, and 0.95 for class 3 neurons. In class 4 neurons the response to large stimuli increased linearly with the increase of the angular velocity, while no systematic relationship between R and v was valid for small moving stimuli (<5°)3. If the contrast or the size of the stimuli was changed the exponent c was not changed; but k depended on both parameters and on the direction of the contrast against the background. The power function was no longer valid if stimuli considerably larger than the ERF were used. The exponent c was independent of the type of the movement (linear, non-linear, irregular movement); it was also independent of the direction of the motion.4. A model of the receptive field is demonstrated. In this model an RC-filter function within the bipolar cells is assumed. The bipolar cells with different filter function activate different classes of ganglion cells. Different time constants of the bandpass filter at the bipolar cell level are the main cause for the different exponents of the power function between angular velocity and neuronal response.