Response properties of descending object motion-detecting neurones in the ventral nerve cord of the common brown butterfly, Heteronympha merope

This study examined the response properties of descending neurones which were selectively excited by object stimuli moving backwards over the ipsilateral eye of the butterfly, Heteronympha merope. These neurones, termed BOMD neurones, exhibited the following characteristics. (1) The response was highly directional. The preferred stimulus is a small bar moving in the backward direction, regardless of whether the background is stationary or in motion. The optimal height of the bar is about 5°. (2) The BOMD neurones responded to high velocities of movement. The lower velocity limit for a 3 × 5° bar is between 25 and 30°/s. The optimal velocity for the bar is about 80°/s, but responses persist at velocities as high as 200°/s. (3) Although movement of the background grating did not evoke any response on its own, it altered the BOMD response to a small moving object in at least two ways. (1) The grating produced strong inhibition when it moved backwards, and less inhibition when it moved in other directions. (2) The influence of the grating was contrast-frequency dependent, with maximal inhibition occurring between 9 and 12 Hz as would be expected from the known contrast-frequency optimum for motion perception.