High-precision Repetitive Firing in the Insect Optic Lobe and a Hypothesis for its Function in Object Location

WE have encountered, in the optic lobe of the insect Calliphora erythrocephala, a layer of elements firing spontaneously at a very constant rate of about 50 spikes per sec. The observed waveform depends on the position of the pick-up electrode and on the temperature, bnt in a given spike-train the rise-time is constant within 20 µsec. A photograph of a typical spike is shown in Fig. 1, In these experiments (more than a hundred), the animals were intact except for a small window cut in the back of the head, and were able to survive for many hours. Several different types of electrodes have been used for recording, for example, Pt-Ir glass-coated electrodes and either KCl-filled or metal-filled pipettes (tip diameter 10–15µ). Both single-ended and differential pre-amplifiers, with direct as well as capacitive interstage coupling, were used. We used batteries as well as mains-fed power supplies. Checks were performed with electrometer tube input whereby the grid current was less than 10−12 amp.