CHARACTERIZATION OF COLUMNAR NEURONS AND VISUAL SIGNAL-PROCESSING IN THE MEDULLA OF THE LOCUST OPTIC LOBE BY SYSTEM-IDENTIFICATION TECHNIQUES

We describe visual responses of seventeen physiological classes of col umnar neuron from the retina, lamina and medulla of the locust (Locust a migrator ia) optic lobe. Many of these neurons were anatomically ide ntified by neurobiotin injection. Characterisation of neuronal respons es was made by moving and flash stimuli, and by two system identificat ion techniques: 1. The first-order spatiotemporal kernel was estimated from response to a spatiotemporal white-noise stimulus; 2. A set of k ernels to second order was derived by the maximal-length shift registe r (M-sequence) technique, describing the system response to a two-chan nel centre-surround stimulus. Most cells have small receptive fields, usually with a centre diameter of about 1.5 degrees, which is similar to that of a single receptor in the compound eye. Linear response comp onents show varying spatial and temporal tuning, although lateral inhi bition is generally fairly weak. Second-order nonlinearities often hav e a simple form consistent with a static nonlinear transformation of t he input from the large monopolar cells of the lamina followed by furt her linear filtering.