Experimental setup and visual stimulation.

A. Experimental setup with display showing stationary background clutter. CRT, cathode ray tube display; asterisk, fronto-dorsal corner mark of CRT; MM, micromanipulator for positioning of intracellular electrode; ELi, intracellular electrode; ELref, reference electrode; LOC, locust (viewed from dorsal); arrow, stimulated (left) eye. Note the slight tilt of the CRT and the mesh wire shielding which prevented inductive interference between display and measurement circuit. B. Region in the left antero-lateral visual field covered by the stimulus display. Arrow, position of stimulated (left) eye; asterisk, fronto-dorsal corner mark of CRT; LOC, locust (viewed from lateral). Dashed horizontal lines tagged H and L indicate high- and low-elevation trajectories for translations of single small-field squares. sr, br: commonly applied square- and bar-shaped stimulus; about 2°x1.5° and 2°x60°, respectively. Albeit both types of stimulus are shown together here, they were never presented simultaneously. B'. Terminology for directions of translational motion, illustrated according to the orientation of the diagram in B. Abbreviations are given in parentheses. Forward (backward) direction of translation is abbreviated "f" ("b") and high (low) elevation is abbreviated "H" ("L"). Thus, the term "b, L" ("f, H") refers to the backward (forward) translation of a 2°x1.5° square -29.5° (+25.5°) in elevation. See S1 Fig for commonly applied sequences of such simple stimuli. C, C'. Components (C) and time course (C') of a more complex stimulus sequence designed to mimic sudden forward small-field motion against a backward optic-background flow. The sequence begins with a stationary display of the "background clutter" pattern, followed by repeated backward translations of the entire pattern (indicated by horizontal arrows) in an optic-flow like manner at 70°/s (dashed horizontal line in C'). In the late phase of the sequence, an individual object in the background flow (short vertical arrows in C) pops out of it twice by changing its direction of motion, i.e. the object suddenly moved forward against the backward background flow (absolute velocity 80°/s; velocity relative to background flow -150°/s). The object, a black, rectangle square of about 2° visual angle azimuthal extent and 1.5° elevational extent, is identical to the one used in the simple stimulus regimes and moves at the same high elevation of 25.5° also applied in the latter. For further details on stimuli, see Materials and Methods.