Receptive field characteristics of a directionally selective movement detector in the visual system of the blowfly

SummaryAction potentials evoked by stepwise-moving visual stimuli have been recorded extracellularly from the horizontally selective movement detectors in the lobula complex of the blowflyCalliphora erythrocephala (M.).Regarding the relation between the ommatidial mosaic structure of the compound eye and the behaviour of the detector, the following experimental results are obtained:a)Maximum reactions are obtained for stimuli which move with a stepwidth equal toδϕh, whereδϕh is the effective interommatidial angle.b)When using stepwise moving single contrast borders the reaction to a moving Dark over Bright contrast border is always greater than or equal to the reaction evoked by a moving Bright over Dark contrast border.c)From experiments with spatially small stimuli in which only a few rows of adjoining ommatidia are stimulated, the conclusion is drawn that the sensitivity distribution of the detector is spatially modulated with a period which equals the interommatidial angleδϕ.d)If the spatial wavelengthλs of a square wave grating is adjusted in such a way thatδϕh<λs<2δϕh, the reaction of the detector is reversed: movement in the preferred direction results in inhibition and movement in the null direction results in excitation.