Inhibition and Excitation in the Locust DCMD Receptive Field: Spatial Frequency, Temporal and Spatial Characteristics

Abstract

ABSTRACT The descending contralateral movement detector (DCMD) of the locust responds vigorously to small target (ca. 5°) stimuli; this response is inhibited by simultaneous or subsequent rotation of a radial grating (windmill) pattern (subtending 19-90° of visual angle) and suppressed by earlier rotation, The excitation produced in the DCMD by rotation of a radial grating pattern depends only on the spatial frequency of the stripes of the pattern, and is independent of pattern size, and of temporal frequency over the range of low values used. The inhibition produced by this same stimulus similarly depends only on the spatial frequency of the stripes of the pattern, independent of pattern size, and of temporal frequency over the range of low values used. As the radial grating excitation decreases with increasing spatial frequency, the inhibition increases until limited by optical and neural resolution. For spatial frequencies of the radial grating pattern below 0·05 cyc/deg the radial grating patterns become excitatory. Above 0·05 cyc/deg they are inhibitory. This is the point in spatial frequency below which inhibitory grating ‘backgrounds’ become excitatory targets. Inhibition decreases as the size of the radial grating pattern is decreased below 19° visual angle; at 8° or less no inhibition can be found at any spatial frequency. Inhibition is greater in the posterior than anterior regions of the receptive field, and greater in the ventral than the dorsal regions. Inhibition decreases as the distance between small target and the radial grating is increased, but this is influenced by the local variations of excitation and inhibition. Habituation is often greater for small target and low-frequency radial grating response than for inhibited small target and high frequency grating response. These results substantiate previously proposed lateral inhibition models of the acridid movement detector system.