Spatial frequency peaks in the responses of the locust DCMD axon to rotation of periodic patterns, lines and dot arrays

Abstract

Responses in the locust descending contralateral movement detector (DCMD) neurone to rotation of various types of test pattern were recorded. Peaking of response at high spatial frequencies was found for both radial and chordal patterns of uniform stripes, but the type of result was different in the two cases. The distance of the pattern from the eye had a marked effect on the result obtained for radial patterns, which could not be accounted for simply in terms of the reduction in whole-pattern subtense which accompanied greater distances. Reduction in pattern area, by masking at a fixed distance, produced a steep fall in response only when the reduction reached one-eighth of the total area. Single line objects were more effective than complete gratings for comparable subtenses, and the response to such objects rose smoothly and monotonically with subtense. Parallel pairs of thin lines were not discriminated from solid lines of the same subtense at angles below about 0.2°. Above this angle there was marked discrimination, the solid lines becoming much more effective. This result was in general true for both black lines on white and white lines on black. However, in contrast to the monotonic rise shown by solid lines for either contrast, there was an initial peak in the paired-line response, occurring in the range 0.12° (white on black) to 0.2° (black on white). Single discs also showed a smooth rise with increase in subtense. Circles (‘hollow discs’) were less effective than discs. Rotation of arrays of black dots produced peaking in response at about 0.38°, far exceeding the response for a single dot of the same size. Above 1.5° the response to a single dot exceeded that to an array. It was concluded that, as in the experiments with on/off stimulation with stationary targets reported earlier, spatial frequency peaks occurred also for moving targets, but only when these had a subdivided nature, e.g. periodic lines, paired lines or dot rasters. The peaks found were in the same range as those for stationary targets, i.e. 0.3–0.1°, or about 3–10 cycles/degree.