Background activity and responses to small-field motion in four types of central-complex neuron.

Subfigures show data from a CL1 neuron (A-D), TB1 neuron (A'-D'), CPU1 neuron (A''-D'') and CPU2 neuron (A'''-D''') of the central complex. A-A'''. Traces illustrating background activity. B-B'''. Spike count distribution and ISI histogram of background activity (trial duration for spike count 1 s). Asterisks and x-marks indicate the location of spike rates observed at the peak (or nadir) of the trial-averaged responses shown in D-D'''. C-C'''. Responses to presentations of a translating black, rectangular small-field square (about 2° visual angle azimuthal extent and 1.5° elevational extent) against a blank background showed fast adaptation to motion along the same trajectory. Stimulus types and periods, indicated below the recording traces, also hold for raster plots in D-D''', unless indicated otherwise. Stimulus abbreviations: f (b), forward (backward) small-field translation; H (L) trajectory with high (low) elevation. D-D'''. Raster plots and trial-averaged, Gaussian-smoothed PSTHs of additional responses. Spike rates occurring at the peak (or nadir) of the averaged response (marked by asterisks or x-marks) are located at the edges of the spike count distribution obtained from long-term background activity (see B-B'''). This indicates that responsiveness is robust across the trials shown here, but response amplitudes vary and individual responses resemble sections of background activity. In particular, arrows in A' and C' mark prominent bursts in background activity and in the phasic response period for the TB-neuron. In the two subtypes of CPU neuron (D'' and D'''), responses are independent from direction of motion. Dashed lines indicate periods of rebound-like increase in spike rate in D and a period of tonic increase in spike rate in C'. Bars, 1 s; 10 mV.