Fig. 2. Identification of DCMD activity within the mesothoracic ganglion. (Ai) Multichannel silicon microprobes were inserted ventrally into the mesothoracic ganglion (outline shows scaled frontal view). Each of the probe's two tines contained two tetrode arrays (T1–T4). The arrangement of the recording sites was as follows: site 1–4 on T1, sites 5–8 on T2, sites 9–12 on T3, sites 13–16 on T4). Thus, sites 1–8 recorded activity from the left side of the ganglion and sites 9–16 recorded activity from the right side of the ganglion. (Aii) Recordings from a sample preparation showing waveforms overlaid in a 1.3 ms time window. The colours of the waveforms correspond to clusters identified in Aiii, which shows a plot of the peak of the waveforms of recording site 3 vs recording site 2 (see details in text for recording parameters). Well-defined clusters were easily identified in all preparations. (Aiv) 5 s time window showing the activity of the discriminated units during the presentation of a simulated looming bird. The blue and red units had the largest spike amplitudes and showed an increase in firing rate during an approach, characteristic of locust DCMDs. The arrow indicates time of collision. (B) Identification of the right and left DCMDs based on physiological responses to looming stimuli. The blue (right DCMD) and red (left DCMD, see Materials and methods for designation) rasters represent the spike times, during different approaches, of the units shown in A. In this example, each raster was time-aligned to the parameters of the approaching `bird', where 0=time of collision. The top two rasters show responses to a `bird' approaching from 0° azimuth (0°), which would be seen by both eyes. The middle rasters show responses to a `bird' approaching from +45° azimuth (+45°) and the bottom rasters show responses to a `bird' approaching from –45° azimuth (–45°). Because approaches from ±45° azimuth would occupy the field of view of the ipsilateral eye only, these criteria could be used to discriminate unambiguously right and left DCMD activity for all runs for each animal. B_b and B_w as in Fig. 1.