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FIGURE 2.6-6 A polar plot of the tilt response of a protocerebral positional unit from Willey (1981). The dotted circle shows the average random ring rate of the PPU when the animal is level. There was complete suppression of the POBL ring for tilts in the sector 280° to 260°. The animal was always tilted ^ = 45° at 3.2 °/s in this and subsequent plots from Willey (1981).

Willey (1981) recorded from single PP units that showed single, very sharp directional responses. For example, see Figure 2.6-7. This unit had a strong, sharp o

FIGURE 2.6-7 An Arenivaga PPU with a very sharp directional sensitivity.

peak at . = 135°. Its background ring was strongly suppressed for tilts over . = 135° . 360° . 30°. In Figure 2.6-6, the response of the unit is not quite as sharp as the one shown in Figure 2.6-7. Its peak angle is at . = 270°, and there is strong suppression of the POBL ring for . = 285°. 360° . 255°. The fact that its peak angle is at 270° suggests that its response is in some way due to the ring of the LCPI and LIPI VNC neurons which both re for . = 270°. Other sharp unimodal response units are shown in Figures 2.6-8 and 2.6-9. Some PP units were unusual in having two or more response peaks (frequency maxima) as a function of . . One such response is shown in Figure 2.6-10. Note that the peaks are very sharp compared with the cosine(2x) response seen on the four VNC positional interneurons (RCPI, RIPI, LCPI, LIPI). They are also about 180° apart (. = 135° and 330°). A smaller, broader response peak at . = 195° is also present. Note that the peak at . = 135° is the same angle that produces maximum ring in the RIPI VNC giant ber . One possible explanation of the three-peak behavior is that three PP units having exactly the same pulse height were discriminated by the window, giving a superimposed response. Or the response may be the real (counterintuitive) behavior of a single PPU. Of special interest is the sharpening of the directional responses.

Two PP units showed broad, single peak responses. The unit shown in Figure 2.6-11 had a peak at . = 195°, and showed a small suppression of ring belo w the POBL rate over 300° to 45°. Figure 2.6-12 shows the response of another broad, unimodal unit having its peak around 315°. Some small suppression was seen for . between 15° and 90°.

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225 %.

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