Fig. 3. Responses of a POL1-neurone (A) and a polarization-sensitive photoreceptor (PS10) of the dorsal rim area (B) to rotating e-vector orientation at very low light intensities. The dark-adapted cells were stimulated with a small-field blue stimulus (2° diameter, 443nm for A, 1° diameter, 440nm for B) positioned in the centre of their visual fields, and the e-vector orientation was rotated forwards and backwards by 360° (see ascending and descending line at the bottom of the graphs). Light intensity is indicated at the upper right of each response trace. Light intensity increases in steps of approximately 1 log unit from the bottom to the top traces. The bottom trace in A shows the threshold response of the POL1-neurone (half-maximal response). Note that the photoreceptor absorbs only approximately 1 photons^-1 in a random fashion at the same light intensity (see quantum bumps in bottom trace of B), i.e. no response modulation can be detected. (A) The first 2s of the POL1-neurone traces show spontaneous activity in the dark. (For details of the electrophysiological recording technique, see Blum and Labhart, 2000; Petzold, 2001.)