The results of these experiments are shown in Figure 2B (Callipho

The results of these experiments are shown in Figure 2B (Calliphora)

and Figure 2C (Drosophila). Lobula plate tangential cells respond to single ON or OFF steps imposed on a uniformly illuminated background with an increase in firing rate or a depolarization (see responses to the appearance of the first stripe). The direction selectivity of the motion detection circuit can be observed by comparing the responses to the second stripe with the responses to the first one. For ON-ON and OFF-OFF stimuli (first and second row in Figures 2B and 2C), the response amplitudes are larger when the stimulus sequence is in the Baf-A1 in vivo cell’s PD (red lines) than when the sequence is in the cell’s ND (blue lines). The opposite effect is observed for ON-OFF and OFF-ON stimulus sequences (third and fourth row in Figures 2B and 2C): here, the response to the second stimulus is smaller than the response to the first one when the sequence is in the cell’s PD, and larger than the first one when the sequence is in the cell’s ND. This effect SAHA HDAC order is called “PD-ND inversion” and is illustrated more clearly when the response

to an ND sequence is subtracted from the response to the corresponding PD sequence (black lines in Figures 2B and 2C): for ON-ON and OFF-OFF sequences, a positive signal is obtained; for ON-OFF and OFF-ON sequences, the resulting signal is negative. All this holds true for recordings from the H1 cell in Calliphora as well as for recordings from VS cells in Drosophila (compare Figure 2B with Figure 2C). While the responses to ON-ON and OFF-OFF stimuli can be explained by both a 4- as well as by a 2-Quadrant-Detector (Figures 1B and 1C, respectively), the responses to sequences of opposite sign (ON-OFF, Thiamine-diphosphate kinase OFF-ON) are hard to reconcile with a 2-Quadrant-Detector.

However, the phenomenon of the PD-ND inversion is in agreement with predictions from the Reichardt Detector (Figure 1A) and its mathematical equivalent, the 4-Quadrant-Detector (Figure 1B): for ON-OFF or OFF-ON sequences, signals of opposite signs are multiplied, leading to the observed PD-ND inversion. Therefore, given the splitting of the photoreceptor output into ON and OFF components, these results seem to rule out the 2-Quadrant-Detector (Figure 1C) and rather imply a motion detection circuit of the 4-Quadrant type (Figure 1B). However, the above reasoning rests on two tacit assumptions: (1) information about the absolute brightness is fully eliminated, and only information about the change of the stimulus brightness is passed on to the rectification stage and the subsequent motion detection circuits; and (2) the threshold for the rectification stage is set at exactly the zero point of the incoming signal. As soon as we drop one of these assumptions, the signal separation becomes less strict, and a 2-Quadrant-Detector might respond to stimulus sequences of opposite sign as well.

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