Figure 2.

HSE responses during stimulation with reconstructed optic flow, as experienced by freely walking flies approaching a goal. The external texture cues were either stationary or moving (see insets above data plots). The gray vertical lines indicate the touchdown of the left midleg. Red shaded areas indicate phases of relatively straight walking. Blue shaded areas mark the recorded membrane potentials of the right and the simulated left HSE-cell during stop phases, in which retinal image motion is evoked only by the motion of external textures and related to bar positon. (A) Original and stride-removed yaw orientation of the head relative to a horizontal axis in the walking area. Positive values indicate a leftward orientation and negative values indicate a rightward orientation relative to a horizontal axis in the walk area. Yaw saccades are indicated by the black arrows. (B) Angular velocities of the yaw orientations depicted in (A). Positive values represent leftward turning velocities while negative values symbolize turning velocities to the right. Exemplary yaw saccades are indicated by black arrows. (C) Average responses of left HSE-cells (resting potential subtracted) to behaviorally generated optic flow. Note that left HSE responses are approximated by recording from right HSE cells stimulated with mirror-symmetrical movies. In the upper row, responses to the original and stride-removed stimulus movies under the respective visual condition are shown. The lower row illustrates responses to original movies and to movies with modified texture properties. (D) Azimuthal position of the bar relative to the head yaw orientation of the fly. The dotted lines represent the motion direction of the bar or background texture, respectively. Note that the angle subtended by the fixation point and an arbitrary other point on the screen is given by the arctangent of the ratio between the metric distance between these two points and the distance of the observer from the point of fixation. Hence, the angular retinal position of the bar’s center converges to the angular position of one of its edges, if the fly fixates and approaches the other edge. (E) Average responses of right HSE-cells to behaviorally generated optic flow. As in (C), the upper row, illustrates responses to the original and stride-removed stimulus movies while the lower row illustrates the responses to the original and modified movies. Sample size: stationary condition: 8 flies, except stride-removed left: 5 flies and stride-removed right: 7 flies; moving bar condition: 7 flies; moving background condition: 7 flies except stride-removed left: 3 Flies.