Fig. 11. Model for eliciting collision-avoidance and landing responses. A fly estimates the optic flow experienced during flight using a two-dimensional array of motion detectors (i). Local motion information is then spatially pooled such that the image expansion in both the lateral and frontal fields of view is calculated (ii). The outputs of each of these three expansion calculations are then temporally integrated (iii) and passed through a threshold detector (iv). Expansion detected in a lateral field of view triggers a collision-avoidance response in the opposite direction, while frontal image expansion causes a landing response (v). Lateral expansion on one side inhibits the opposite expansion pathway, preventing a saccade from being immediately followed by another saccade in the opposite direction.