Figure 1. Motion detection and the fly optic lobe.
A. A half Hassenstein-Reichardt correlator (HRC) sensitive to rightward motion. An object moving from left to right first activates input 1 and then input 2. The signal from input 1 is delayed (τ) and arrives at the correlation stage (M for multiplication) close in time to the signal from unit 2, nonlinearly enhancing the signal. For leftward motion, the signals are separated in time by the delay and no motion signal is generated. In the full correlator model, two mirror symmetric correlators are subtracted, producing responses that have opposite signs for opposite directions (see Figure 4A).
B. Light edge (L1) and dark edge (L2) motion sensitive pathways in the Drosophila optic lobe. L1 and L2 lamina monopolar cells in the lamina provide inputs to two distinct motion sensitive pathways that selectively respond to moving light edges and dark edges, respectively. L1 and L3 also contribute to the pathway detecting moving dark edge (not shown). T4 and T5 in the lobula complex are the main inputs to LPTCs, and are themselves direction selective. T4 neurons respond selectively to moving light edges and T5 neurons respond to moving dark edges. Mi1 and Tm3 are the main postsynaptic targets of L1 while Tm1 and Tm2 are the main postsynaptic targets of L2. The axons of Mi1 and Tm3 contact T4 in the most proximal medulla layer, whereas Tm1 and Tm2 contact T5 dendrites in superficial lobula layers (modified from ref. 30).
C. In vivo electrophysiology set up: A window is cut in a dorsal region of the head cuticle of an immobilized live fly to expose the cell bodies of medulla neurons to a glass pipette used to perform the recordings. Grey-scale images are displayed on a screen positioned in front of the fly, using a digital light projector (DLP) coupled to a coherent fiber optic.