The insect antennal lobe is the first brain structure to process olfactory information. Like the vertebrate olfactory bulb the antennal lobe is substructured in olfactory glomeruli. In insects, glomeruli can be morphologically identified, and have characteristic olfactory response profiles. Local neurons interconnect glomeruli, and output (projection) neurons project to higher-order brain centres. The relationship between their elaborate morphology and their physiology is not understood. We recorded electrophysiologically from antennal lobe neurons, and iontophoretically injected a calcium-sensitive dye. We then measured their spatio-temporal calcium responses to a variety of odours. Finally, we confocally reconstructed the neurons, and identified the innervated glomeruli. An increase or decrease in spiking frequency corresponded to an intracellular calcium increase or decrease in the cell. While intracellular recordings generally lasted between 10 and 30 min, calcium imaging was stable for up to 2 h, allowing a more detailed physiological analysis. The responses indicate that heterogeneous local neurons get input in the glomerulus in which they branch most strongly. In many cases, the physiological response properties of the cells corresponded to the known response profile of the innervated glomerulus. In other words, the large variety of response profiles generally found when comparing antennal lobe neurons is reduced to a more predictable response profile when the innervated glomerulus is known.

Translatory movement of an animal in its environment induces optic¯ow that contains information about the three-dimensional layout of the surroundings: as a rule, images of objects that are closer to the animal move faster across the retina than those of more distant objects. Such relative motion cues are used by¯ies to detect objects in front of a structured background. We confronted¯ying¯ies, tethered to a torque meter, with front-to-back motion of patterns displayed on two CRT screens, thereby simulating translatory motion of the background as experienced by an animal during straight¯ight. The torque meter measured the instantaneous turning responses of the¯y around its vertical body axis. During short time intervals, object motion was superimposed on background pattern motion. The average turning response towards such an object depends on both object and background velocity in a characteristic way: (1) in order to elicit signi®cant responses object motion has to be faster than background motion; (2) background motion within a certain range of velocities improves object detection. These properties can be interpreted as adaptations to situations as they occur in natural free¯ight. We con®rmed that the measured responses were mediated mainly by a control system specialized for the detection of objects rather than by the compensatory optomotor system responsible for course stabilization.

Object detection on the basis of relative motion was investigated in the¯y at the neuronal level.