Evidence for velocity-tuned motion-sensitive descending neurons in the honeybee.

Behavioural experiments suggest the existence of two functionally distinct movement-sensitive pathways in honeybees: one mediates optomotor behaviour, consisting of reflexive turning responses preventing deviations from course, and the other controls flight speed. The first consists of direction-selective neurons responding optimally to a particular temporal frequency of motion, regardless of the pattern's spatial structure. The temporal frequency dependence matches the temporal tuning of the optomotor output. Behavioural experiments suggest the second pathway contains velocity-tuned cells, which generate equal-sized responses for any given image velocity, for patterns with a range of spatial structures. Here, recordings were made from direction-selective neurons in the honeybee's ventral nerve cord. Neurons were tested for responses to motion at velocities of 40-1000 deg s(-1) using four gratings with spatial periods of 11-76 degrees. In addition to temporal frequency-dependent optomotor neurons, direction-selective cells were found that had the same shaped velocity-response functions for all four patterns. The velocity-tuning properties of these cells suggest a possible role in monitoring flight speed because their velocity tuning matches the image velocities encountered during free flight and landing behaviour.