A Neural Investigation into the Maximal and Optimal Speeds of Prey Detected by Visual Neurons in Aeshna Dragonflies

Within the dragonfly’s brain are a group of neurons (small target movement detectors) which are believed to provide visual information to the target-selective descending neurons (TSDNs) which have been found to directly control wing movements in response to visual stimuli. Three different sized stimuli at 7 different positions were projected onto a movable mirror which reflected the projection onto a screen for the dragonfly. The mirror was then rotated at increasing speeds to move the stimulus up and down across the screen at various speeds. We recorded and analyzed the neuronal spikes in response to this movement via a hook electrode adjacent to the TSDNs of a dragonfly secured to a mount. Across all dragonflies tested, the frequency of neuronal spikes increased as the size of the stimulus increased. This increase of neuronal spikes was also observed in response to targets that were closer to the center of the screen versus the peripheral positions. We also found that the TSDNs selective for upward moving targets (MDT1) responded with increasing frequency with increasing speed until a peak is hit at a relatively slow speed, after which the frequency of spikes precipitously decreases. The TSDNs responsible for downward moving targets (MDT4) had a relatively lower frequency of spikes that appeared constant across most speeds but still remained responsive to our fastest stimuli. Surprisingly, we found that the TSDNs responded reliably to targets moving at speeds as high as 10,000°/s