Dragonflies are commonly known for their beauty and color but the neuroscience community adore them for several other characteristics, one of which is their near instantaneous ability to respond to changes in the movement of a target. This speed at which the dragonflies are able to communicate this visual information into mechanical adjustment of their wings can be attributed to several neurons. Within the dragonfly’s brain are a group of neurons (small target movement detectors) which are believed to provide information to the target-selective descending neurons (TSDNs) which have been found to directly control wing movements in response to visual stimuli. TSDNs have also been found to prefer a dark target on a light background such as a fly against the sky during the day (as would be seen in its natural environment) or a black square on a white background (as seen in an experimental environment). By extension, the neurons within this group contribute significantly to the extremely successful rate that dragonflies have for catching prey they pursue. This level of success brought to question whether dragonflies were selective for the speed at which their target moved and how these neurons communicate this information.
To do this, we utilized a hook electrode adjacent to the TSDNs and stabilized the dragonfly on a mount. 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 moved up and down at increasing speeds to move the stimulus up and down across the screen at various speeds. The neuronal spikes in response to this movement was recorded and analyzed. 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), on the other hand, had a relatively smaller frequency of spikes that appeared constant across most speeds but also remained responsive our fastest stimuli.