Mapping predictive facilitation in a dragonfly target neuron

• ¹ The University of Adelaide, School of Medical Sciences, Australia

Dragonflies are masters of aerial pursuit, executing prey capture flights with a 97% success rate. In perching Libellulids, such flights are brief (average 184ms, [1]), although target motion is also tracked from the perch prior to take-off. However many dragonflies also engage in longer duration territorial and courtship pursuits of conspecifics, that can last tens of seconds. These chase sequences include changes in velocity and direction, as well as the possibility of target occlusion by matched background texture. Recent work shows that responses of dragonfly small target motion detector (STMD) neurons may be facilitated over prolonged time courses (up to 500ms) of continuous motion [2,3]. We hypothesize that such facilitation may play a role in increasing robustness by predictively increasing the gain of detectors in the direction of future travel.

We tested this in CSTMD1, a dragonfly neuron recently shown to express a form of selective attention for one target in the presence of a distractor [4]. We presented single target stimuli that moved along an initial ‘priming’ path for 500ms before undergoing spatial, temporal or combined discontinuities in their trajectories. We quantified the facilitation state by comparing the neuronal response (spike rate) in a 200ms window following the discontinuity with that for a naive control, tested at the same receptive field location. We found that facilitation is initially spatially localized: only the smallest spatial displacement tested in the direction of target travel (4°) gives significantly stronger responses than control (Fig 1, dashed line). When larger spatial displacements were combined with a delay in reappearance, however, responses were significantly elevated, even for a 20° displacement with a 500ms delay in reappearance. Backward displacements (i.e. across previously traversed location) yield strongly inhibited responses. This suggests that facilitation is mediated by a process of local gain modulation that actively spreads from the last seen location of a stimulus and in the approximate direction of travel. Such predictive modulation of local target salience may be a key mechanism for selective attention during target tracking.