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Lookup NU author(s): Dr Roger Santer
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Flying Locusta migratoria respond to rapidly approaching looming stimuli by ceasing flight for a period of several hundred milliseconds. This behaviour is elicited during the final milliseconds of a looming stimulus and occurs most commonly in response to the speeds of looming stimuli which the locust should naturally encounter during flight. I have termed this reaction ‘evasive gliding behaviour’ and suggest that it is a suitable strategy for the evasion of fast aerial predators, like the carmine bee-eater bird, which prey heavily upon locusts and capture them in flight. The stimuli to which evasive gliding occurs match those to which the descending contralateral movement detector (DCMD) neuron responds. Prematurely ending a looming stimulus prior to collision alters both the DCMD spike train and the occurrence of evasive gliding behaviour. Recordings from the DCMD in tethered flying locusts reveal that evasive gliding behaviours are correlated with bursts of high frequency DCMD spikes. I suggest that the DCMD is a suitable interneuron for the mediation of this behaviour and that high frequency spikes produced in this cell may underlie an evasive glide. These high frequency spikes may be the behaviourally relevant signal produced by the DCMD neuron. DCMD spikes are known to induce large monosynaptic EPSPs in the flight motor neurons which are involved in the production of an evasive gliding behaviour. I have used computational techniques to show that rhythmical activity in these flight motor neurons is capable of gating the DCMD signal into a flight motor neuron response modulation which matches several important features of the evasive gliding behaviour itself. Several properties of the DCMD neuron suit it to a role in mediating such evasive flight behaviours. These include the ability of the neuron to discriminate potentially colliding from near-miss looming stimuli. I have used computational techniques to demonstrate that this ability results from lateral inhibitory interactions presynaptic to the lobula giant movement detector (LGMD). These lateral inhibitory interactions occur between small medullary neurons and participate in a critical race between excitation generated by a looming image and inhibition conveyed laterally from excited processing channels. I have used a computational model of the LGMD to show that the relative speeds of excitation and lateral inhibition must be tuned in order to allow the LGMD to distinguish potentially colliding from near-miss looming stimuli. However, removal of these lateral inhibitory connections produces a more robust collision tuning over a range of image speeds. I propose that although lateral inhibition contributes to the detection of objects approaching on a collision course, this was not the function for which these connections originally evolved. The DCMD does not respond to whole-field visual stimulation as would result from intentional or unintentional deviations in the locust’s flight path. The suppression of the DCMD’s response results from feed-forward inhibition generated by sudden and intense whole-field stimulation which suppresses a DCMD response during rapid or acceleratory image movements, and lateral inhibition which suppresses the cell’s response during sustained whole-field image movement at a constant speed. I conclude that the DCMD is a pathway suited to the mediation of predator evasion behaviours and I present evidence supporting its involvement in triggering avoidance reactions.
Author(s): Santer RD
Publication type: Report
Publication status: Published
Series Title:
Type: PhD Thesis
Year: 2003
Institution: University of Newcastle
Place Published: Newcastle upon Tyne
