Sensory coding of complex visual motion in the locust (Locusta migratoria)
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The visual environment of any animal is a complex amalgamation of sensory information (Lochmann and Deneve, 2011); however, it is adaptive for an animal to only react to salient cues (Zupanc, 2010). For many organisms, the detection of an approaching object, such as an oncoming conspecific or a predator, is particularly important. An approaching object with constant velocity is called looming, and has been widely studied for evoking avoidance behaviours in a number of animal species (Gibson, 1958). The migratory locust, Locusta migratoria, has been used extensively as a model system for visually guided behaviour, due to its robust collision-avoidance behaviours and its tractable nervous system (Schlotterer, 1977). The Lobula Giant Movement Detector (LGMD) and the Descending Contralateral Movement Detector (DCMD) constitute one pathway in the locust visual system that integrates the entire field of view that has been implicated in coordinating these types of behaviours (Santer et al., 2006). Previous studies have found that the LGMD/DCMD pathway responds to many visual stimuli, including complex scenes (Rind and Simmons, 1992), approaching paired objects (Guest and Gray, 2006), objects with compound shapes (Guest and Gray, 2006), and objects that follow compound trajectories (McMillan and Gray, 2012). These findings suggest that this pathway is capable of encoding complex motion such as exists in the locust’s natural environment. In my first objective (Chapter 2), I tested the response of the locust DCMD to increasingly complex motion. Using computer generated disks that followed compound trajectories with different velocities, I demonstrate that the DCMD is capable of encoding the location, trajectory, and velocity of an approaching object through aspects of the response profile over time. The motor systems of invertebrates are often controlled by ensembles of neurons working together (Dubuc et al., 2008; Hedrich et al., 2011; Gonzalez-Bellido et al., 2013). The locust visual system has at least five identified descending neurons, beyond the DCMD, that respond to visual motion (Rowell, 1971; Griss and Rowell, 1986; Gray et al., 2010). Due to the tractability of extracellular recordings of the DCMD, these neurons remain relatively little studied. Furthermore, their responses to stimuli have not been investigated concurrently. With recent advancements in multichannel recordings and spike sorting algorithms, it is now possible to explore the responses of multiple neurons in the locust system together. In my second objective (Chapter 3), I recorded from the connective of the locust using multichannel electrodes while challenging it with a wide array of visual stimuli. Preliminary results of these experiments identified as many as five neuronal units with distinctive firing patterns, some which appear to be novel. Together, these results illustrate that the locust visual system is more complex than previously thought, through both the abilities of a single neuron to encode many aspects of visual motion and the presence of multiple unique, visually-sensitive neurons.
DegreeMaster of Science (M.Sc.)
CommitteeMarchant, Tracy; Howland, John
Copyright DateSeptember 2013