| Abstract No.: | A-D1156 |
| Country: | Canada |
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| Title: | A SINGLE MOTION-SENSITIVE PATHWAY ENCODES CHANGES IN AN APPROACHING OBJECT’S TRAJECTORY. |
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| Authors/Affiliations: | 1 Glyn McMillan*; 1 Jack Gray;
1 University of Saskatchewan, Saskatoon, SK, Canada
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| Content: | Objectives: Animals must respond to important aspects of a complex visual environment (such as predators, conspecifics, or stationary objects) by generating suitable adaptive behaviours. In the locust, the lobula giant movement detector (LGMD), and its postsynaptic partner, the descending contralateral movement detector (DCMD), constitute one motion sensitive pathway in the visual system that responds preferentially to objects that approach on a direct collision course. Previously described LGMD and DCMD responses suggest that this pathway is also affected by more complicated movements in the locust’s visual environment. The approach of paired objects in the azimuthal position and approaches at different time intervals affect DCMD firing rate properties. Another study demonstrated that the DCMD responds to object deviations away from a looming trajectory by generating a distinct peak in the firing rate irrespective of object size or time of deviation. As a follow up on these findings, and in support of the idea that this single, motion sensitive pathway is able to encode complex aspects of visual scenes, we presented locusts with trajectories that switched from non-looming to looming.
Materials and Methods: Twenty intact locusts were each presented with 29 object trajectories; including a straight (control) loom at the beginning and end of each experiment and a randomized presentation of 27 different trajectories composed of looming, non-looming, and non-looming to looming trajectories at three distances from the locust. Object motion began either in the anterior or posterior region of the locust’s visual field. Each object (a single 7 cm black disc) was projected onto a specialized dome screen toward the right eye of the locust and DCMD activity was recorded using extracellular hook electrodes from the left nerve cord connective.
Results: Initial and final control looms were not significantly different in time of peak, peak amplitude and peak width at half maximum. Characteristic and consistent DCMD responses (peak firing rates before the perceived time of collision) occurred whenever the object was on a collision course. Objects that deviated to a collision course generated a second observable peak that appears to be independent of the distance away from the locust, time of deviation, or direction of initial motion. Object motion that began from the anterior of the animal generated a larger peak response and faster rise time to peak relative to the same object traveling from the posterior visual field. Responses were more robust during close object motion, while overall responses diminished at greater distances.
Conclusion: Distinctly different responses to different trajectories demonstrate that the DCMD is able to encode complicated aspects of a visual stimulus. The DCMD response may contain directional information as well as information regarding time of object deviation and perceived time of collision. Encoded within the DCMD firing rate, subtle and dramatic changes in rate may provide crucial cues that allow the animal to perform suitable avoidance behaviours or continually modify an existing, adaptive behaviour, such as flight.
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