The turtle in Fig. 1 is trying to catch the fish that is moving past from left to right. The turtle fist notices the fish at point x1 at time t1. It watches the fish until it reaches point x2 at time t2, and then moves towards the fish to grab it with its mouth. However, the fish keeps moving and reaches point x3 by the time, t3, when the turtle completes its head movement. Thus, the turtle will miss the fish if it bases its head movement on the position of the fish and plan its head movement to reach point x3 at time t3. However, the fish has a stake in the event and will try to evade capture by making escape movements that appear unpredictable to the turtle.
Fig. 1: Turtle extrapolates motion in catching fish
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You can watch a movies of a turtle catching fish at http://visual.cs.utsa.edu/research/turtle/movies/index.htm
Turtles engage in motion extrapolation in order to capture moving prey. Location of prey and other visual activities cause waves of activation response to sweep across the turtle visual cortex as relected by the local field potential (LFP) measured in awake and behaving turtles in their environment. Fig. 2 shows the increase in LFP right before and during capture.
Fig. 2: LFP reflects prey tracking activity
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Fig. 3 shows another sequence of video frames of prey capture. The sequence starts with the turtle on the opposite side of the tank, looking in another direction. The turtle turns its head to track the prey, then moves its body to better align itself in preparation for a capture attempt. The sequence ends with the turtle missing the fish.
Fig. 3: Sequence showing prey tracking and miss
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The turtle shown in Fig. 3 has marks on its head an body in order to make the measurement of its position easier to measure from the video frames. These points give a coordinate system for head and body position as explainted in Fig. 4.
Fig. 4: Coordinate system for measuring head and body position
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Fig. 5 shows the measurement of the head and body position during the sequence of Fig. 3 using the coordinate system explained in Fig. 4.
Fig. 5: Trace of head and body position during prey tracking
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Fig. 6 shows the curves of Fig. 5 as a time series. The time series makes it clear that snout position leads body position.
Fig. 6: Time series of prey tracking during capture attempt
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