Computation of different optical variables of looming objects in pigeon nucleus rotundus neurons

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Three types of looming-selective neurons have been found in the nucleus rotundus of pigeons, each computing a different optical variable related to image expansion of objects approaching on a direct collision course with the bird. None of these neurons respond to simulated approach toward stationary objects. A detailed analysis of these neurons' firing pattern to approaching objects of different sizes and velocities shows that one group of neurons signals relative rate of expansion τ (tau), a second group signals absolute rate of expansion ρ (rho), and a third group signals yet another optical variable η (eta). The ρ parameter is required for the computation of both τ and η, whose respective ecological functions probably provide precise 'time-to-collision' information and 'early warning' detection for large approaching objects.

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Figure 1: A schematic diagram of a spherical object of diameter d directly approaching an animal's eye (a).
Figure 2: Based on the differences in the time course of the neuronal responses relative to the moment of collision, the looming sensitive neurons in nucleus rotundus have been classified into three distinct classes.
Figure 3: Quantitative examination of the timing of the responses of same three neurons shown in Fig. 2.
Figure 4: Quantitative examination of the timing of the response for the population of nucleus rotundus looming-sensitive neurons when presented with approaching objects that varied in size or velocity.


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The authors wish to thank T. Kripalani and S. David for excellent technical assistance and D. Fleet and N. Troje for helpful discussions and comments on the manuscripts. HJS was supported by a Postgraduate Scholarship from the Natural Science and Engineering Research Council (NSERC) of Canada. This work was supported by an NSERC grant OGP0000353 and an Alexander von Humboldt Research Award to BJF.

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Correspondence to Hongjin Sun.

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