Abstract
The polarization pattern of the sky is exploited by many insects for spatial orientation and navigation. It derives from Rayleigh scattering in the atmosphere and depends directly on the position of the sun. In the insect brain, the central complex (CX) houses neurons tuned to the angle of polarization (AoP), that together constitute an internal compass for celestial navigation. Polarized light is not only characterized by the AoP, but also by the degree of polarization (DoP), which can be highly variable, depending on sky conditions. Under a clear sky, the DoP of polarized sky light may reach up to 0.75 but is usually much lower especially when light is scattered by clouds or haze. To investigate how the polarization-processing network of the CX copes with low DoPs, we recorded intracellularly from neurons of the locust CX at different stages of processing, while stimulating with light of different DoPs. Significant responses to polarized light occurred down to DoPs of 0.05 indicating reliable coding of the AoP even at unfavorable sky conditions. Moreover, we found that the activity of neurons at the CX input stage may be strongly influenced by nearly unpolarized light, while the activity of downstream neurons appears less affected.
Highlights
Spatial orientation and navigation require the perception and integration of environmental stimuli
Time points of action potentials during each 360° rotation were assigned to the respective orientation of the polarizer and these angles were doubled to allow using circular statistics on these axial data (Zar 1999)
We investigated the influence of blue light with different degree of polarization (DoP) on the mean vector length r, the response amplitude A, and the firing activity of the neurons
Summary
Spatial orientation and navigation require the perception and integration of environmental stimuli. Many animals rely on sky compass cues, including celestial bodies such as the sun or moon, the chromatic gradient and the polarization pattern of the sky. We used linear-circular correlation analysis (CircStat; Berens 2009) to determine whether the modulation of spike rate was correlated to changes in AoP. Time points of action potentials during each 360° rotation were assigned to the respective orientation of the polarizer and these angles were doubled to allow using circular statistics on these axial data (Zar 1999). A was calculated as follows according to Labhart (1996) and Pfeiffer et al (2011): i∑ =18 A = ||ni − n||, i=1 where ni is the number of spikes in bin i and n is the number of spikes during the 360° rotation divided by the number of bins. Firing rates at Фmax and Фmin were estimated by fitting a bimodal von Mises distribution model to the binned data (Fitak and Johnsen 2017) and taking the model’s firing rate at the respective angles
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