Neural basis of the polarization compass in the central complex of the locust brain

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Insects and vertebrates are highly mobile organisms and, accordingly, show sophisticated performance in spatial orientation and navigation. These include oriented long-range seasonal migrations, landmark and sun compass orientation, and various forms of spatial learning. In mammals the hippocampal-entorhinal system plays a key role in spatial orientation. Neurons coding for spatial locations (place cells, grid cells) and directions (head-direction cells) contribute to internal map-type spatial memories. In insects, accumulating evidence points to the central complex as the brain area controlling spatial aspects of behaviour, including sky compass orientation and spatial memory tasks. Many insects detect the position of the sun indirectly by the sun-dependent polarization pattern in the blue sky. In desert locusts the orientation of sky polarization is represented topographically in the central complex suggesting that it acts as an internal sky compass. Work proposed here will further analyse this compass-like arrangement and explore the underlying neural network. Characterization of receptive fields for polarized light will show whether the central complex codes for a 360° representation of azimuthal (horizontal) space across the brain midline or a double representation of directions over a range of 180°. Furthermore, the underlying neural mechanisms of this polarotopy will be explored. Specifically, feedback circuits between both hemispheres of the protocerebral bridge, a major part of the central complex, will be analyzed for their possible role in generating and stabilizing the internal compass. The data are highly relevant not only for a better understanding of spatial orientation in insects, but also in comparison to similar topographic organizations in vertebrate brains, like the spatial arrangement of orientation columns in primary visual cortex.