Integration of polarization and intensity contrast information in a highly visual animal
Polarization vision subserves diverse biological functions, such as navigation, communication and target-motion detection. Regarding the detection of biological targets, studies on semi-terrestrial crabs suggest that polarization and intensity contrast are processed in separate visual channels. Information about the polarization contrast of targets would be extracted independently of intensity contrast, and the two signals combined downstream in the visual system. However, understanding how the information about these visual attributes is processed and integrated has been limited, as it is technically challenging to present visual stimuli in which both the polarization and the intensity contrast of a stimulus are controlled. Here we developed a monitor screen that allows us to present stimuli in which both contrasts can be controlled. Thus, to study how polarization and intensity information is processed to increase target detection, we presented moving stimuli with controlled polarization and intensity contrast while recording the cardiac response of the semi-terrestrial crab Neohelice granulata as a sensitive readout of its visual perception. Our results suggest that Neohelice possesses similar sensitivity to vertically and horizontally polarized light; thus, previously reported responses of the animals to polarized stimuli in which figure and background have the same intensity are likely accounted for by the comparison of two polarization channels. In addition, we determined that a moving polarization-only stimulus has a salience equivalent to that of an intensity-only stimulus with a Michelson contrast of 0.51. Finally, we studied how polarization and intensity contrast information is integrated, and found that polarization contrast increases the salience of an intensity-contrast-based target mostly when its intensity contrast is low, i.e. when information about intensity contrast is more ambiguous.