Stokes' parameter responses for individual R1 cells measured in the dorsal and ventral hemispheres and row 5 of the mid-band.

The Stokes' parameters, {S1,S2,S3}, are the rectangular coordinates of the Stokes' vector in the Poincar�� sphere, see Figure 1 (unity radius for normalized Stokes' vectors). The length of the vector is the degree of polarisation, , the spherical coordinates and indicate the type of polarisation. For linearly polarised light, �� = 0��; for circularly polarised light �� = 90��. The R1 cells act as partially polarising detectors, with mid-band cells being better polarisers than hemispheric cells. The ability of each cell to distinguish along one of the Stokes' axes is given by the discrimination, D = (Si/P)2, where i��{1,2,3}. Dorsal photoreceptor cells respond most strongly to diagonal/anti-diagonal linear polarisation; ventral photoreceptors to horizontal/vertical linear polarisation; and mid-band photoreceptors to right/left circular polarisation.