Fig. 2. The target was constructed of clear acrylic covered with colorless, polarization-active Mylar. (A) Unmodified photograph of the transparent target viewed under polarized light. (B) Polarization contrast image of the target (trans-illuminated by vertically polarized light) generated by taking two photographs through a polarizing filter that was rotated by 90° between exposures. Each pixel brightness is equal to 255[(I_v–I_h)/(I_v+I_h)], where I_v and I_h are the pixel values when the transilluminated target is viewed through a vertical and horizontal polarizer respectively. (C) Same as in B but with wax-paper diffuser depolarizing the light. Because the neural responses of P. clarkii to changing e-vector depend on the rate of change (Glantz, 2001), and possibly on color, it is impossible to simulate how the moving target appears under each condition. Thus B and C are probably only an approximate indication of the contrast of the target under polarizing and non-polarizing conditions, respectively. The bar on right shows the contrast scale.