Fig. 11 A. Geometric horopter with asymmetric fixation (dashed lines). Note that both the Vieth-Müller circle and the vertical horopter line remain fixed in space as fixation moves around Vieth-Müller circle. B. Geometric horopter with asymmetric fixation away from both horopter lines. Horopter (full line) becomes a one-turn helix winding around the symmetric horopter (dashed lines).