Besides the phase and disparity of the surround mask, its spatial layout relative to the target could be an important factor. In this experiment, the five different layouts illustrated in
Figures 1b–
f were used to test for the three aspects of the surround geometry: (a) surround area, (b) surround collinearity with the target, and (c) surround symmetry (unilateral vs. bilateral). To test the effect of area, surround suppression from the full annulus (
Figure 1b) was compared with that from one half of the surround (partitioned in four different ways, as shown
Figures 1c–
f). To test for collinearity effects, suppression from the bow-tie mask collinear with the target, C (
Figure 1e), was compared with suppression from the bow-tie mask flanking the target, F (
Figure 1f). Also, suppression from the half-annulus mask shown near the end of the target, E (
Figure 1c), was compared with that from the half-annulus mask shown near the side of the target, S (
Figure 1d). Finally, to test the effects of symmetry, suppression from the two unilateral masks, E and S, was compared with that from the two bilateral masks, C and F.
The five surround layouts were tested in consecutive experimental runs. The results for four subjects are shown in
Figure 5. One can see that the only important aspect of the surround was its area (A vs. C, F, E, and S). On average, the amount of surround suppression dropped by a factor of 2.7 when the surround was halved. The suppression was unaffected by either the surround collinearity (C vs. F and E vs. S) or its symmetry (E and S vs. C and F).