In
Experiment 2II, we used the widely adopted digits that shared spatial frequency and had a similar shape, as well as some letters that were similar to them. By topological definition, the difference between the figures of b, p, d, q (four flankers with a hole); 6, 9 (two targets); and those of 5, 2, Ǝ, Ε (four flankers without hole) is the number of holes. The target was always a digit 6 or 9 (
Table S3). The response key was up arrow for Target 6 and down arrow for Target 9. The differences of flankers either between categories or within category could be well controlled. The flankers shared the same area, luminous flux, nearly the same spatial frequency, and perimeter length. Meanwhile, it could still be argued that, because the S carries a horizontally oriented bar in the middle, there is more horizontal-edge energy and higher horizontal spatial frequencies in the S than in the ring; a neuron merely preferring horizontal edges or horizontal higher spatial frequencies could distinguish the S from the ring without the need to pay explicit attention to topology. To address this issue, the target figure 6 or 9 and the flankers share exactly the same horizontal line segments. Both the target and the topological equivalent flanker had a hole, which could eliminate the possible explanation that the reduction of crowding effect was induced by a hole in the flanker. If both flankers had no hole, that constituted Condition a. If both flankers had one hole, that constituted Condition b. Moreover, if one flanker had no hole and one flanker had one hole, it constituted either Condition c or Condition d. The best performance was observed in Condition a, that is, topological difference between the target and the flankers (Condition a vs. Condition b,
t(15) = 11.05,
p < 0.001, Cohen's
d = 2.76; Condition a vs. Condition c,
t(15) = 5.77,
p < 0.001, Cohen's
d = 1.44; Condition a vs. Condition d,
t(15) = 7.25,
p < 0.001, Cohen's
d = 1.81). The worst performance was observed in Condition b, that is, topological equivalence between the target and the flankers (Condition b vs. Condition c,
t(15) = –8.65,
p < 0.001, Cohen's
d = –2.16; Condition b vs. Condition d,
t(15) = –5.40,
p < 0.001, Cohen's
d = –1.35). Consistent with
Experiment 2I, the performance in Condition c was better than that in Condition d (
t(15) = 5.51,
p < 0.001, Cohen's
d = 1.38) (
Figure 4).