Here is a more beautified version of it that is easier on the eyes. Representing the stage at which the player remained pure long enough that their Willpower can fortify parts of the brain against the tadpole's influence and thus unlock Purity bonuses as a reward;

  • Remaining pure throughout ACT 1 fortifies the first tier of the brain
  • Remaining pure throughout ACT 2 fortifies the second tier of the brain
  • And denying the Astral tadpole in ACT 3 fortifies the third tier of the brain

[Linked Image from i.imgur.com]

And the way we actually get to spend points on these is by crushing the tadpoles, thus giving some depth to that part of the story as well.