Edges and corners
A note for Lesson no. 3, "Little Brains Wire Themselves to Their World," in Lisa Feldman Barrett.
Some context from page 48–50 is:
When you expose her to boxes and buildings, you’re training her visual system to see edges and corners.
See Müller-Lyer illusion (from the webnotes for How Emotions are Made: The Secret Life of the Brain), and the references therein.