Examples, I can definitively give.

When they’re neutral and calm, the curls remain as is.

Source: Hetascanlations, World Stars Chapter 20.

When they’re distressed, their curls tend to crinkle and
become box-like.

Sources: Hetascanlations, World Stars Chapter 73 + Hetarchive (Bamboo Thicket, eng trans., Jan
16th, 2015). 

When they’re fond of someone/ sharing an affectionate
moment, their curls will become hearts.

Source: Volume 1 + Hetarchive, Comic Diary 6.