UX designer Curt Alredge wanted to test if the assertion made by Aubrey Johnson was true:

Hollow icons create more work for users and ultimately create cognitive fatigue

Icon differences

Icon Recognition Test is a web app game that will test your skills in identifying hollow and non-hollow icons.

Research has shown that users begin to map the meaning of icons to their positions in the interface – was a line from Alredge’s article which I agree with mostly, especially as I took the test. If an icon appeared nearby an item I was looking for, I would tend to look for the next icon in search near there. If it was close by, quicker correct answer.