This is a working memory exercise called "n-back". For true torture try "dual n-back" where you also have to match a tone, shape or color. https://en.m.wikiped

This is a working memory exercise called "n-back". For true torture try "dual n-back" where you also have to match a tone, shape or color. https://en.m.wikipedia.org/wiki/N-back https://en.m.wikipedia.org/wiki/N-back