Decades ago, it was surmised that the human brain could hold 7 pieces of data (plus or minus 4 pieces.) This is where our modern phone numbers were derived. As it turns out, chunking information allows us to hold more than 7 numbers; each 'chunk' would comprise a single piece of data, allowing even untrained subjects to improve to 70 digits or more after a week of practice in chunking.
However, this is true ONLY if all pieces are held in working memory at once; introduce recall, and the information you're able to hold drops steeply.
The n-Back test requires you to hold several pieces of data - pictures - in your memory, IN ORDER, and recall the picture you saw a few pictures before the current picture shown. It's harder than it sounds. Click below to try it: