From one piece

One piece brings back the whole

Key ideaOne piece brings back the whole memory

Six pictures are stored in this board (the small pictures above). Now we show only the top-left quarter of the cake and leave the rest as blue “don’t know” dots. The dots send signals along their links — switch on, switch off — and the board fills in the rest by itself. The cake comes back. It is like seeing one corner of a photo and remembering the whole day.

Why?A memory is not kept in one place, like a single photo in a drawer. It is spread out as links between the nerve cells that were active together. That is why one piece can pull out the rest along those links. It is why an old photo, a few notes of a song or a smell can bring back a whole day.

Go furtherThe board is a small learning model that really computes. We showed only 144 of its 576 dots, and when it finished, 100% of the dots matched the original picture. The idea was proposed by the physicist John Hopfield in 1982, so it is called a Hopfield network. In the brain, part of the hippocampus is often named as a place that does this.