The Grandmother cell comes to the neural net

We really don’t know how concepts are stored in the brain (or where) despite a lot of work.  Sure, if you destroy an area of the brain, you have a neurologic deficit, but if you pull out the plug of a lamp it does dark, but that the plug isn’t making the light. There has been a huge amount of human experimentation on subjects awaiting resection of an epileptic focus (to find and remove it).  Patients can be awake while all this is going on as the brain has no pain endings.

Is the concept of grandmother stored diffusely among many different neurons (or even glial cells) or is there a single cell storing it, so that if you lose that cell you lose the concept of grandmother.  Certainly the epilepsy work has found some very specific cells. In one fantastic (unreplicatable) paper, a neuron was found that responded to 7 different images of Jennifer Aniston, but not to other pictures of actresses (Julia Roberts) or even pictures of Aniston with another actor. [ Nature vol. 435 pp. 1102 – 1107, 2005 ].  Another neuron would respond only to another actress (Halle Berry).  The direction of presentation of the actress didn’t matter — face on, side view, etc. etc.

Nearly the same thing has been found with neural nets [ Proc. Natl. Acad. Sci. vol. 117 pp. 30071 – 30078 ’20 ].  A trained network contains units responding to high level visual concepts which aren’t explicitly labeled in the training data.  This paper was able to activate and deactivate any unit in the net to see where things were stored.

In a network with 512 output units, removing the 20 most important units for each class to be identified reduced accuracy to chance (53%).  Removing the 492 least important units only reduces class accuracy by 4%.

Not quite a grandmother cell, but getting close.

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