It’s fun to see the surprising images of human hands produced by generative image models. Why are there so many fingers – or so few? It turns out that this issue may not be unique to AI. Today I learned that the dreaming human brain also has trouble generating images of five-fingered hands.

Line drawing generated by DALLE-2, February 2024

I was reading about dreaming for a writing project I’m working on and stumbled across this comment on Reddit:

I find the best method [to lucid dream] is to randomly throughout the day is to look at your hands and count your fingers. Youll tend to do it in a dream and by the time you get to the 4th finger youll realize its a blurry mess of 5-6 fingers.

KneeDeepInTheDead, Reddit, 11 years ago

As soon as I saw this I thought about generative models and their issues with fingers, so I tried to find out whether other people have reported this same phenomenon in their dreams. I couldn’t find any formal scientific studies on the topic, but there are a bunch of random people on the Internet who claim that their sleeping brains generate unusual hand images:

This article: “It’s extremely rare to have 5 fingers in a dream. Most of the time, your hand will directly look really weird and it will be obvious that you have more or less than 5 fingers.”

This thread: “I generally have 6 fingers on each hand in my dreams. I have no idea why. They also look sort of misshapen… kind of squiggly, I guess. Trust me, if you look at your hands in your dreams, you’ll know its a dream.”

This WikiBooks article: “I counted my fingers. It looked as though I had the right amount, but as I actually counted them, I discovered that I had seven the first count, and six the next.”

Colored pencil drawing generated by DALLE-2, February 2024

This article on Why AI-generated hands are the stuff of nightmares says, “These models are divorced from reality, they don’t have any context and they don’t actually have any knowledge or ability to consider the context of an image. They just sort of combine all of the junk that we’ve given it.” Dream researchers hypothesize that dreaming is related to memory consolidation – i.e., combining all the junk we’ve given our brain during our waking hours.

Also, thinking more about brains, dreaming is the only time that the brain has to actually generate a hand image on its own, from its “parameters” if you will. All the rest of the time, when a person is awake, there’s a totally different process going on: the eye (like a camera) captures light information, which the brain interprets and occasionally stores. Sensory perception is the “dataset creation” phase, while dreaming is the “generation” phase. True, there are plenty of people with more or fewer than five digits – for example, people with polydactyly, oligodactyly, or amputations. But over 99.9% of people have 5 fingers on each hand. So why isn’t that the default for brain-generated or neural-network-generated images?

I’m very interested in understanding why brains and models have a hard time generating five-fingered hands, and if there’s any similarity in the underlying reasons. These are both open research questions that will likely take many years to answer. Do human brains and deep neural networks leverage similar high-level representational shortcuts? Past research has shown similarities between low-level filters in CNNs and human visual processing. Maybe there are even more sophisticated similarities present.

We tend to call deep neural networks silly when they do things like produce pictures of twenty-fingered hands – but if both the human brain and the models are doing the same thing, maybe there’s something “deep” going on after all.

And as for the models, the phenomenon won’t go on forever. Model developers are actively working on making models produce five-fingered hands. In the spring of last year, Midjourney released an update that significantly changed hand generation to make five-fingered hands more likely. Similarly, DALLE-2 is now capable of generating fairly average looking hands for certain prompts:

One human hand, generated by DALLE-2, February 2024

So there you have it! An intriguing and unexpected similarity between the brain and deep neural networks: a five-fingered hand is hard to find.