Only a minority of neurons in the brain fire in response to a given stimulus, leading to the speculation that many neurons never fire at all and are leftovers from an evolutionary past:
I didn’t find Ovsepian’s evolutionary argument that persuasive, because I’m skeptical of the whole ‘dark matter’ concept. Still, the idea that remnants of our ancestors’ brains still exist in our own skulls is a rather fascinating one. It suggests Jurassic Park-like scenarios in which someone succeeds in reactivating these lost neural functions – and there is evidence that this can happen in very specific cases such as reflexes. In general, though, I don’t think we can say that the majority of our neurons are fossilized.Neuroskeptic, “Silent Neurons: The Dark Matter of the Brain?” at Discover Nagazine
Notice that the neurons aren’t being called “junk neurons,” as in the exploded concept of vast libraries of “junk DNA.” Quite the contrary, they are given the somewhat glamorous cachet of “dark” neurons, as in “dark matter.” Perhaps something has been learned from the collapse of the concept of “junk DNA.”
The neurons from the deep dark past story sounds like the myth of the triune brain (that is, our mammal brain is built on top of a reptile brain, and our human brain is built on that). It sounds really convincing if you believe it anyway but it doesn’t represent how the brain is organized very well. A lot of this stuff would make great sci-fi, of course.
See also: Life Forms Are Not Machines And Neurons Are Not Neural Networks
Unique Type Of Cell Found In Human Brain: Rosehip Neurons
Researcher: Y chromosome not a genetic wasteland after all
Do We Have A Reptilian Brain? At Last, A Question We Can Easily Answer.
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