In the (excellent) What We Owe the Future, William MacAskill discusses some ways of considering and weighing values on different lives, one of which was counting neurons. I don't have the book in front of me, but from my notes at the time:

>There are 8 billion humans and 135 billion farmed animals, but in total humans have 700 million trillion neurons while farmed animals have in total only 20 million trillion. Still, there are maybe 600 trillion wild fish, which collectively have 12 billion trillion neurons. And that isn't even counting nematodes.