The parts of the human brain that process the unpleasantness of pain have no equivalent in the brains of fish. So, does that mean fish aren't able to feel pain? Not necessarily, according to a new paper.
This is a debate with a long history – one that shows no signs of being drawn to a conclusion anytime soon – but this new paper points to modern neuroscience research that pain effect and awareness can still be present in humans even when pain-processing regions of the brain are injured.
In other words, if human brains can adapt to do without a part of the neural pain chain, then maybe fish don't need all of the links either. Finding out one way or the other could change everything from fishing practices to personal diet choices.
"While our study cannot prove that fish feel pain, we can assert that arguments relying on a lack of certain brain structures to deny pain in fish look increasingly untenable," says Kenneth Williford, a professor of philosophy at the University of Texas at Arlington.
Or to put it another way: Claiming that fish don't feel pain due to the absence of certain brain regions is like concluding they can't swim because they don't have the arms and legs that humans do, according to the authors of the paper.
The team used several cases of brain injury in humans to strengthen their argument, including one with a patient called Roger. While a key pain processing part of Roger's brain had been destroyed by disease, he was even more sensitive to pain than the average person.
This sort of "neural resilience" – the ability of the human brain to rewire itself to ensure that key functions carry on running – is important, the researchers say. In humans at least, no single cortical region seems to be responsible for pain.
Whether or not the same can be said about fish remains an unanswered question. Fish can't use language to tell us whether they're suffering, and they don't give an awful lot away in their expressions, either. However, they do have plenty of the necessary systems in place, including pain receptors.
The topic ties into a philosophy called Neo-Cartesianism, a school of thought that argues any suffering we might see in an animal is only apparent and can't be connected with mental anguish, at least not in the human sense.
It follows that if fish, and animals in general, aren't experiencing pain in the same way that we do, then the way we treat them is less important from a moral standpoint. Here, a related question is just how much self-awareness animals have, considering their simpler brain structures.
Once we dig deeper, the debate over the experience of pain in animals ends up straddling both philosophy and science; for now, it does appear there isn't enough scientific evidence to make concrete claims either way.
The science is catching up, though. A study from earlier this year was the first to find strong evidence of invertebrates feeling pain in a similar way to mammals. The jury is still out, but the evidence continues to come in.
"That many animals consciously suffer negative pain affect remains well supported by a general analogy argument and, in turn, this provides a parsimonious explanation of many animal behaviors," write the researchers in their paper.
"Proponents of Neo-Cartesianism have not offered good evidence to the contrary. Taken together, the considerations canvassed here constitute a strong empirical, cumulative case against Neo-Cartesianism or, at the least, shift the burden of proof to the Neo-Cartesianism camp."
The research has been published in Philosophical Psychology.