In humans, sensory nerve fibres send electrical signals from the site of the stimulus to the spinal cord and then to the brain. Signals reached the thalamus, via the brain stem.
The thalamus, a sort of relay station, forwards signals for processing in different areas of the brain. One of these areas is the limbic system, which is the emotional centre of the brain. The neocortex, on the outer surface of the brain, handles conscious thought in the perception of pain.
All of these areas are present in fish, or the fish have the same function elsewhere in their brain.
Fish pain specialist, Dr Braithwaite, told the Los Angeles Times that, although simpler than our own, the brain of a fish has been found to be more similar than once thought. In the amygdala and hippocampus, our brains handle emotion, learning, and memory. The equivalent area has recently been found in the forebrains of fish. When this part is damaged, they lose their fear, forget how to navigate mazes, and are impaired emotionally.
"Their brains are not as different from ours as we once thought. Although less anatomically complex than our own brain, the function of two of their forebrain areas is very similar to the mammalian amygdala and hippocampus - areas associated with emotion, learning and memory. If these regions are damaged in fish, their learning and emotional capacities are impaired; they can no longer find their way through mazes, and they lose their sense of fear."
Anglers say because fish are cold-blooded, they cannot feel pain, ignoring the fact that arrangements of temperature control in animals have nothing to do with pain perception. They have even tried to claim that other animals do not feel pain as we do because they do not have our larger brain. Science is steadily moving to understand the mind of the fish, dispensing with these ridiculous notions.
For example, scientists at the University of Sevilla in Spain (Salas, 2006) say that studies using more sensitive techniques and sophisticated experimental procedures have revealed that the forebrain of fish, like that of mammals, is involved in emotional and social behavior, as well as in learning and memory.
Birds and amphibians, do not have a neocortex, but feel pain.
Norwegian scientists (Nordgreen, 2007) applied electrical shocks to the tail of anaesthetised salmon. They were able to trace this painful stimulation to the telencephalon in the brain.
Researchers (Rodriguez, 2006, Nordgreen, 2007) say that the telencephalon in fish is probably equivalent to the hippocampus and amygdala in humans. The outer layer of the telencephalon, the pallium, may be equivalent to the neocortex (Braithwaite, 2010)
Scientists at the University of Sevilla (Portavella, 2004), in an experiment, damaged the telencephalon of goldfish. This resulted in the fish being unable to learn to avoid an electric shock. Similar damage to the amygdala and hippocampus of mammals produces the same effect. The scientists think that fish and mammals have inherited the same brain functions from an ancient ancestor.
The German magazine, Der Spiegel in 2011 reported research that also showed that the human limbic system has its equivalent in the fish’s telencephalon. However, this area was not previously found in fish before. Rather being in the inside, as in the human brain, it moves to the outside, after embryonic development.
The thalamus is found in fish and humans. It is a major relay station from all of the senses to other parts of the brain. It screens, sorts and pre-processes information. It is also an important structure for pain perception in mammals (Nordgreen, 2007).
Pathways to cortical areas via the thalamus have been characterized in the zebrafish brain (Rink and Wullimann, 20041, quoted in Sneddon, 20112).
The simplified diagrams below show the different arrangements in human and fish brains.