What Is the Paleocortex?

The paleocortex is a structure in the brain. It is primarily associated with an organism's olfaction, or sense of smell. It is a component of the cerebral cortex, an important part of the central nervous system of all mammals, including human beings. The prefix “paleo,” meaning “old,” refers to the fact that this part of the cerebral cortex arose in the evolutionary history of mammals and their ancestors prior to the evolution of the neocortex, or “new” cortex, which is a part of the brain found only in mammals that is closely connected to higher mental functions such as reasoning, learning, and perception.

The cerebral cortex, which derives it name from Latin words meaning “head” and “shell” or “rind,” is a multilayered structure important to mental functions such as thought, motor control, and sensory perception that comprises the outermost layer of the brain. It also contains the neocortex and the archicortex. The paleocortex and archicortex, which is involved in memory formation and emotions, are also collectively referred to as the allocortex.

The paleocortex is made of three to five horizontal layers of nerve cells in the gray matter of the brain. It contains several constituent parts related to the sense of smell, including the olfactory bulb, olfactory tubercle, and piriform cortex. These structures receive sensory information from the olfactory epithelium, a type of tissue in the nasal cavity that detects the presence of chemicals, and interpret it. They may have other roles as well; for instance, there is some research suggesting that the olfactory tubercle plays a role in the brain's reward system. The olfactory system in the paleocortex is closely connected to parts of the limbic system, such as the hippocampus and amygdala, that are important to memory and emotion.

In the human brain, the paleocortex constitutes only a small percentage of the cerebral cortex, compared to many other mammals, with the neocortex comprising the great majority of the human cerebral cortex. This is because human intelligence requires a proportionately larger neocortex in order to make complex reasoning possible. Humans are also less reliant on their sense of smell than many other mammals, which reduces the importance of the olfactory system, and more more reliant on their sense of sight, which in the mammalian brain is processed in a part of the neocortex called the visual cortex. The trade-off for this is that the human sense of smell is markedly inferior to that of many other animals. Our brains can process only a small fraction of the amount of input that can be processed by the brain of an animal such as a dog, and we have correspondingly fewer scent receptors in our noses.