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As you might imagine, larger animals have larger brains. However, this does not mean that animals with larger brains are smarter than animals with smaller brains. For example, a larger brain is necessary to control larger muscles in larger animals and a larger brain is necessary to process more sensory information from the skin in larger animals - this has nothing to do with intelligence.
|Brain Weight (grams)||Species|
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During the course of evolution, the brain areas that show the most changes are the cerebral hemispheres (the red areas in the drawings): the more recently evolved animals have a larger proportion of the brain taken up by the cerebral cortex. In the "higher" animals (especially the higher mammals), the surface of the cerebral cortex becomes folded. This creates grooves on the surface of the brain called sulci (singular = sulcus). The bumps or ridges on the surface of the brain are called gyri (singular = gyrus). The folding of the cortex increases the cortical surface area. The cerebral cortex, made up of four lobes is involved in many complex brain functions including memory, perceptual awareness, thinking, language and consciousness.
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Parts of the cerebral cortex in the parietal lobe are involved with processing information related to touch. One such area is the primary somatosensory cortex which is located behind the central sulcus. Neurons in the primary somatosensory are activated when the skin is touched. However, the body is NOT represented in the cortex in proportion to the amount of skin. A map of the human somatosensory cortex was drawn by Dr. Wilder Penfield, a neurosurgeon, in the 1950s.
After stimulating the cortex of patients undergoing brain surgery for epilepsy, Dr. Penfield asked the patients what they felt. By observing the location on the brain that caused patients to feel sensations on different parts of their bodies, Dr. Penfield was able to draw a map of the brain. As you can see in this figure above, even though the arms and trunk make up most of your body, they are not given much cortical tissue. However, the face and hands take up a good portion of the primary somatosensory cortex. This is because the amount of primary somatosensory cortex is directly related to the sensitivity of a body area and the density of receptors found in different parts of the body. The areas of skin with the higher density of receptors (like the face, hands and fingers) have more cortical tissue devoted to them. If you were "built" in proportion to the amount of cortex devoted to each part of your body, you would look a bit distorted: you would have a big head and hands and a small torso and tiny legs. This distorted body map is called a homunculus which means "little man."
Think about how sensitive your fingertips are compared to your leg. For a demonstration of the sensitivity of different body areas, test your two point discrimination.
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