This is the 2007 version. Click here for the 2017 chapter 02 table of contents.

Hemispheric Specialization

The brain has two halves or hemispheres. Viewed from above, as in the following diagram, the two sides look like mirror images. This is called bilateral symmetry. The word bilateral means two-sided, while symmetry means the two sides look the same. So bilateral symmetry refers to the fact that the brain consists of two halves that are nearly mirror images of each other.

The brain seen from above

What is bilateral symmetry? What is the main alternative to bilateral symmetry? What is crossover organization?

Almost all the animals on earth show bilateral symmetry. The exceptions, such as starfish and jellyfish, are circular. They have radial symmetry (identical parts radiating from the center) rather than bilateral symmetry.

In animals with bilateral symmetry, the right hemisphere generally controls the left side of the body, while the left hemisphere controls the right side of the body. This feature is called crossover or contralateral organization.

In humans and many other species, one side or the other may be dominant in motor activity (movement). This is the phenomenon known as handedness. About 13% of the human population is naturally left-handed. For the other 87%, the right arm works better for skilled hand movements.

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