The Left-Handed Brain

What does it mean to be left-handed? Well, first and foremost, it is a behavioral preference. Left-handers have the subjective feeling that they prefer to write or perform other complex fine motor tasks with their left hand, rather than their right.

In addition to this subjective preference, there are also measurable skill differences between lefties and righties. For example, left-handers are typically much faster and more accurate when writing with their left rather than their right hand.

So what causes these differences? Interestingly, not the hands themselves. It is impossible to tell whether somebody is a left-hander or a right-hander just by looking at their hands. There are no discernible differences between the bones, muscles, tendons, and other parts that make up the hands of left-handers and right-handers.

Instead, it is well known that handedness is caused by the brain (Schmitz et al., 2019). Handedness represents one form of functional hemispheric asymmetries—e.g., left-right differences in the brain. Specifically, in left-handers, the motor cortex in the right side of the brain (the left side of the body is controlled by the right side of the brain, and vice versa) is dominant for fine motor behavior. In contrast, in right-handers, the left motor cortex is better at fine motor tasks such as writing.

This finding leads to an intriguing question: Are there any differences in brain structure between left- and right-handers?

A research team from Montreal, Canada, has begun to tackle this question with new neuroimaging analysis techniques (Germann et al., 2019). The authors analyzed MRT (magnetic resonance imaging) images from more than 700 individuals. These images are obtained using an MRT scanner (a machine commonly found in hospitals) that uses magnetic resonance to create images of the structure of the brain.

Unlike other researchers who have investigated structural brain differences between left-and right-handers, Germann et al. (2019) analyzed the MRT images using an analysis method that relied on so-called local landmarks, or distinct structures in the brain. Unlike many fully automated algorithms, this approach allows for a better integration of individual differences in brain structure into the analysis. As all brains are slightly—or more than slightly—different, this approach is thought to yield more accurate results than other analysis methods.

The authors found differences between left- and right-handers in three main brain regions. The main finding was that the area of the motor cortex that controls the hand is bigger on the left side in right-handers, but bigger on the right side in left-handers. In men only, the authors also found similar results in two other parts of the brain that are involved in motor control: the striatum and the white matter of the cerebellum. The striatum is part of the basal ganglia, a group of clusters of neurons that are involved in the control of voluntary movements. The striatum itself is thought to contribute to the planning of motor actions. The cerebellum (Latin for “little brain”) is important for the coordination of motor behavior.

Taken together, it appears that there are clear anatomical differences between the brains of left-handers and those of right-handers. It is important to note that left-handers' motor areas are not better or worse than right-handers in any way—they are just mirror images of one another.