Evidences Supporting the Right Hemisphere’s Role in Controlling the Left Side of the Body

Understanding the intricate relationship between the brain's hemispheres and body control is crucial for comprehending brain function and disability. The right hemisphere primarily controls the left side of the body, and this phenomenon is substantiated by several lines of evidence that span from historical case studies to cutting-edge neuroscientific research.

Historical Insights and Case Studies

Historically, the concept that the right hemisphere controls the left side of the body was confirmed through case studies of individuals who suffered brain injuries. For example, when the right side of the brain was damaged, patients often experienced partial loss of control over the left side of their body, and vice versa. This observation, known as contralateral control, supports the notion that the left hemisphere controls the right side and the right hemisphere controls the left side of the body.

Modern Neuroscientific Evidence

Today, modern neuroscientific methods offer a deeper understanding of this phenomenon. Here are the primary lines of evidence:

1. Awakened Brain Surgeries and Motor Cortex Mapping

In many brain surgeries, patients can remain awake and fully responsive during the procedure. Neurosurgeons often use electrodes to stimulate specific regions of the brain while the patient performs tasks, such as playing guitar (Figure 1). This allows scientists to map out which parts of the motor cortex control specific body movements. For instance, stimulating the left motor cortex may cause muscle contractions in the right hand, confirming the contralateral control (Figure 2).

2. Pathophysiological Evidence from Brain Injuries

Brain injuries, such as strokes, tumors, or traumatic injuries, provide additional evidence. Damage to the motor cortex on one side of the brain typically results in weakness or paralysis on the opposite side of the body (Figure 3). Right-sided brain injuries can lead to left-sided paralysis, and this is known as contralateral weakness or paralysis (paresis).

3. Brain Imaging Techniques

Modern non-invasive imaging methods such as PET scans and functional MRI (fMRI) allow scientists to observe brain activity in real-time. When a person performs a specific task, such as moving their left hand, the right motor cortex "lights up" (Figure 3). This imaging provides clear evidence that the right hemisphere is responsible for controlling the left side of the body.

4. Neuroanatomical Evidence from Fiber Tracing

Neuroanatomists study the brain by dissecting it and tracing the nerve fibers that connect different regions. They have found that about 90% of the nerve fibers cross over from one side of the brain to the other in the brainstem, specifically in the medulla oblongata (Figure 4 and 5). This crossover is known as decussation. Using modern techniques like diffusion tensor imaging (DTI), scientists can visualize these fiber tracts and confirm the contralateral control of the brain.

Conclusion and Final Thoughts

The evidence supporting the right hemisphere's role in controlling the left side of the body is robust and diverse. Historical case studies, clinical observations, modern imaging techniques, and anatomical studies all provide compelling evidence for the concept of contralateral control. Understanding this relationship is essential for diagnosing and treating various neurological conditions.

Visual References and Fig Legends

Fig. 1. Musician playing guitar during open-brain surgery. The brain has no pain receptors and anesthesia is not necessary for many brain surgeries.

Fig. 2. Brain mapping during open-brain surgery. The patient can be awake and report sensations or perform movements on command as the surgeon stimulates small brain regions with an electrode array, revealing what parts of the brain control what body movements or other functions.

Fig. 3. Brain mapping by fMRI. The computer-generated image from the MRI machine “lights up” to show what parts of the brain are most active in any given task state.

Fig. 4. Diffusion tensor imaging DTI. This technique traces white matter tracts (nerve fibers) to create a “map” of brain connections called the connectome. The technique of using DTI for this is called tractography.

Fig. 5. DTI of decussation. This DTI image shows where nerve fibers cross between the right and left sides of the brain at the DEC label. In this image, the fibers are going from primary motor cortex (PM) to the cerebellum (SCP, DN), but the principle is basically the same for fibers that go to the muscles of the lower body.