Lesioning Research: Definition, Method & Examples (2023)

The brain is one of the most complex systems in human nature. Our brains are capable of making billions of connections. But what happens when an accident or disease damages the delicate structure of this system? Lesioning research seeks to answer this question.

• We'll begin by defining lesioning research.

• Then, we will present several examples of lesioning research.

• After this, we will get a deeper understanding of what the lesion research method is.

• As we move forward, we'll take a look at the disadvantages of the lesion method.

• Finally, we will uncover the advantages of lesion production.

Lesioning Research Definition

In the 19th century, researchers didn't have the same fancy technology and advanced medical knowledge we have today to study the brain. The only approach they could take to study the brain was to observe cases in which the brain was damaged due to an accident or disease.

Lesions are damaged parts of the brain resulting from injury or disease.

Accidents and lesioning research in the 19th century provided the first evidence that complex cognitive processes (e.g. language) depend on different regions of the brain. Most of what we know about the brain started with discoveries that were made through lesion research.

Lesions can be caused by injuries resulting from accidents, but the most common causes of lesions are disease and disease-like events, such as strokes, haemorrhages, or tumors. One major advantage of lesioning research is that researchers are able to observe a situation they wouldn't be able to replicate in the lab. As you may expect, they can't cause permanent brain damage to someone solely for research purposes.

Although, later, we will learn about some ways in which lesions are caused temporarily or to reduce symptoms of a disorder.

Brain lesions from accidents, Freepik.com

Lesions Research Examples

Lesioning research has come a long way over the years. With new technology, researchers are learning more and more about how brain lesions affect our functioning.

Neuroscientists are researching how lesions in a common functional brain network linked to morality may be connected to lesion-induced criminal behavior (or acquired sociopathy).

Let's take a look at some lesions research examples including the work of Paul Broca, and the case of Phineas Gage.

Paul Broca

In 1861, French physician Paul Broca developed a theory of the brain based on lesioning research. He stumbled upon his insight that the brain is segmented while working with two patients, who had lost their ability to speak following an injury to the posterior inferior frontal gyrus, that is, at the bottom of the frontal lobe closest to the back of the head.

These two patients, named Leborgne and Lelong, both had trouble forming words and responding to questions. 51-year-old Leborgne suffered multiple neurological problems, and 84-year-old Lelong had experienced a stroke a year before meeting with Broca. When they both died, Broca was able to perform an autopsy which revealed lesions in the same region in the frontal lobe, the posterior inferior frontal gyrus. Today, through lesioning research, this area is called "Broca's area."

Phineas Gage

Lesion Research Method

So, how do researchers use lesioning research to study the brain? Early lesion research methods used a more causal logic to understand the brain. If region A is damaged and function B has been affected or no longer occurs, then region A must be responsible for function B.

If you damage the back of your brain near the occipital lobe and experience vision loss, then the occipital lobe must be responsible for controlling vision.

Brain mapping, Freepik.com

While this logic is not entirely wrong, it's not entirely correct either. While there may be specific regions in the brain that are heavily involved in certain functions, there are usually several areas of the brain that are also active. A more accurate lesion research method used today is to observe how certain areas of the brain are important to certain functions.

To understand which areas are important for which functions, researchers engage in a practice called brain mapping.

Brain mapping is a technique used in neuroscience, in which researchers map functions onto spatial representations of the brain.

Take Parkinson's disease: Parkinson's is a disease that affects movement, causing tremors as well as slowed or rigid movements. "Classical" Parkinson's results in the loss of nigral cells in several areas of the brain. However, lesioning research has revealed that damage in similar areas of the brain may result in lesion-induced parkinsonism, causing similar symptoms.

Thanks to modern technology, lesioning research has a lot more tools at its disposal to map the brain. Early lesioning research relied on either external representations of the injury (i.e. a gash in the right side of the brain) or autopsies. During an autopsy, researchers are able to cut into a person's brain to discover possible lesions. However, this method made it difficult to connect brain regions to more complex functions such as personality or decision-making (because the person was dead).

Now we have brain imaging such as computed tomography (CT or CAT scan), magnetic resonance imaging (MRI), or functional MRI (fMRI). Most lesioning research methods used today often incorporate the use of other types of brain imaging. With these scans, we can get a fuller picture of how the brain responds to lesions we can see externally such as strokes and haemorrhaging.

Disadvantages of the Lesion Method

While lesioning research has led to important discoveries about the brain, there are disadvantages to using it. One of the disadvantages of the lesion method is that it is not a controlled experiment. We mentioned earlier that one of the advantages of the lesion method is that it can give researchers an opportunity to observe a situation they otherwise would not have been able to replicate in the lab. However, you're also not able to control certain aspects of a lesioning case study such as sampling methods (you can't pick who gets a traumatic brain injury (TBI), and who doesn't). This can lead to unreliable results that can't be applied to the whole population.

Another disadvantage of the traditional lesion method is that it can only tell us what part of the brain is related to a loss of functioning. But it can't tell us much about how the brain responds to the damage through neuroplasticity.

Neuroplasticity refers to the brain's ability to change and adapt its structure and function.

This is why the lesioning method is usually used alongside some type of brain imaging, especially an fMRI which can provide information about both the structure and function of the brain. Researchers can learn the most about how the brain heals and adapts after an injury such as a stroke.

Brain imaging, Pixabay.com

Advantages of Lesion Production

So far, we have only discussed lesioning research on cases caused by accidents or disease. What we haven't talked about yet are cases in which lesions are intentionally caused by a physician through lesion production and brain manipulation.

Brain manipulation methods temporarily impact or "damage" neurological activity in order to observe changes in functioning.

Brain manipulation methods include pharmacological agents, optogenetics (controlling neurons with light), transcranial magnetic stimulation (TMS), and chemogenetics. While these procedures may be more invasive, there are several advantages of lesion production.

Brain manipulation studies allow more ethical options to study accidents and lesions in a clinical setting, since brain manipulation methods are temporary. Researchers can draw causal conclusions between behavior and brain function. This is especially applicable for brain surgeries. Neurosurgeons can use brain manipulation to figure out the risk factors of a procedure and how it may impact a person's functioning. If a person has a brain tumor, for example, neurosurgeons may use brain manipulation to figure out what areas of the brain they need to stay away from during the procedure.

If the tumor is located near Broca's area, a neurosurgeon may use brain manipulation before the surgery to figure out where not to cut, or else the patient's ability to use language may be affected.

Brain manipulations are temporary and reversible. But other times, lesions in the brain are intentionally produced by researchers to reduce symptoms of psychological or neurological disorders.

Severe epilepsy patients may get a "split-brain procedure" in which the corpus callosum (the nerve fibers of the brain connecting the left and right hemisphere) is cut.

Lesion studies help neurosurgeons understand more about the brain, allowing them to safely perform procedures as drastic as the split-brain procedure.