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TMS Therapy News: Using TMS to Learn More about the Senses

August 19, 2019

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The human brain has some truly amazing capabilities. For instance, if one of the senses—sight, smell, sound, taste, or touch—is deprived, a different sense can improve to compensate. 

In order to learn more about the process by which this occurs, researchers used transcranial magnetic stimulation (TMS) to activate sense-related areas of the brain

Continue reading to find out more about:

  • The senses and the brain
  • What TMS is, and how it works
  • The study and its results

The Senses and the Brain

How the Brain Processes Sensory Information

There are specific areas in the brain designed to process sensory input from the outside world. This is known as the sensory cortex. Each sense has its own portion of the sensory cortex:

  • Sight: Visual cortex
  • Smell: Olfactory cortex
  • Sound: Auditory cortex 
  • Taste: Gustatory cortex 
  • Touch: Somatosensory cortex
Diagram of the brain featuring the cortexes related to the different senses
There are primary and secondary cortexes for each of the senses. Sensory information will pass through the former for initial recognition, then continue to the latter for further processing. Image courtesy of Wikipedia

An external signal is likely to activate more than one sensory cortex at a time. This is commonly referred to as “cross-talk” because it requires communication among different parts of the brain. In order to combine information from multiple senses, the signal is sent to an association area. 

Sensory Compensation 

According to Scientific American, “A large body of evidence shows when the brain is deprived of input in one sensory modality, it is capable of reorganizing itself to support and augment other senses.” This is due to neuroplasticity, or the brain’s ability to adapt and readjust itself. 

For instance, it is common belief that a blind person can hear better than someone with sight. Such a phenomenon is known as sensory compensation, in which the brain uses one sensory cortex to help process information that would typically go to a different sensory cortex. In the example of the blind person, this would be the visual cortex accommodating the auditory cortex. Likewise, it has been shown that the visual cortex, rather than the somatosensory cortex, is activated when a blind person reads Braille. 

All About TMS

What Is It? 

TMS is a cognitive treatment method that uses a conductive coil to issue controlled electromagnetic pulses. These pulses stimulate a targeted area of the brain, igniting cellular activity and, essentially, changing the brain’s chemistry to facilitate improved function. 

Technician applies TMS to patient's head uses the electromagnetic coil
The strength of the TMS magnet is equivalent to that of an MRI machine. Likewise, the TMS machine makes similar clicking noises. Image courtesy of The Spokesman-Review

How Does It Work?

Once prescribed by a doctor, TMS is issued by a specialist once a day up to five days a week for around six weeks. TMS is a non-invasive procedure, which means the patient does not need to be sedated during a session and can resume daily activity afterward (driving home, going back to work or school, etc.).  

TMS is not painful, though some patients do report a slight headache or pain in the scalp after a session. However, any physical discomfort should be light and short-lived. 

The Study

Background

Researchers were curious about subjectivity in terms of activation when it comes to sensory compensation, formally known as cross-modal plasticity. Blind test subjects and normally-seeing control subjects were trained to use a tongue display unit (TDU), which the researchers define as “a tactile vision sensory system.” 

Previously, they had concluded that the blind test subjects rerouted the tactile information to the visual cortex (similar to the Braille example). In this study, the researchers “used TMS to stimulate the visual cortex before and after TDU training in a group of early blind (EB), late blind (LB), and blindfolded [normally-seeing] subjects while noting the sensory experiences reported by the subjects.” 

Different models of tongue display units
These are examples of TDU models. They use external sensors and stimulating electrodes to sense pressure and produce feedback. Image courtesy of Fine Art America

Results 

The results show that applying TMS to the blind test subjects’ visual cortexes “induced tactile sensations in the tongue that were somatotopically organized.” This means that, though targeting the visual cortex, the TMS produced somatosensory (touch-related) effects. 

Yet, applying TMS to the blindfolded, normally-seeing test subjects’ visual cortexes “evoked only visual phosphenes” (meaning they saw a flash of light). In other words, the blind subjects felt a sensation on their tongues, while the normally-seeing subjects saw a flash of light. 

Figure from the study showing the results
In this figure (fig. 4 from the study’s published report), the active cortexes are denoted by the green coloring. Without the TMS, the somatosensory cortex was only significantly active cortex. With TMS, both the somatosensory cortex and the visual cortex were significantly active. Image courtesy of Proceedings of the National Academy of Science of the United States

This leads the researchers to believe that subjectivity in perception depends on the stimulated sensory channel rather than the activated cortex. The researchers understand their results to be empirical evidence lending support to the theory of cortical deference, which is the explanation behind cross-modal plasticity. The results are also significant to understanding how the senses function when one is damaged or missing. 

The Versatility of TMS  

TMS is predominantly used to treat depression. Right now, the FDA has only approved it to treat Major Depressive Disorder. However, as supported by this study, more and more potential uses for TMS seem to be arising. 

This is probably because TMS affects the brain, and the brain is as versatile as it is complex. In other words, it’s an organ with a lot of use, so it’s only logical that a corresponding treatment could have a wide variety of applications as well. 

In addition using TMS as a treatment method, TMS can act as a tool for scientific experiment, as seen with the study discussed in this post. 

To stay up to date on the latest TMS therapy news, follow the Transformations blog for summaries of scientific studies and other TMS-related articles. 


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