In the lab, researchers directly and indirectly stimulate the brain to study how it works, according to our partner
The Conversation
.
For example, they use transcranial magnetic stimulation (TMS), which causes a temporary change in the activity of neurons by producing a localized magnetic field.
This analysis was conducted by
Laurie Galas
, PhD student in cognitive sciences,
Laetitia Grabot
, postdoctoral researcher in cognitive neurosciences, and
Laura Dugué
, teacher-researcher in cognitive neurosciences (all three at Paris Cité University).
Any good sci-fi story mixing myth and fact about the brain has already tried to make us believe that it is possible to manipulate our thoughts, modify our memories or make us act like puppets, thanks to all kinds of fantastic machines.
But what about in real life: is it possible to modify, or in other words, to stimulate the activity of our brain?
In the laboratory, cognitive neuroscientists use a variety of direct and indirect stimulations to change the activity of our brain in order to study how it works.
What does stimulating the brain mean?
The brain is constantly "stimulated" by information that arrives from our sensory organs, such as the light captured by our eyes.
For example, when you look at a dog in a park, the image of that dog forms in your eye on the retina.
The visual information present on the retina is then transmitted to the brain via the optic nerve.
The stimulation (visual here) is therefore indirect.
The information passes through the sensory organ of vision, the eye, which modifies the stimulation before it reaches the brain.
In other words, the retina will play a role in what we will perceive.
The effect that the sensory organs could have on the stimuli can interfere with scientific experimentation in certain specific cases.
This is why cognitive neuroscientists use techniques that help avoid these effects.
Transcranial magnetic stimulation (TMS), also used for the treatment of neurological or psychiatric disorders, makes it possible to act directly on our neurons.
It causes a temporary modification of their activity (usually a few milliseconds) by producing a localized magnetic field.
This technique does not require any surgery since it consists in positioning near the head a device composed of a coil which creates this magnetic field.
If the TMS is applied to the back of your skull, at the level of your visual neurons, it will stimulate these neurons and mimic their natural activity, as if they were receiving light from the eye.
Thus, for example, you will be able to perceive a luminous flash, which however does not exist in the outside world.
This illusory flash, called the phosphene, appears differently from person to person.
Some describe it as a change in texture, others as a white spot or as a set of white and black dots that appear locally in the field of vision.
The fact that TMS has a direct action on our neurons, precise in time and spatially localized without requiring any surgery, makes it a technique of choice for studying brain activity.
In indirect visual stimulation, information first passes through the eye before reaching the brain.
On its way, it may undergo modifications, particularly in the retina, before being transferred to the area of vision via the optic nerve.
There is therefore an intermediate stage between visual stimulation and the area of the brain processing information.
During direct magnetic stimulation, it is the area of vision that will be directly stimulated.
Why stimulate the brain?
It is interesting to stimulate the brain to study it well.
Let's continue the example of the study of visual perception.
In the laboratory, researchers can study it indirectly.
An object is presented to you on a screen, and the resulting activity of your brain can be visualized through an MRI recording (3D image of the brain and brain areas involved) or EEG (recording of the electrical activity of the brain) by example.
This type of study makes it possible to know which areas of the brain are activated in response to a given stimulation and what is the temporal dynamics of their electrical activity.
But as we have seen, this approach has limits, in particular because of the role of the sensory organs in perception.
TMS, on the other hand, will make it possible to directly stimulate a specific area of the cortex of a few cm³, allowing excellent spatial (and temporal) resolution and thus avoiding the effect of these sensory organs.
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Direct and indirect stimulation of the brain - Dugué Lab/Eddy Malrat
If the vision of an object activates the population of neurons A, this does not mean that the converse is necessarily true: when I activate the population of neurons A, will I see this object?
TMS makes it possible to study this causal link between our perception and cerebral activity.
Thus, studies have shown that if TMS activates neurons that usually respond to the perception of an object located in the upper right of your field of vision, it will result in the perception of an illusory activity in this same part of the field. vision (top right).
TMS therefore makes it possible to specifically target the population of neurons at the origin of the perception of this object.
In short, activating the brain makes it easier to study.
TMS makes it possible to study the direct link between neuronal activation and perception.
It can therefore complement other brain imaging techniques.
For example, MRI gives a very precise spatial image of the brain, unlike EEG which records its activation more precisely over time.
TMS does not make it possible to obtain an image of the brain in itself.
It is only there to modify its activity and therefore allows us to confirm the results observed in EEG or MRI.
It is therefore one more tool in the panoply of the researcher to better understand the mechanisms of the brain.
Can we change our thoughts by stimulating the brain?
Activating neurons absolutely does not mean that you can do what you want with your brain with TMS!
Our daily environment is made up of thousands of stimulations per minute.
Thoughts, memories, actions involve neural networks and activation patterns far more complex than the response created by a simple TMS pulse on a part of the head.
OUR “BRAIN” FILE
Moreover, researchers do not know all the subtleties of the neural dynamics behind a given thought… It is therefore difficult to know how to modify cerebral activity to influence our minds.
The TMS is still very far from resembling a science fiction device capable of transforming anyone into a puppet – but proves to be a formidable tool for unraveling the many mysteries of the brain…
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