13 February 2018 Maurizio Mauri

LeDoux’s “double path”: the two different ways of reacting to brain stimuli

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Starting to write this piece to explain what happens inside us, or better, inside our brain, when we experience emotions like fear, joy, love, anger, hatred, etc., on the one hand I feel “invited to a wedding”, because I seem to have to write one of the most beautiful pieces in the world, being so strong the passion and interest that I personally feel for these topics. On the other hand, I feel at the same time in difficulty, being a difficult challenge to collect to briefly summarize the great secrets of the human brain. But as we know, passion is difficult to stop, so let’s try. The question we will try to answer is the following: do we have control of our emotions, or are emotions controlling us?

Neuroscientist LeDoux and studies on the Amigdala

Let’s start, as a good journalist would do, from the title of this piece: “The double way of LeDoux”. Here is a second question: what is “the double way”? Or better yet, we could start with a third question: who is Joseph LeDoux? Joseph LeDoux is a neuroscientist. The neurosciences represent a rather recent disciplinary field: the term was coined in 1962 by an American, Francis O. Schmitt, neurobiologist and professor at the “Massachusetts Institute of Technology” (MIT). In 1962 he began the “Neuroscience Research Program” (Neuroscience Research Program – NRP), in which for the first time in history the scientific basis for the study of the brain is integrated, that is thanks to the multidisciplinary contribution of mathematicians, physicists, chemists, biologists, engineers and psychologists, who conduct research on the brain, neurons, nerves, and behavior. Their approach is based on the physical-chemical and bio-chemical principles of mental phenomena, such as memory, learning and consciousness. Even today, inside the MIT university campus, the “building 46”, in “Vassar Street” number 43, is a unique department, but at the same time “triune”: that is formed by three research institutes working at unison to reveal the secrets of the brain: 1) the “Brain and Cognitive Science Dpt.”; 2) the “Picower Institute of Learning and Memory”, and finally the “Mc Govern Institute for Brain Research”.

Let’s return to our title: who is Joseph LeDoux? He is a neuroscientist: a scholar of brain mechanisms. Unfortunately (and fortunately), neurosciences represent, like so many scientific disciplines, an enormous field of knowledge and fields of application. Thus, Joseph concentrated much of his life on a small cluster of neurons within the brain: the amygdala. In ancient Greek, amygdala means “almond”. The first scholars of the brain have used this term because, from an anatomical point of view, the shape and size of this brain center resemble those of an almond. For many years LeDoux has studied this “brain center” (the technical term, which a doctor would use, is “brain nucleus“) in the brain of mice.

Why did the studies start with mice and did they switch to the human brain only afterwards? This question is legitimate, because one way to begin to understand more and more of the brain in general is to study it not only in humans, but also in animals. Given that studying the brain in people requires many expedient and precautions, the possibility of studying the brain in animals has been fundamental for understanding the brain mechanisms “in vivo”, while the animal is still alive. Even this issue deserves to be investigated, but it would take us too far from our goal. Suffice it to say here that thanks to animal brain studies, in particular that of mice and primates (one above all, the “macaque Rhesus”), understanding of the brain has been deepened considerably.

The double way: the brain processes stimuli in two different ways

LeDoux has discovered that the amygdala in the mouse’s brain is involved especially when the mouse experiences emotions, such as anger and fear. In particular, one of the emotions that LeDoux has analyzed even more deeply was fear. From here originates the title of a presentation made for “Big Think”: “Neuroscience of Fear” (“The neuroscience of fear”) in which it explains how this emotion was understood in neuro-biological terms. By synthesizing to the maximum, the mouse amygdala mediates fear responses. Just to give an example: put in front of a snake, the mouse perceives a danger. Reacts trying to get away. But it can also respond by implementing a reaction that in English is called “Freezing”. It “freezes” in the sense that the mouse is completely immobilized. This response, translated into behavior, is also “dictated” by a group of neurons located in the amygdala of his brain. Thanks to the studies of LeDoux, it was possible to understand and explain the behavioral reactions of fear, “dictated” by the amygdala.

Obviously, the amygdala is also present in the human brain. Indeed, there are two: one in the right hemisphere, and one in the left, as shown in the image below.

Curiously, the amygdala in the right hemisphere is activated more than the one in the left hemisphere when a human being feels a strong emotion, such as a strong fear. Also on this phenomenon, defined as “hemispheric lateralization” of emotions (according to which the right hemisphere, very generally, is particularly involved and active when the person feels an emotion, compared to the left), another piece could be written, but we remain adherent to our title.

The most interesting thing discovered by LeDoux is that the amygdala is part of a circuit: it is one of the points through which the information processed by the brain passes. In other words, it is a “station”, a “stage” of a “path”, a real “way”: finally we come to explain the title of our piece! As explained by the image below, taken from the New York Times website where an interview with Joseph LeDoux was published in 1996, the year in which he theorized the presence of the so-called “double wayof information processing by the brain.

Looking at the image, it is possible to notice that the amygdala is involved in two paths or ways. The first (highlighted in red and that LeDoux has labeled as “Low Road“) starts from the external stimuli received by the sense organs (eyes, ears, etc.). From the sense organs, which “translate” sensations and perceptions into electrical impulses, the information is subsequently conveyed to the first “stage” of the processing path: the thalami (cerebral nuclei in the center of the brain, two “mini brains” hidden in the depths of the brain!). From the thalami, other electrical impulses restart to finally reach the amygdala (or better, the amygdales), for a subsequent elaboration of the information. From here, new electrical impulses impart the “orders” and the “commands” brought to the effector organs (lungs, heart, muscles, etc.) due to the prompt reaction of the organism in terms of behavior and action aimed at expressing emotion.

The second “path” (or “way”) in which the amygdala is involved (in the image above, highlighted in blue and defined as “High Road“) always starts from the external stimuli that are received by the sense organs and translated into electrical impulses always conveyed to the first “station” of information processing, represented by the thalami. This time, however, new electrical impulses are sent from the thalams to the cortex (instead of the amygdales, as was the case instead in the first way). From the cortex, new electrical impulses are then sent to the amygdales, which process and restart new impulses towards the effector organs for a reaction of the organism.

Why “low” road and “high” road?

To understand why LeDoux has defined these two ways with these “labels”, we can try to show the above scheme through real images taken from the human brain. Taking as an example the phenomenon of vision, whereby light (in particular, in physical terms, photons) affects the retinas present in our eyes, which thus receive information from the external world and convert it into electrical impulses through special cells (cones and rods) that are conveyed into the optic nerves. Below we give a schematic image of a brain seen from above (horizontal section), and how information from the eyes reaches the thalami through the optic nerves, and then passes to the occipital cortex (in the back of the skull) used to decode the vision (Primay visual cortex).

Below, we provide an image that falls “in vivo” inside a portrait brain while a person is exposed to visual stimuli, the transmission of visual inputs from the eyes to the thalami, through the cranial nerves (highlighted in green) that they cross in the optic chiasm before reaching the thalami.

Below is an image of how visual inputs are transferred from the thalamus to the visual cortex (the second part of the “high way”), through neural connections.

Let’s try to represent the same phenomenon seen above with regard to vision, this time with images that always reflect the human brain, but seen from the side (sagittal section of the brain).

Looking at the image above, it can be seen that the view, through the eyes, detects a danger (“danger” – for example the vision of a tiger), and transmits the information to the talami (Thalamus). At this point, the information can take two paths: through the “low road“, it is sent directly to the amygdala, thanks to the “shortcut” highlighted in yellow; the amygdala, in turn, sends the responses that the body puts in place (for example “running away from the tiger”). This way is called low because it involves the “lower parts” of the brain. Below we show an image of the brain seen from the front (coronal section) taken by fMRI (functional magnetic resonance) of a person while he is feeling fear.

As you can see by looking at the image, the two amygdales that are activated when the person feels fear can be located in the lower parts of the brain.

Alternatively, the information received by the thalamus can pass, via the “high road”, from the thalamus to the visual cortex, thus remaining in the “upper parts” of the brain. Now you can understand why LeDoux used these terms.

Two ways, two different speed of reaction to stimuli

Let us now consider different characteristics of the two “ways” of information processing. Let me give you some small neuroscience pills to better understand how the two ways work. Let’s start from the high road. First of all you should know that from the moment an external stimulus (for example, the famous tiger) appears in our visual field, at the moment when we become aware of the stimulus, they pass approximately 300 milliseconds, practically a third of a second. In other words, we perceive the world with a constant delay of one third of a second. Why spend 300 milliseconds? This is the time necessary to: receive, in the retinas of the eyes (cones and rods) and decode the light stimuli, then transmit impulses through the optic nerve, elaborate them summarily at the level of the thalamus, and make them reach the occipital cortex, to then send impulses to the temporal cortex used for the “identification of the stimulus” (what) and to establish where it is located in the space (where). Below we give an image that shows how visual information, after being processed by the visual occipital cortex, is then processed by the temporal cortex (the flow of information is called “ventral visual stream”, precisely because it passes into the ventral part of the brain) which establishes the identity of the stimulus; at the same time, information is also processed by the visual cortex from the parietal cortex (this flow is instead called “dorsal visual stream”, because it passes along the back of the brain), which processes where the stimulus is in space.

So many neuroscientific researches have shown that consciousness correlates with the activation of cortical areas (the surface areas of the brain). To become aware of the famous tiger, from the world outside the temporal cortex that identifies the tiger it takes 300 milliseconds, on average.

If instead we consider only the “low” road, which takes the information from the sense organs (the eyes that have detected the presence of the famous tiger), it summarily processes them at the thalamus level and then sends the information to the amygdala the which controls the reactions (automatic) in response to the stimulus, it is more rapid (around 150 or 200 milliseconds), even if less precise (LeDoux defines it as “durty”, dirty) compared to the “high” road, which being more slow in terms of milliseconds (because the cortex, to process all available information takes more time), is however more precise and systematic.

Why is the discovery of the “two ways” so important?

For several reasons.

– The first concerns emotions. Knowing that emotions (in particular fear, anger, stress, anxiety, etc.) particularly involve the amygdala, and that this in turn is part of a brain circuit that can do without the involvement of the cortex (site of the awareness), allows us to understand why it is not so easy to always be aware of one’s emotions. Psychologists know this well, but ordinary people are often not aware of all these facets.

– The second reason concerns the possibility of explaining strange phenomena, such as when walking in a jungle and seeing a branch on the ground that has the shape of a snake, first we react by taking a backward shot, and then we realize that it is a branch and we say “but go away …!”. Now we can understand how ever the threatening stimuli can elicit an “instinctive”, emotional reaction, that is, devoid of more refined and articulated and complex elaborations, which require not only more time, but also more cognitive effort.

– The third reason concerns the fact that these two systems not only follow different circuits, but also different logics. Robert Zajonc, a famous psychologist for his experimental studies (among others we recall the studies on subliminal perception) has hypothesized that these two systems follow different processing criteria: the high road, with a greater cognitive role, follows criteria of “inferanda” , that is, it processes information according to systematic and deductive logic, while the low road follows “preferanda” criteria, following logics related to pleasantness, the hedonic quality of the stimulus, regardless of cognitive evaluations.

Kept in mind these last considerations, we are able to understand how it can happen to experience a strong emotional reaction, positive or negative, and how difficult it is to control it or not, depending on the capacity of the cortex (the “high” road) to interfere with (brake) or better interact with the “low” road. If a person makes us so angry, the motivation to hit the source of our anger is born in us. It is an automatic, instinctive reaction, which uses the presence of the low road, reminiscent of when we were organisms that fought in very hostile environments for our own survival: the automatic emotional responses were the only ones available, and they actually made us survive (at least they brought us here).

The cerebral cortex and the cognitive evolution of man

Only in more recent times, in evolutionary terms, did the “cerebral cortex” appear, the seat of the most noble cognitive functions, such as consciousness, verbal language, mathematical calculation, etc. If you have noticed, the (neo) bark (“neos” in ancient Greek means “new”) completely envelops the “internal” brain. Some neuroscientists call it “neo-cortex”, to distinguish it from the older one (the more “internal” brain) that we share with less developed organisms, such as reptiles.

It is only thanks to the intervention of the ability to curb our response, to control it, to integrate it with the cognitive responses (of the “neo-cortex”) with the more emotional ones (low way, or “primitive brain”) that we can distinguish from organisms less evolved with which we share the “primitive” or “animal” part.

But this is already another story, which I promise to tell in my next piece: the presence of more brains within our brains. And meanwhile, I hope to make you make another piece of travel, to be able to involve you in what represents our destiny, as men, carved at the entrance to the oracle of Delphi, where the myth tells that the foundation of the literary tradition was born: “ΓΝΩΘΙ ΣΕΑΥΤΟΝ”, “know thyself”. The myth tells two stories: according to one, more recent, the oracle was given to Zeus by other deities; according to the other, more ancient, it was necessary to face the dragon Python, who was guardian of the oracle, owned by a single deity, Gaea. Only through the struggle with the guardian could one get hold of the oracle, and know the truth. If we really know we have an animal part within us, we just have to know it, to be able to control it in the best possible way by integrating it with what we have (hardly) conquered with evolution.

I end this piece with a last image, below, which depicts the god Apollo with the lyre, god of the sun and of music and of all the arts, obviously, also of science: symbol of the light that pierces the darkness.

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