The brain accelerates the perception, knowing what will happen next

Your expectations shape and accelerate your perception. A new model explaining this effect suggests updating the theory of signal processing.

If you expect a certain taste, and your tongue feels different - it will seem unpleasant to you. If the taste is expected, you will feel it faster.

Imagine that you took a glass, and you think that apple juice is inside, and then, after sipping, you discover that it is ginger ale. Even if you usually love soda, this time the taste seems nasty to you. This is because context and internal states, including expectation, influence how animals perceive and process information from their senses, explains Alfredo Fontanini , a neuroscientist at Stony Brook University in New York. In this case, waiting for the wrong stimulus leads to surprise and a negative reaction.

However, this influence is not limited to the quality of perception. Among other effects, setting the senses to wait for input data, good or bad, can increase the speed at which an animal is detected, defined, and reacted.

Many years ago, Fontanini and his team discovered direct evidence of this acceleration effect in the gustatory crust, the brain region responsible for taste perception. Since then, they have been trying to find a structure in the cortex that makes this effect possible. And so they succeeded. In April 2019, they published their discoveries in Nature Neuroscience magazine: a network model with a specific architecture that not only offers new ideas about how expectations work, but also enters the territory of broader questions about how neuroscience should relate to perception. Moreover, the conclusions in some ways coincide with the theory of decision making, which asserts that the brain does not build decisions gradually, but accepts them in a hurry.
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Accelerated feelings and active states

The taste, the least studied sensation, was the ideal starting point. After the taste appears in the language, it takes several hundred milliseconds before the activity of the gustatory crust begins to reflect the input data. “In terms of the brain’s work, this is terribly long,” said Don Katz , a neuroscientist at Brandeis University in Massachusetts (in whose laboratory Fontanini was undergoing post-doctoral studies). “In the visual cortex, everything happens for a small fraction of this time,” which is why it is much harder for the eyes to recognize the effect of waiting, which the scientists wanted to study.

In 2012, Fontanini and colleagues conducted an experiment in which rats heard a sound (“preliminary hint”), and then received a small dose of food through a tube in the mouth. Her very taste could be sweet, salty, sour, or bitter, and in the prompt there was no information about his character.

Still, it was found that, on the whole, the expectation of taste made the neurons of the gustatory crust recognize the stimulus almost twice as fast as when the rats received food without hearing a preliminary sound. The delay fell from about 200 ms to 120 ms.

Fontanini wanted to know which network would theoretically allow for coding acceleration. He attracted a specialist who had not previously worked in the field of taste: a colleague from Stony Brooke, a neuroscientist Giancarlo la Camère , who had previously worked on modeling spontaneous brain activity that occurs even in the absence of stimuli.


Alfredo Fontanini and Giancarlo la Camera

In the past few decades, there has been a growing conviction that most of the activity of sensor networks is generated internally, and not caused by external stimuli. If we compare the activity of the visual cortex of an animal in complete darkness with its activity, when the animal is examined, it will be difficult to find differences in them. Even in the absence of light, sets of neurons in the visual cortex begin to be activated jointly, simultaneously, or with predictable periodicity. This interlocking trigger is located in the so-called. a metastable state from several hundred milliseconds to several seconds, and then the activity configuration changes to another. Metastability, or the tendency to jump from one fleeting state to another, continues after the onset of the stimulus, however, some states often appear more often in connection with a particular stimulus, and therefore are considered “coding states”.

La Camera and others (including Katz) have already simulated metastability, creating what is called a clustered network. Inside it, the excitatory neuron groups are closely related to each other, and the inhibitory neurons are randomly connected to the excitatory ones, which has a wide muffling effect on the entire system. “Such a clustered architecture is fundamentally important for creating metastability,” said Fontanini.

Fontanini, la Camera and their colleague, a postdoc of Luca Matscukato (now working at the University of Oregon) found that the same network structure is also needed to recreate the effect of waiting. In a metastable model of a clustered architecture, researchers conducted a simulation of a warning tip, followed by a certain taste stimulus. As a result, they successfully reproduced the accelerated coding scheme that Fontanini observed in rats in 2012: the transitions from one metastable state to another were accelerated, which enabled the system to quickly transition to coding states. The results show that simply by creating a network to demonstrate metastable activity patterns, “one can capture many neurological responses by stimulating taste sensations,” said Fontanini.

When researchers attempted to model warning tips and stimuli on a network that does not have clustered neurons, they were unable to repeat the 2012 results. Therefore, "this effect is possible only in networks of a certain type," said Katz.

Less stressful walk

The discovery seemed remarkable, first, because it gave an idea of ​​what kind of architecture to look for in a real gustatory crust — and perhaps in other parts of the crust responsible for the senses. So far, neuroscientists have been arguing about how taste is processed: some say that some specific neurons can encode “sweet” and others “salty”, creating characteristic neurological responses for certain tastes. Others associate it with broader activity patterns; most neurons respond to most tastes, and the resulting neurological scheme roughly matches one or another taste. The work of Fontanini and colleagues supports the latest theory, predicting exactly how this structure should look. Already only clusters "describe the many properties of the taste of the crust," said Fontanini, "spontaneous activity, the sequence of responses to taste, the effect of expectation." He hopes to continue to dig up the history of the formation of these clusters, and what other types of nervous activity they influence.

The work also describes the neural basis for expectations in the brain. A warning hint does not just excite certain neurons or causes a set of states, which then encodes the stimulus. Instead, waiting changes the dynamics — specifically, the switching speed — of the entire system.

Fontanini and La Camera compare this dynamic with a ball moving over a landscape filled with trenches. These grooves indicate reaction states, and waiting tilts the terrain so that the ball quickly falls into the first trench. It also smoothes hilly terrain, in which the ball needs to move from one state to another, facilitating this transition and obstacles getting stuck.

That is, the wait makes the network not so sticky. It allows you to make a easier walk towards the states that actually encode taste, but does not give such stability that the system is stuck in one state. This problem often haunts such clustered networks: because of this grouping, some “trenches” turn out to be too deep, and the system reinforces incorrect information. But these discoveries say that to solve this problem “you don’t need a complex system,” said Georg Keller, a neuroscientist who studies the work of sight at the Institute of Biomedical Research. Friedrich Miescher in Switzerland.

Fontanini and La Camera hope that this mechanism will be able to explain the work of other processes that take context beyond expectations, such as attention and learning. But perhaps “the most important consequence of our work will be to shift attention from the static response of neurons that encode certain reactions to their dynamic behavior,” said La Camera.

And although the approach to the study of neuroscience through dynamic systems cannot be called new, it was difficult to test and simulate it. Experts usually lean toward the hierarchical structure of sensory perception: the cortex builds up and integrates features to form perceptions, sending signals to other layers of the network that integrate even more information until the brain decides or chooses behavior.

But this is not the case. The team’s results speak in favor of another signal-processing idea, in which “everything happens at the same time, even before the stimulus signal arrives,” said Leslie Kay, a neuroscientist from the University of Chicago who practices smell. “The information learned turns out to be in the field of the cortex, forms a system of interconnected groups of neurons denoting this information, and then you influence it with the help of expectations, which results in what this system knows.”

Sudden rush

It follows from the model that the decision-making process is not a smooth construction based on the information received, but something like a sequence of insights, a leap of neural fluctuations. Katz used the same model as Fontanini with la Chamber to support the idea that a decision "happens in a sudden rush."

The connection between these “completely different angles of taste” - the work of Fontanini on the processing of sensations from the senses and his study of their further processing - leaves Kac in a state of “joyful anticipation”.

It also emphasizes the need to move away from concentrating on individual neurons that respond to certain prompts, and move toward internal states and dynamics in order to better understand the functioning of sensory networks - even in the case of the most basic sensory stimuli. “It’s much easier to say that a neuron increases the number of activations,” said Anan Moran, a neuroscientist at Tel Aviv University in Israel. But in order to understand how organisms work, “one cannot take into account only the stimulus, one has to reckon with the internal state,” he added. “And this means that our previous understanding of the mechanism used by the brain to implement sensations, actions, and so on, needs to be reconsidered.”

“Most of what happens in the gustatory crust before it reaches the stimulus is related to its processing by arrival,” said Katz. In this case, a study of how these internal states change under the influence of experience or clues has opened up to us new information about the connectivity of the network.

Now, Moran said, such contextual dependence must undergo other studies of perception and thinking. “The final frontier is the visual system. Such work can tell us something interesting about the processing of visual information. "

“We do not yet have a good, unified model uniting all this activity,” he added. But this is a “good starting point."