During the last 20 years, neuroscience research has revealed that the cerebral cortex constantly generates predictions on what will happen next, and that neurons in charge of sensory processing only encode the difference between our predictions and the actual reality.

Did you hear that?

Now, a team of neuroscientists of TU Dresden headed by Prof Dr Katharina von Kriegstein presents new findings that show that not only the cerebral cortex, but the entire auditory pathway, represents sounds according to prior expectations.

For their study, the team used functional magnetic resonance imaging (fMRI) to measure brain responses of 19 participants while they were listening to sequences of sounds. The participants were instructed to find which of the sounds in the sequence deviated from the others. Then, the participants’ expectations were manipulated so that they would expect the deviant sound in certain positions of the sequences.

The neuroscientists examined the responses elicited by the deviant sounds in the two principal nuclei of the subcortical pathway responsible for auditory processing: the inferior colliculus and the medial geniculate body. Although participants recognised the deviant faster when it was placed on positions where they expected it, the subcortical nuclei encoded the sounds only when they were placed in unexpected positions.

The brain is always predicting

These results can be best interpreted in the context of predictive coding, a general theory of sensory processing that describes perception as a process of hypothesis testing. Predictive coding assumes that the brain is constantly generating predictions about how the physical world will look, sound, feel, and smell like in the next instant, and that neurons in charge of processing our senses save resources by representing only the differences between these predictions and the actual physical world.

Dr Alejandro Tabas, first author of the publication, states on the findings: “Our subjective beliefs on the physical world have a decisive role on how we perceive reality. Decades of research in neuroscience had already shown that the cerebral cortex, the part of the brain that is most developed in humans and apes, scans the sensory world by testing these beliefs against the actual sensory information. We have now shown that this process also dominates the most primitive and evolutionary conserved parts of the brain. All that we perceive might be deeply contaminated by our subjective beliefs on the physical world.”

New possibilities in neuroscience

These new results open up new ways for neuroscientists studying sensory processing in humans towards the subcortical pathways. Perhaps due to the axiomatic belief that subjectivity is inherently human, and the fact that the cerebral cortex is the major point of divergence between the human and other mammal’s brains, little attention has been paid before to the role that subjective beliefs could have on subcortical sensory representations.

Given the importance that predictions have on daily life, impairments on how expectations are transmitted to the subcortical pathway could have profound repercussion in cognition. Developmental dyslexia, the most wide-spread learning disorder, has already been linked to altered responses in subcortical auditory pathway and to difficulties on exploiting stimulus regularities in auditory perception.

The new results could provide with a unified explanation of why individuals with dyslexia have difficulties in the perception of speech, and provide clinical neuroscientists with a new set of hypotheses on the origin of other neural disorders related to sensory processing.

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In more dire circumstances, however, the brain’s fear response can be critical for survival. “Being able to fear is the ability to sense the danger and is the driving force to figure out a way to escape or fight back,” said Cold Spring Harbor Laboratory Professor Bo Li.

VIDEO: Watching a mouse think about fear and pleasure

Li’s team is probing the brain circuits that underlie fear, using sophisticated neuroscience tools to map their connections and tease out how specific components contribute to learning fear. A deeper understanding of these circuits could lead to better ways to control the overactive or inappropriate fear responses experienced by people with anxiety disorders.

Many of their studies begin with the amygdala, an almond-shaped structure that is considered the hub for fear processing in the brain. While the amygdala was once thought to be devoted exclusively to processing fear, researchers are now broadening their understanding of its role.

Amygdala: beyond fear response

Li’s team has found that the amygdala is also important for reward-based learning, and as they trace its connections to other parts of the brain, they are uncovering additional complexity. Li said: “It is important for formation of fearful memory, but it’s also important for interacting with other brain systems in a different behavior context. We think that this circuit that we discovered that plays a role in regulating fearful memory is only a tip of the iceberg. It is indeed important for regulating fearful memory, but probably is also involved in more complex behavior.”

Li and his colleagues were surprised recently to find that the amygdala communicates with a part of the brain best known for its role in controlling movement. The structure, called the globus pallidus, was not known to be involved in fear processing or memory formation. But when the researchers interfered with signaling between the amygdala and the globus pallidus in the brains of mice, they found that the animals failed to learn that a particular sound cue signaled an unpleasant sensation. Based on their experiments, this component of the fear-processing circuitry might be important for alerting the brain “which situations are worth learning from,” Li said.

Li’s team and collaborators at Stanford University reported recent findings in the Journal of Neuroscience.

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