2023-07-04 12:12:35
Silence please! Neuroscience has shown that the absence of noise is essential for us to concentrate, learn, strengthen memory, create our inner world and recharge ‘batteries’, but until now it was not known how the brain managed to identify these sound pauses. A study co-led by Manuel Sánchez-Malmierca, director of the Institute of Neurosciences of Castilla y León (INCYL), has discovered the silence neurons, those that detect the omission of sounds within an auditory sequence. He has called them “skip neurons” and they are located in the auditory cortex.
The key finding of this study, carried out in rodents, has been to identify “that a subset of neurons of the auditory brain responds to the omission of an expected stimulus (a sound within a sequence)”, explains Malmierca, who directs the laboratory of Cognitive and Auditory Neuroscience of Incyl.
Specifically, the investigationpublished in ‘Science Advance’, has shown that robust neural activity appears in response to the omission of a sound in a regular sequence of repetitive stimuli. And this response is consistent with the so-called “prediction error signals”, understood as the difference between what the brain expects to perceive and the stimuli it actually perceives.
According to predictive coding theory, the brain constantly generates “downstream” predictions, which are compared to sensory signals coming to us through the senses (upstream, from outside to the brain).
Incoming sensory signals that match the predictions are suppressed, while unexpected stimuli, which don’t match the predictions the brain generates, trigger a prediction error signal, which propagates to generate new and updated predictions, he explains. Sanchez Malmierca.
In this way, sensory information is continuously shared between the lower levels of sensory input and the higher levels, providing up-to-date predictions about the next expected sensory inputs.
important for survival
“A strength of our study has been using two animal species (mice and rats) and two different recording preparations in two different laboratories. This approach has allowed us to distinguish both the effect of the species and the brain state in the omission responses”, highlights the first author of the study, Ana Lao-Rodríguez.
Thus, they have been able to verify that this omission response does not depend on the size of the auditory cortex (different in mice and rats) and that it is constant in different states of consciousness. In the same way, the researchers from Salamanca have observed that the omission response is present even when attention is not focused on listening to the sounds in the environment, and that it increases when attention is focused on sound stimuli.
“The fact that we have also detected skip responses in the inferior colliculus, a very important subcortical structure of the auditory pathway, as well as in the auditory cortex, indicates that the auditory brain internally generates a prediction about future sensory inputs,” he adds. David Pérez-González, who has also participated in the study.
The inferior colliculi are involved in reflex movements elicited by auditory stimuli. An example of these movements is the reflex to turn the head towards the source of a sudden and unexpected sound.
Prediction provides key survival advantages, and cognitive studies have shown that the brain makes multilevel predictions. But evidence for predictions is difficult to obtain at the neural level because of the complexity of separating neural activity resulting from predictions made by the auditory brain and neural responses to external stimuli.
“We have overcome this challenge by recording individual neurons from cortical and subcortical auditory regions during unexpected omissions of stimuli interspersed in a regular sequence of tones. And thanks to this we found a subset of neurons that responds reliably to the omitted tones”, points out Sánchez Malmierca.
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