Source: M. Belén Pardi
The brain encodes the information our senses collect. In order to perceive and interact with the environment, these sensory signals must be interpreted in the context of past experiences stored in the brain and current goals of the individual. A team led by Prof. Dr. Johannes Letzkus, Professor at the University of Freiburg Medical School and Head of the Research Group at the Max Planck Institute for Brain Research in Frankfurt am Main, identified a major source for this experiment – based on so-called top-down information. Scientists published their findings in the journal Science.
The neocortex is the largest and most powerful region of the human brain. All of its important cognitive functions are made possible by the convergence of two distinct streams of information: a “bottom-up” stream, which represents signals from the environment, and a “top-down” stream, which conveys internally generated information about past experiences and current goals. The issue of exactly how and where this internally generated information is processed remains largely unexplored, Litzkos says. This motivated him and his team to search for the sources of these signals from top to bottom. Scientists have successfully identified a region of the thalamus, a brain region deeply implanted within the forebrain, as a major candidate region for such internal information.
Based on this, Dr. M. Pelin Bardy, postdoctoral fellow in Litzkus’s lab, developed a strategy for measuring the responses of individual thalamic synapses in the mouse neocortex before and after the learning model. “While inappropriate neutral stimuli were encoded by small, transient responses in this pathway, learning strongly boosted their activity and made the signals faster and more sustainable over time,” says Bardy. “We were really convinced when we compared the strength of the acquired memory to the change in activity. Thalamus: This revealed a strong positive association, indicating that input from the thalamus prominently symbolizes an acquired behavioral link to the stimuli.
In further computer experiments and modeling, carried out jointly with the team of Dr. Henning Sprekeler of the Technische Universität Berlin, researchers discovered a previously unknown mechanism that can accurately organize this information and identify a specialized type of neuron in the outer layer of the neocortex that dynamically controls the flow of these. Signals from top to bottom. This confirms the scientists’ assumption that the hypothalamic projections on the sensory neocortex serve as a major source of information about past experiences associated with sensory stimuli. “These top-down signals are disrupted by a number of brain disorders such as autism and schizophrenia,” Litzkos explains. “We hope the current findings will also enable a deeper understanding of the non-adaptive changes underlying these extreme conditions.”
This work was supported by the German Research Foundation, the European Research Council, the Max Planck Society and the Human Frontier Sciences Program.
Pardi, MB, Vogenstahl, J., Dalmay, T., Spanò, T., Pu, D.-L., Naumann, LB, Kretschmer, F., Sprekeler, H., Letzkus, JJ (2020): A thalamocortical A top-down circuit for associative memory. in a: Science. DOI: 10.1126 / science.abc2399