Researchers from Charité – Universitätsmedizin Berlin and the Max Planck Institute for Biological Intelligence, which is currently being established, have for the first time identified the precise connections between sensory neurons within the retina and the superior colliculus, a structure in the midbrain. The most recent advancement in electrode technology is the creation of neuropixel probes. Neuropixel probes, which are used to record nerve cell activity and are densely packed with recording points, have made it possible to gain these most recent insights into neuronal circuits. In a paper that was published in Nature Communications, the researchers explain a basic idea that applies to the visual systems of both mammals and birds.
The superior colliculus, a midbrain structure, and the visual cortex, a part of the primary cerebral cortex, are both essential for processing visual stimuli. Visual perception and information processing are extremely complex processes. The structures of the evolutionarily older midbrain are in charge of visually guiding reflexive behaviors, whereas the visual cortex is in charge of general visual perception. The principles and mechanisms guiding visual processing within the visual cortex are well understood. Our understanding of this area has improved as a result of research done by a team of scientists led by Dr. Jens Kremkow, which culminated in the creation of an Emmy Noether Junior Research Group at Charité’s Neuroscience Research Center in 2017. (NWFZ). Increasing our knowledge of the nerve cells involved in the visual system is the main goal of the research team, which is supported by the German Research Foundation (DFG). One of the many questions that still need to be answered is how visual information is processed in the superior colliculi of the midbrain.
Sensory retinal ganglion cells in the eye’s retina react to outside visual stimuli and transmit the information they gather to the brain. Direct signaling pathways guarantee that the midbrain also receives the visual information that is received by the retinal nerve cells. “The functional relationship between nerve cells in the midbrain and nerve cells in the retina has largely remained unknown up until this point. In a similar way, we don’t know much about how synaptic inputs are handled by neurons in the superior colliculi.Dr. Kremkow, the study’s principal investigator, says This information is essential to understanding the mechanisms underlying midbrain processing. It has been impossible to measure the activity of synaptically linked midbrain and retinal neurons in living organisms until now. The research team created a methodology based on measurements obtained using cutting-edge, high-density electrodes known as Neuropixels probes for their most recent study. Specifically, Neuropixels probes are tiny, linear electrode arrays with roughly 1,000 recording sites along a short shank. These instruments, which have 384 electrodes and can record the electric activity of many neurons in the brain at the same time, have changed the way neuroscience is studied.
Now, scientists at Charité and the Max Planck Institute for Biological Intelligence have identified the pertinent midbrain structures in mice (superior colliculi) and birds using this new technology (optic tectum). In both types of animals, the visual processing of retinal input signals is carried out by both brain regions, which share a common evolutionary history. They discovered something unexpected as a result of their work: According to Dr. Kremkow, “Typically, this type of electrophysiological recording measures electrical signals from action potentials that originate in the soma, the neuron’s cell body.” ” However, we discovered signals in our recordings that looked different from typical action potentials. We then looked into what was causing this phenomenon and discovered that the ‘axonal arbors’ (branches) of the retinal ganglion cells were the source of the action potentials that were responsible for the input signals in the midbrain. Our research suggests that axons, the nerve cell projections that transmit neuronal signals, can be used to record electrical signals using the new electron array technology. This is an entirely new discovery. ” In a first for the world, Dr. Kremkow’s team was able to record the activity of both synaptically connected target neurons in the midbrain and retinal nerve cells simultaneously.
The functional wiring between the midbrain and the eye had been unknown until now. At the single-cell level, the researchers were able to demonstrate that the spatial arrangement of the inputs from retinal ganglion cells in the midbrain constitutes an extremely accurate representation of the original retinal input. The midbrain’s structures, according to Dr. Kremkow, “effectively provide an almost exact replica of the retinal structure.” He keeps going: “Another novel discovery we made was that retinal ganglion cells only supply a small subset of the midbrain’s sensory neurons with very strong and focused synaptic input. These neural connections allow for a very organized and useful connection between the midbrain’s corresponding regions and the retina of the eye. ” This new understanding will help us better understand the phenomenon of blindsight, which is seen in people whose visual cortex has been harmed by trauma or tumors, among other things. People with this condition don’t have conscious perception, but they still have some ability to process visual information. This gives them an intuitive sense of stimuli, shapes, movement, and even colors, which seems to be linked to the midbrain.
Dr. Kremkow and his team collaborated with a team from the Max Planck Institute for Biological Intelligence, where a Lise Meitner Research Group led by Dr. Daniele Vallentin focuses on neuronal circuits responsible for the coordination of precise movements in birds, to test whether the principles initially observed in the mouse model could also apply to other vertebrates and, therefore, whether they could be more general in nature. According to Dr. Vallentin, “Using the same types of measurements, we were able to demonstrate that the spatial organization of the nerve tracts connecting the retina and midbrain follows a similar principle in zebra finches.” This finding was unexpected, she continues, “given that birds have significantly higher visual acuity and that there is a significant evolutionary gap between birds and mammals.” The retinal ganglion cells in the superior colliculi and optical tectum appear to have similar spatial organization and functional wiring, according to the researchers’ observations. As a result of their findings, they came to the conclusion that the principles they had discovered must be essential to visual processing in the mammalian midbrain as a result of their findings. It’s even possible that these ideas are universal and hold true for the brains of all vertebrates, including those of people.
The research team will continue to investigate how sensory signals are processed in the vision system, specifically in the regions of the midbrain, and how they affect visually-guided reflexive behavior, according to Dr. Kremkow. “Now that we understand the functional, mosaic-like connections between retinal ganglion cells and neurons within the superior colliculi,” he says. The team also wants to find out if the new technique can be applied to other brain structures and if it can be used to measure axonal activity in other parts of the brain. This would create a wealth of new opportunities to investigate the underlying mechanisms of the brain, should it turn out to be feasible.
