Reconstruction of a single cell with all imaged orientation-tuned synapses overlaid onto the orientation preference map; the cell is indicated by a triangle and its synapses are indicated by circles — both are colored by their preferred orientation. Credit: Max Planck Florida Institute for Neuroscience
Study points to an active role for dendrites in cortical processing. The cerebral cortex is the largest and most complex area of the brain, with 20 billion neurons and 60 trillion synapses-a neuronal network whose proper function is critical for sensory perception, motor control, and cognition. The part of the cerebral cortex devoted to vision has played a key role in elucidating fundamental principles that are used by cortical circuits to encode information.
As edges supply a wealth of information about our visual world, neurons in visual cortex respond selectively to edge orientation, some preferring vertical edges, while others prefer horizontal, and all angles in between. Individual neurons also exhibit considerable diversity in their degree of selectivity, some responding to a narrow range of orientations, others to a broad range. These differences in selectivity are critical for accurately encoding visual information in natural scenes, but underlying mechanisms remain unclear.
The team addressed this issue using new microscopic imaging that allowed them for the first time to assess the input/output functions of individual cortical neurons in the living brain. By using in vivo 2-photon calcium imaging, they were able to characterize the orientation tuning and spatial arrangement of synaptic inputs to the dendritic spines of individual neurons in ferret visual cortex, and compare dendritic spine and cell body responses.
They were able to predict orientation preference of individual neurons simply by adding up responses of their dendritic spines. However, the responses of the dendritic spines did not account for the degree of orientation selectivity exhibited by individual neurons. In looking for factors that could account for differences in selectivity, they noticed spines with similar orientation preference were often spatially clustered along the dendrite and that neurons that had a greater number of these clusters exhibited greater selectivity. They also discovered this functional clustering was correlated with localized dendritic events that are likely to enhance the inputs from the clustered spines.
So not only did the researchers solve the riddle of orientation selectivity, they provided evidence that dendrites endow neurons with more computational power than previously thought. While this study focused specifically on information coding in visual cortex, it is likely that functional clustering of inputs within the dendritic field is a common principle influencing neuronal input/output functions throughout the cerebral cortex, significantly enhancing the brain’s information processing capabilities. https://www.maxplanckflorida.org/news-and-media/news/new-insights-into-neural-computations-in-cerebral-cortex/
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