PINS: Functional Synaptic Architecture of Visual Cortex

David Fitzpatrick PhD Scientific Director and CEO, Max Planck Florida Institute for Neuroscience Abstract: Ultimately, understanding cortical circuit function and development requires elucidating the functional synaptic architecture of individual neurons—characterizing the functional properties of a neuron’s synaptic inputs, their strength, their source, how they are arranged within the dendritic field, and how this relates to somatic output. To address this challenging problem, we have combined in vivo functional imaging of dendritic spines with correlative light and electron microscopy to explore the functional synaptic architecture of excitatory inputs to individual orientation tuned pyramidal neurons in layer 2/3 of visual cortex of the ferret. Individual pyramidal neurons receive a functionally diverse set of excitatory synaptic inputs that includes preferred orientations that do not elicit a somatic response. Surprisingly, we find no correlation between ultrastructural measures of the synaptic strength of individual spines (spine size and postsynaptic density area), and degree of similarity of spine tuning to that of the soma. Instead, tuning curves based on the relative number of active spines and the aggregate PSD area strongly predict somatic preferred orientation. Our analyses also reveal a high degree of order in the microscale (<10 um) functional clustering of synaptic inputs within the dendritic field relevant to the representation of orientation and visual space, and provide novel insights into the synaptic basis for binocular integration. The functional synaptic architecture of cortical circuits harbors a wealth of under-explored specializations that are critical for building coherent cortical representations. Relevant Publications: Wilson, D. E., Whitney, D. E., Scholl, B., & Fitzpatrick, D. (2016). Orientation selectivity and the functional clustering of synaptic inputs in primary visual cortex. Nature neuroscience, 19(8), 1003-1009. Scholl, B., Wilson, D. E., & Fitzpatrick, D. (2017). Local order within global disorder: synaptic architecture of visual space. Neuron, 96(5), 1127-1138. Scholl B, Thomas CI, Ryan MA, Kamasawa N, Fitzpatrick D. Cortical response selectivity derives from strength in numbers of synapses. Nature. 2020 Dec 16. doi: 10.1038/s41586-020-03044-3. Epub ahead of print. PMID: 33328635.