Tabular data for the following experiments: Analysis of synaptic localization of pSYN, synaptic density, and association of synaptic pSYN to pSYN positive neurites_AS: Synaptic localization of pSYN, synaptic density, and puncta size changes (relative to Lewy neurites), as well as the association of synaptic pSYN with pSYN-positive neurites, were quantified. Basically, it has the following dataset: Large-area quantification of how synaptically associated p-a-syn pathology impacts synaptic abundance Spatial relationship between p-a-synuclein aggregates and cortical synapse loss SEQUIN analysis for 1MPI and 6MPI using synapsin and PSD-95 Axonal and dendritic imaging analysis one month after injection of alpha-synuclein pre-formed fibrils SEQUIN for VGLUT1, VGLUT2 and VGAT positive synaspes_SS: Synaptic loci frequency distributions and synapse densities for VGLUT1- and VGLUT2-positive excitatory synaptic loci and VGAT-positive inhibitory synaptic loci were quantified. VGLUT1 and VGLUT2 colocalization with endogenous α-synuclein_SS: Colocalization of VGLUT1 and VGLUT2 with endogenous α-synuclein was quantified in the cortex. Electron Microscopy analysis of excitatory synapses_JR_DR: Ultrastructural measures were quantified, including synaptic vesicle area, intervesicular distance, and synaptic cleft distance. pSyn pathology negatively impacts excitatory transmission_JG: Electrophysiology data from cortical slices were analyzed to assess how pSyn pathology affects synaptic transmission. pSyn pathology progressively increases_SS: Lewy pathology progression at 1, 2, and 3 months post–striatal PFF injection was quantified.