From each animal’s brain tissue, data were pooled from EM images captured across two grids, so as to avoid over-representation of data from any one grid. A minimum of 200 asymmetric synapses was sampled from the dorsal tip of the LA of each animal, and these were separated into two groups: axo-spinous synapses versus axo-shaft synapses. Typically, about 180 of the encountered synapses were on spines, and 12 or more of the asymmetric synapses were along the dendritic shafts. Because the number of asymmetric synapses occurring along dendritic shafts was small, the number of axo-shaft synapses analyzed per animal was kept constant at 12, by randomly sampling 6 of the synapses from one grid and randomly sampling 6 of the synapses from the second grid.
