The sensory cortex contains many columns of vertically arranged neurons that span the width of the cortex, share connectivity, and, importantly, respond to a single peripheral receptive field. This arrangement suggests homogeneity. But that assumption fails upon closer analysis, since cell type, synapse organization, and gene expression differ widely between and within species.
Casanova et al recently reported on their work comparing post-mortem tissue samples of macaques, chimpanzees, and humans in Nissl-stained pyramidal cell columns of the primary visual cortex. They measured the average distance between the central axes of the columns, finding that chimps had the longest average distance (~24-25 micrometers), then humans (~18-19 micrometers), then macaques (~16 micrometers). Also, humans were found to have increased variability in the diameter of their minicolumns as compared to the two other species. There are some evolutionary implications of these findings, namely that the human minicolumns may have been susceptible to reorganization during human evolution.
Casanova MF, et al. 2009 Morphometric variability of minicolumns in the striate cortex of Homo sapiens, Macaca mulatta, and Pan troglodytes. Journal of Anatomy 241:226-234. doi: 10.1111/j.1469-7580.2008.01027
Rakic P. 2008 Confusing cortical columns. PNAS 105:12099-12100. doi: 10.1073/pnas.0807271105