Clustering of ipsilateral cortico-cortical projection neurons to area 7 in the rhesus monkey.

The distribution within individual cytoarchitectonic areas of the cells of origin of ipsilateral cortico-cortical fibres to area 7 of the parietal lobe of the monkey has been studied. After injections of horseradish peroxidase into area 7, labelled cells in a variety of cortical areas were plotted and their distribution along the length of the cortex analysed. Significant clustering of labelled cells was seen wherever their numbers were sufficient for meaningful statistical analysis. In most cases, further analysis using correlation techniques revealed a significant banding of labelled cells across the cortex. Typical bands varied from 500-800 microns in width, and formed an irregular branching and rejoining pattern across the cortex. The intra-area distribution of cortico-cortical projection neurons is comparable to the distribution of neurofibrillary tangles in the cortex in Alzheimer's disease.