Pyramidal neurons in superficial layers of cerebral cortex have extensive horizontal axons that provide a substrate for lateral interactions across cortical columns. These connections are believed to link functionally similar regions, as suggested by the observation that cytochrome-oxidase blobs in the monkey primary visual cortex (V1) are preferentially connected to blobs and interblobs to interblobs. To better understand the precise relationship between horizontal connections and blobs, we intracellularly labeled 20 layer 2/3 pyramidal neurons in tangential living brain slices from V1 of macaque monkeys. The locations of each cell body and the cell's synaptic boutons relative to blobs were quantitatively analyzed. We found evidence for two cell types located at characteristic distances from blob centers: (1) neurons lacking long-distance, clustered axons (somata 130–200 μm from blob centers) and (2) cells with clustered, long-distance axon collaterals (somata <130 μm or >200 μm from blob centers). For all cells, synaptic boutons close to the cell body were located at similar distances from blob centers as the cell body. The majority of boutons from cells lacking distal axon clusters were close to their cell bodies. Cells located more than 200 μm from blob centers were in interblobs and had long-distance clustered axon collaterals selectively targeting distant interblob regions. Cells located less than 130 μm from blob centers were found within both blobs and interblobs, but many were close to traditionally defined borders. The distant synaptic boutons from these cells were generally located relatively near to blob centers, but the neurons closest to blob centers had synaptic boutons closer to blob centers than those farther away. There was not a sharp transition that would suggest specificity for blobs and interblobs as discrete, binary entities. Instead they appear to be extremes along a continuum. These observations have important implications for the function of lateral interactions within V1.
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