CYTOCHROME-OXIDASE BLOBS AND INTRINSIC HORIZONTAL CONNECTIONS OF LAYER 2 3-PYRAMIDAL NEURONS IN PRIMATE V1/

Pyramidal neurons in superficial layers of cerebral cortex have extens ive horizontal axons that provide a substrate for lateral interactions across cortical columns. These connections are believed to link funct ionally similar regions, as suggested by the observation that cytochro me-oxidase blobs in the monkey primary visual cortex (V1) are preferen tially connected to blobs and interblobs to interblobs. To better unde rstand the precise relationship between horizontal connections and blo bs, we intracellularly labeled 20 layer 2/3 pyramidal neurons in tange ntial 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 lo ng-distance, clustered axons (somata 130-200 mu m from blob centers) a nd (2) cells with clustered, long-distance axon collaterals (somata <1 30 mu m or >200 mu m from blob centers). For all cells, synaptic bouto ns close to the cell body were located at similar distances from blob centers as the cell body. The majority of boutons from cells lacking d istal axon clusters were close to their cell bodies. Cells located mor e than 200 mu m from blob centers were in interblobs and had long-dist ance clustered axon collaterals selectively targeting distant interblo b regions. Cells located less than 130 mu m from blob centers were fou nd within both blobs and interblobs, but many were close to traditiona lly defined borders. The distant synaptic boutons from these cells wer e generally located relatively near to blob centers, but the neurons c losest to blob centers had synaptic boutons closer to blob centers tha n those farther away. There was not a sharp transition that would sugg est specificity for blobs and interblobs as discrete, binary entities. Instead they appear to be extremes along a continuum. These observati ons have important implications for the function of lateral interactio ns within V1.