ANATOMICAL DEMONSTRATION OF OCULAR DOMINANCE COLUMNS IN STRIATE CORTEX OF THE SQUIRREL-MONKEY

The squirrel monkey is the only primate reported to lack ocular domina nce columns. Nothing anomalous about the visual capacity of squirrel m onkeys has been found to explain their missing columns, leading to the suggestion that ocular dominance columns might be ''an epiphenomenon, not serving any purpose'' (Livingstone et al., 1995), Puzzled by the apparent lack of ocular dominance columns in squirrel monkeys, we made eye injections with transneuronal tracers in four normal squirrel mon keys. An irregular mosaic of columns, averaging 225 mu m in width, was found throughout striate cortex. They were double-labeled by placing wheat germ agglutinin-horseradish peroxidase into the left eye and [H- 3]proline into the right eye, The tracers labeled opposite sets of int erdigitating columns, proving they represent ocular dominance columns. The columns were much clearer in layer IVc alpha (magno-receiving) th an IVc beta (parvo-receiving). In the lateral geniculate body, the par vo laminae showed extensive mixing of ocular inputs, suggesting that i ncreased label spillover contributes to the blurred columns in layer I Vc beta. The cytochrome oxidase (GO) patches were organized into disti nct rows, but they bore no consistent relationship to the ocular domin ance columns. These experiments indicate that ocular dominance columns are less well segregated in squirrel monkeys than macaques, but they are present. This fact is pertinent to a recent study reporting that o cular dominance columns are absent in normal squirrel monkeys, but ind uced to form by strabismus (Livingstone, 1996).