1. Lettvin, J. Y., Maturana, H. R., McCullock, W. S., and Pitts, W. H., Proc. Inst. Radio Eng., 47, 1940 (1959).
2. Barlow, H. B., and Hill, R. M., Science, 139, 412 (1963).
3. Maturana, H. R., Twenty-third Intern. Union Physiol. Sci., Leiden (1960).
4. Grusser-Cornehls, U., Grusser, O., and Bullock, T. H., Science, 141, 820 (1963).
5. Wagner, H. G., MacNichol, E. F., and Wolbarsht, M. L., J. Opt. Soc. Amer., 53, 66 (1963).