Figure 1: Pitts and McCulloch’s model of the visual cortex. Impulses relayed by the lateral geniculate move down to layer IV, where they branch laterally and excite small cells singly and larger cells by summation. Large cells thus represent larger visual areas. From layer IV, impulses go to higher layers. From there, they converge on large cells of the third layer that relay impulses to the parastriate area 18. On their way down, they contribute to summation on the large pyramids of layer V, which relays them to the superior colliculus. Reproduced, with permission of Springer, from “How we know universals: the perception of auditory and visual forms” (W. Pitts and W.S. McCulloch), (6) Bulletin of Mathematics and Biophysics 1947;9:127–47. Copyright © 1947 University of Chicago Press.