In a follow-up experiment, Hubel and Wiesel (1963) explored the cortical responses present in kittens after binocular deprivation; they found it difficult to find any active cells in the cortex, and the responses they did get were either slow-moving or fast-fatiguing. Furthermore, the cells that did respond selected for edges and bars with distinct orientation preferences. Nevertheless, these kittens developed normal binocularity. Hubel and Wiesel first explained the mechanism, known as orientation selectivity, in the mammalian visual cortex. Orientation tuning, a model that originated with their model, is a concept in which receptive fields of neurons in the LGN excite a cortical simple cell and are arranged in rows. This model was important because it was able to describe a strong critical period for the proper development of normal ocular dominance columns in the lateral geniculate nucleus, and thus able to explain the effects of monocular deprivation during this critical period. The critical period for cats is about three months and for monkeys, about six months.