Figure 1. The primary visual pathway and examples of the activity-dependent development of brain circuits. (a) Retinal ganglion cells mediating perception of a visual hemifield project their axons to the contralateral lateral geniculate nucleus (LGN). Relay cells of the LGN innervate layer 4 of the visual cortex. (b) Early in development axons from the ipsilateral and contralateral eyes are intermingled in the LGN. Later, axons from the two eyes segregate to form eye-specific laminae and, shortly thereafter, sublaminae that receive inputs from either ON- or OFF-centre retinal ganglion cells. (c) Similarly, axons from both LGN laminae initially overlap at their target, layer 4 of the visual cortex, and are subsequently refined such that they occupy distinct ocular dominance columns. (d) Maps of the orientation preference of neurons (depicted as shades of blue) in visual cortex show relatively little clustering of neurons with like orientation preferences in early development. As the circuitry matures, maps reveal extensive clustering of similar orientation preferences in a pinwheel-like organization. (e) Developing pyramidal neurons in the superficial layers of visual cortex extend their axons horizontally in a relatively crude manner, showing little preference for any particular area within their reach. As development proceeds, the axonal arborization is elaborated in areas of similar orientation preferences, and branches are removed from dissimilar areas.