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Fig. 17.2. Example Topographica model. In Topographica, models are composed of interconnected sheets of neurons. Each visual area is represented by one or more sheets in this example model of the early visual system: For instance, the eye is represented by an array of photoreceptors plus two sheets representing retinal ganglion cells. Each of the sheets can be coarse or detailed, plastic or fixed, as needed for a particular study. Afferent and feedback connections link the different areas, and units within each area can be connected using lateral connections. The afferent connections are shown for one sample neuron in each sheet, and also the lateral connections for the sample neurons in V1 and higher areas. Similar models can be used for topographic maps in auditory, somatosensory, and motor cortex.