(A) Left shows a cytochrome oxidase (CO) stain obtained from rat S1 by Zheng et al., 2001, with black lines to delineate barrels and to measure departure (Honda-δ; see Senft and Woolsey, 1991) from a perfect Voronoi tessellation. Right shows the initial distribution of axon branching density (a) for one thalamocortical projection, and two molecular guidance fields (ρ), where the domain S has been traced from the CO stain. (B) The strengths of interaction γ with fields and are indicated for each of 41 projections by the lengths of green and blue arrows respectively, assuming that similar fields aligned to the posterior-anterior and medial-lateral axes in the ventroposterior medial nucleus of the thalamus are sampled at the locations of putative barreloid centres (reconstructed from Haidarliu and Ahissar, 2001, their Figure 5b). (C) Results for the example simulation, with parameters , , , , , , and . Colours indicate the thalamic projection for which the connection density is maximal, barrel labels are located at the centroid of each region and black lines delineate boundaries (see Figure 1—video 1). (D) Red dots show the Honda– metric obtained from the simulation approaching that obtained from the real barrels in A (dotted line); black squares show the pattern difference metric , and reveal the emergence of a correspondence between the real and simulated barrel shapes (units mm3); grey hexagons show how selectively each cortical site is innervated; , where . (E) Plotted across the cortical sheet, the selectivity develops to reveal an alignment with the emergent barrel boundary shapes. Greyscale colour indicates values of . All scale bars 1 mm.