SPONTANEOUS PINWHEEL ANNIHILATION DURING VISUAL DEVELOPMENT

Neurons in the visual cortex respond preferentially to edge-like stimu li of a particular orientation(1). It is a long-standing hypothesis th at orientation selectivity arises during development through the activ ity-dependent refinement of cortical circuitry(2-4). Unambiguous evide nce far such a process has, however, remained elusive(5-7). Here we ar gue that, if orientation preferences arise through activity-dependent refinement of initially unselective patterns of synaptic connections, this process should leave distinct signatures in the emerging spatial pattern of preferred orientations. Preferred orientations typically ch ange smoothly and progressively across the cortex(1). This smooth prog ression is disrupted at the centres of so-called pinwheels(8,9), where neurons exhibiting the whole range of orientation preferences are loc ated in close vicinity(10). Assuming that orientation selectivity deve lops through a set of rules that we do not specify, we demonstrate mat hematically that the spatial density of pinwheels is rigidly constrain ed by basic symmetry principles. In particular, the spatial density of pinwheels, which emerge when orientation selectivity is first establi shed, is larger than a model-independent minimal value. As a consequen ce, lower densities, if observed in adult animals, are predicted to de velop through the motion and annihilation of pinwheel pairs.