AXONAL PROCESSES AND NEURAL PLASTICITY .1. OCULAR DOMINANCE COLUMNS

We present two related computational models of ocular dominance column formation. Both address nervous system plasticity in terms of sprouti ng and retraction of axonal processes rather than changes in synaptic strength implied by synapse-specific Hebbian models. We employ statist ical mechanics to simulate changes in the pattern of network connectiv ity. Our formalism uses the concept of an energy function, which we in terpret as related to the levels of target-generated neurotrophins for which afferents compete. In contrast, synapse-specific Hebbian models impose synaptic normalization, for which there is little experimental evidence, in order to induce competition. Our models make many predic tions which require experimental investigation. We suggest that the ab sence of monocular deprivation effects in the optic tectum may be due to a tendency of amphibian retinal ganglion cells to preserve the comp lexity of their terminal arbors. One model raises the possibility that boundaries separating columns in the mammalian cortex are poorly inne rvated if they have been formed by complete but asynchronous retinal a ctivation. Both models exhibit a phase transition, suggesting a discon tinuity in the transition from a binocular cortex to one possessing oc ular dominance columns. Finally, our other model could account for the perpendicularity of ocular dominance columns to the boundary of the p rimary visual cortex while admitting of less ordered central patterns.