Axonal processes and neural plasticity. II: Adult somatosensory maps

Following our recent presentation of an axonal process sprouting and retraction framework for ocular dominance column formation, we now apply it, unchanged, to address issues of adult somatosensory map plasticity. Specifically, we model the rearrangement of S-I in adult rodents following denervation of a row of vibrissae, and the rearrangement of area 3b in adult monkeys following hyperstimulation of a digit. While we do not attempt to capture the rapid changes which occur as the result of unmasking or potentiating existing connections, we demonstrate that axonal process sprouting and retraction is a possible mechanism mediating many of the long-term changes induced by anomalous peripheral activity. A significant feature of our framework, demonstrated by this study, is that it can account for plasticity in both developing and mature systems, and in different sensory modalities. In contrast, synapse-specific Hebbian models with synaptic normalization, which employ anatomically fixed connections capable of changes in efficacy, may not be able to account for both developmental and adult plasticity without the form of the imposed normalization, which enforces competition between afferents, being changed.