How do features of sensory representations develop?

Sensory representations in the brainstem and cortex have a number of features that support the idea that neural activity patterns are important in their development. Many of these features vary across species in ways that could result from perturbances in the balance of the effects of activity patterns and position-dependent gene expression. (1) Most notably, disruptions or septa in sensory maps often reflect actual discontinuities in the receptor sheet, and the discontinuities may be reflected in a series of interconnected maps. Species with different disruption patterns in sensory sheets have different matching disruption patterns in the sensory maps and variant individuals and strains of the same species have matching variations in the receptor disruption patterns and their sensory maps. (2) In addition, mutations that misdirect some of the retinal afferents from one side of the brain to the other create new sensory maps that preserve continuities in the altered pattern of input, while creating new structural discontinuities. (3) Furthermore, functionally different classes of afferents that are mixed in the receptor sheet often segregate to activate separate populations of target cells. (4) Finally, early developing portions of receptor sheets may gain more than their share of territory in sensory maps. These and other variable features of sensory maps are most readily accommodated by theories that involve roles for instruction by evoked and spontaneous neural activity patterns.