Rapid anatomical plasticity of horizontal connections in the developing visual cortex.

Experience can dramatically alter the responses of cortical neurons. During a critical period in the development of visual cortex, these changes are extremely rapid, taking place in 2 d or less. Anatomical substrates of these changes have long been sought, primarily in alterations in the principal visual input from the thalamus, but the significant changes that have been found take 1 week. Recent results indicate that the initial physiological changes in the cortical circuit take place outside of the primary input layer. We now find that rapid plasticity of binocular responses in the upper layers of cortex is mirrored by similarly rapid anatomical changes in the horizontal connections between ocular dominance columns in the upper layers, which reorganize within 2 d.