Recent studies have focused on the mechanisms and conditions yielding the short- and long-term plasticity exhibited by neuronal
responses in the primary auditory cortex of adults. These investigations have examined factors operating at the cellular and intercellular
levels, the effects of global behavioral states and the role of the cholinergic system, which could mediate between the global and local
levels. A behaviorally gated unsupervised Hebbian-covariance rule can explain most of the bottom-up driven changes that were
observed following sensory manipulation. However, additional supervised learning mechanisms are probably required to generate
behavioral improvement. This suggestion has not yet been tested directly.