Localization of a memory trace in the mammalian brain.

The localization of sites of memory formation within the brain has proven to be a formidable task even for simple forms of learning and memory. In order to localize a particular site of memory formation within the brain, the rabbit eyeblink response was classically conditioned while regions of the cerebellum or red nucleus were temporarily inactivated by microinfusions of the gamma-aminobutyric acid agonist muscimol. Cerebellar inactivation completely blocked learning but had no effect on subsequent learning after inactivation, whereas red nucleus inactivation did not prevent learning but did block the expression of conditioned responses. The site of memory formation for this learned response thus appears to be localized within the cerebellum.