Where and how memories are encoded in nervous system
Washington: Scientists have had trouble solving one of the biggest mysteries in the biological research: where and how memories are encoded in a nervous system.
The hippocampus has long been considered a centre in the brain for the long-term storage of spatial associations.
Mazahir T. Hasan at the Max Planck Institute for Medical Research and Jose Maria Delgado-Garcia at the University Pablo de Olavide of Seville, Spain, have been able to provide first experimental evidence that a specific form of memory associations is encoded in the cerebral cortex and is not localized in the hippocampus as described in most Neuroscience textbooks.
The new study is a game changer since it strongly suggests that the motor cortical circuits itself, and not the hippocampus, is used as memory storage.
The new study shows, in the motor cortex this so-called synaptic plasticity no longer functions without the NMDA receptors.
The scientists were thus able to rule out the hippocampus or other regions as the cause for their observations. Based on the new findings, it is the cerebral cortex, not the hippocampus that is the storage site for some forms of memory.
In behaviour tests, so called eyeblink conditioning, animals with and without NMDA receptors in the primary motor cortex had to learn to link a tone with a subsequent electrical stimulus of the eyelid. This association of two sensory inputs involves the cerebellum which coordinates the necessary movements, as well as the hippocampus and the cerebral cortex, which are important learning and memory centres.
Hasan said that after a learning phase, the animals` reflex is to close their eye when they hear just the tone, asserting that without NMDA receptors in the primary motor cerebral cortex, the genetically modified mice on the other hand cannot remember the connection between the tone and electrical stimulus, and therefore they keep their eyes open despite the tone.
He said that they believe that the hippocampus provides the necessary environmental cues, which are transmitted to the cortex where learning-dependent associations take place. Memories are thus stored at various sites in the cerebral cortex on a long-term basis.