Sensory Associations Stored in Cortex, Not Hippocampus: A Study

For the first time, researchers have provided experimental evidence that sensory associations are encoded in the cerebral cortex, rather than being localized in the hippocampus, as is largely believed.

Led by Mazahir Hasan of the Max Planck Institute for Medical Research and José Maria Delgado-Garcia of the University Pablo de Olavide of Seville, the new study offers strong evidence that the motor cortical circuits, and not the hippocampus, is used as memory storage.

Much of modern understanding of the brain and memories is based in the case of Henry Molaison who, back in the 1950s, had significant portions of his hippocampus removed in hopes of curbing his epileptic seizures. After the procedure, Molaison was unable to remember anything new he learned and suffered from severe memory lapses, leading scientists to conclude that the hippocampus functions as the place where the brain stores long-term memory.

However, to what extent Molaison's brain was damaged as a result of the surgery is not clear -- there is no telling, for example, what areas of his brain might have been compromised in the process. For this reason, Hasan and Delgado-Garcia genetically modified mice by turning NMDA receptors, the central molecular elements of the learning process, off only in their motor cerebral cortex and then set about studying the animals' learning behavior.

They found the mice without NMDA receptors in the primary motor cerebral cortex could not remember certain connections, such as closing their eyes when a certain tone was played, while those without the genetic modification could.

Based on these results, Hasan explains that they believe "the hippocampus provides the necessary environmental cues, which are transmitted to the cortex where learning-dependent associations take place," meaning memories are "stored at various sites in the cerebral cortex on a long-term basis."

Knowing this, the researchers explain, could play a pivotal role in the treatment of memory loss due to various neurological diseases, such as amnesia and Alzheimer's disease.