Spatial memory is more vulnerable than object recognition memory when damage occurs in the brain’s memory processing center, the hippocampus, according to memory specialists at the University of California, San Diego (UCSD) School of Medicine in a study published the week of September 27, 2004 in the online edition of the Proceedings of the National Academy of Sciences.
“Our findings have implications for understanding psychiatric and neurological disease that affect the hippocampus and related medial temporal lobe structures,” said the study’s first author, Nicola J. Broadbent, Ph.D., a post doctoral fellow in the UCSD Department of Psychiatry.
Damage to the hippocampus can occur following lack of oxygen such as during cardiac arrest and the degree of damage can vary with the duration of the hypoxia, or oxygen deprivation. Psychiatric illnesses such as schizophrenia, post-traumatic stress disorder and depression are often associated with modest reductions in hippocampal volume. While researchers have known that the hippocampus, together with other medial temporal lobe structures, is important for the acquisition of new memories, little has been known about the effects of small amounts of hippocampal damage or how the extent of damage is related to function.
In studies with rats, the researchers found that spatial memory was impaired with as little as 30 to 50 percent of damage to the hippocampus, while object recognition memory remained intact until the animals were given larger lesions that encompassed 75 to 100 percent of hippocampal volume. Spatial memory includes such tasks as retention of geographical layouts or interiors of a house; object recognition memory calls upon the ability to remember stimuli or objects that were previously experienced.
In the experiments, 148 male rats received either sham lesions (an incision but no damage to the hippocampus) or actual lesions that damaged the hippocampus in various degrees, from 5 to 100 percent. Spatial memory training was conducted daily for five days in a standard water maze with a retractable platform. The object recognition task involved exposure to two identical objects, removal to the home cage for three hours, then a return to the test chamber with one previously seen object and a new object. Rats without damage to the hippocampus typically remember the previously seen object, and spend most of their time investigating the new, and now more interesting object.