Memory is the sum of the mental processes that result in the modification of an individual's behavior in the light of previous experience. There are several different types of memory, In rote memory, one of the least efficient ways of storing information, data is learned by rote and repeated verbatim. Logical memory is far more efficient: only the salient data are stored, and each may be used in its original or in a different context. Mnemonics, which assist rote memory, superimpose what is in effect a logical structure on not necessarily related data. Testing of the efficiency of memory may be by recall (e.g., remembering a string unrelated symbols); recognition (as in a multiple-choice test, where the candidate recognizes the correct answer among alternatives); and relearning, in which comparison is made between the time taken by an individual to commit certain data to memory, and the time taken to recommit it to memory after a delay. Though studies of certain computer functions have thrown light on some of the workings of memory (see also cybernetics), little is known of its exact physiological basis. It appears, however, that chemical changes in the brain, particularly in the composition of RNA, alter the electrical pathways there. Moreover, it seems that some form of initial learning takes place in the nervous system before data are stored permanently in the brain.