Implicit memory refers to the expression of past events on current behavior when people are not trying to retrieve these past events and when they are usually not even aware of the events' influence. This process is different from explicit memory, which refers to conscious attempts to retrieve memories of past events; in implicit memory tests there is no conscious effort to retrieve. The customary use of the terms memory or remembering refers to explicit, conscious recollection during which people attempt to travel back in time to mentally relive or reexperience past events. Many behaviors people perform, however, reflect past learning even when they are not consciously attempting to retrieve; therefore, these behaviors reflect the manifestation of implicit memory. Some of these behaviors involve motor skills. When people tie their shoes or ride a bicycle or walk, they need not consciously retrieve their first attempts to learn these skills. The same is true of other types of learning. It is much easier to read a passage of text that one has read before, even if not consciously trying to remember the original time the passage was read. As these examples indicate, implicit learning is sometimes referred to as occurring rather automatically or at least to having an automatic component.
As a Reflection of Conscious Learning
Implicit memory measures are sometimes said to reflect unconscious learning because densely amnesic brain-damaged patients typically show intact uses of implicit memory. The data in Figure 1 are from a 1984 experiment by Peter Graf, Larry Squire, and George Mandler. They compared brain-damaged participants who displayed serious impairments on explicit memory tests such as recall and recognition to age- and education-matched control participants. In one test condition, both groups of participants studied lists of words and attempted to recall them in any order (free recall). As can be noted on the left of the graph in Figure 1, the patients recalled the words much worse than did the controls. This pattern reflects the patients' deficit on an explicit memory test, in which they were asked to consciously retrieve past events.
In the implicit test, both groups studied the lists of words but were tested by being shown three-letter stems of words with the instruction to produce the first word that came to mind in response to each stem clue. So, if the word chair had been in the list, participants would get cha and be asked to say the first word that came to mind (chain, chapter, challenge and so on–each stem had at least ten possible
completions). If chair had not been presented in the list, people in both groups produced it about 10 percent of the time (the dashed line on the right side of Figure 1). However, if chai had been presented in the list, both patients and the control participants produced the word about 45 percent of the time. The fact that both groups completed the word so much above the base rate reflects priming, the basic measure of implicit memory tests. Priming is defined as the difference between performance on a test when the relevant information has been presented and performance when the relevant information has not been recently presented. Therefore, the amount of priming reflected in the data in Figure 1 was about 35 percent (45% in the primed condition minus the 10% base rate). Although the participants were told to produce any word that came to mind, the presentation of chair in the list primed them to produce that word rather than another on the test.
This priming effect reflects a use of memory, but not a conscious or intentional use of memory. Because the patients were densely amnesic and probably did not remember even studying the list of words, the priming may be said to be unconscious (in this sense). The amnesic patients produced just as much priming as did control participants. Because the patients had suffered brain damage that impaired their use of explicit memory processes, it appears that the brain mechanisms and processes that underlie explicit and implicit memory tests are quite different. Put another way, the results show that memory is not a unitary entity; people with certain types of brain damage can be severely impaired on one type of memory test and unaffected on other types of tests.
Implicit Memory Tests
The study of implicit memory began in psychology in the early 1980s and in the early twenty-first century there is a large amount of literature on the topic. There seem to be at least two distinct types of implicit memory tests, perceptual and conceptual.
Perceptual memory tests. Perceptual implicit memory tests challenge the perceptual system by presenting impoverished test stimuli to which participants respond. The word stem completion test already described (cha) is one such test. Others are word identification (presenting words very briefly and having participants guess what they are), and word fragment completion (naming words from fragments such as l_p_a_t. (That fragment is hard if not recently primed [with word elephant].) If pictures are used as study materials, then the test can involve giving fragmented forms of pictures or having them be gradually clarified through a series of successively fuller fragments until the participant can identify the picture. Again, the measure in all cases is priming–as reflected by more accurate or faster completion of the target when it has been studied relative to when it has not been studied.
Factors that greatly affect priming on perceptual implicit memory tests are often quite different from those that affect performance on most explicit memory tests in both patients and in healthy control participants, indicating further that these two types of tests seem to be measuring different processes. For example, modality of presentation of words strongly affects performance on perceptual implicit tests. Visual presentation of words enhances priming on visual tests, whereas auditory presentation enhances priming on auditory implicit tests (e.g., presenting words describing noise with auditory cues for identification). Modality generally matters little in tests of explicit memory. On the other hand, factors that can have a great effect on explicit memory tests can have little or no effect on priming on implicit tests. For example, when participants read pairs of words (hot and cold) or generate the second word from a clue such as "opposite of cold," they scored better on an explicit test of recognition for the words they generated, but exhibited more priming for the words they just read on an implicit test, in which they had to quickly identify the word. The data are shown in Figure 2.
The results described above can be explained, at a general level, by the theory of transfer appropriate processing. This principle states that performance on memory tests will be enhanced if there is a match between the conditions of study and test, which will permit the study experience to transfer better to the test. For example, if the test involves deciphering a fragmented or briefly presented word given visually (classified as a perceptual test), then performance on this test should benefit from prior visual presentation more than from a prior auditory presentation or from generating the word, as is indeed the case. Practice reading a visual word (versus hearing or generating it) transfers better to a test that also involves reading words.
Conceptually driven tests. Whereas most implicit tests depend on perceptual processing, most explicit memory tests depend heavily on the meaning of the concepts or events that are being remembered. These tests are called conceptually driven tests because, when people are trying to retrieve past events, it is the meaning of the events that is important. Generating a word involves more attention to meaning than simply reading it, and so generating produced greater explicit recognition in results shown in Figure 2. Again, this finding is in accord with the transfer appropriate processing theory. The transfer appropriate processing theory can account for a large body of findings although some problems remain.
Although explicit memory tests are usually driven by meaning or by conceptual information, there is a class of implicit memory tests that is also conceptually driven. These tests are probably the most relevant for education, but they have not yet been studied as much as perceptual implicit tests. One class of conceptual implicit memory tests that has been studied is the general knowledge test. "What animal did Hannibal use to help him cross the Alps in his attack on Rome?" and "What is the name of the ship that carried the pilgrims to America in 1620?" are examples of questions on general knowledge tests. Prior exposure to the words elephant or Mayflower before the questions are asked increases correct answers to these questions, which reflects priming of concepts. Free association tests ("say the first word that you think of to the stimulus word tusk") and category association tests ("list as many African animals as you can in thirty seconds") are other examples of conceptually driven implicit memory tests. These priming effects again seem to be indicative of implicit retrieval because they also appear in brain-damaged patients with severe difficulties in explicit expressions of memory.
In some sense, much of education is intended to permit people the automatic, unconscious retrieval of facts, routines, and principles when they need them. Education is meant to provide learning experiences that will, at least in some cases, last a lifetime. Of course, not all facts and principles will be remembered for that long. Much information learned in the classroom will be forgotten (at least when explicit tests are given). The hope is that one's general knowledge and skills (writing, thinking logically)
will survive. There are no studies of residual, implicit retention of formal education but such studies will surely come in the future.
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HENRY L. ROEDIGER III