As discussed previously, researchers generally conceptualize memory as having three stages (encoding, storage, and retrieval). The types of strategies that people use during the encoding (or acquisition) stage have a profound effect on what is remembered at a later time. Considered here are just a few of the classic variables shown to strongly affect performance on a later memory test.
The level-of-processing effect is one of the most robust and well-known findings in the explicit memory literature. In two seminal papers on this topic in the mid-1970s, Fergus Craik, Robert Lockhart, and Endel Tulving showed that if people are encouraged to think about the meaning of words (from a list of words to be remembered later), they later recall and recognize those words with a higher probability than if they are encouraged to think about the sound (or phonology). For example, for the word "lizard", people could be asked ''Is it an animal?" or "Does it rhyme with wizard?'' The former question would lead to a higher probability of recall and recognition on average, across many words in the study list. Similarly, people remember words encoded with attention to sound better than words that are processed at a more superficial level (e.g., determining whether the word is in uppercase letters). This phenomenon is called level ofprocessing because it was proposed that people must go through the more "shallow" levels (e.g., the letter level) to access the "deeper" levels (e.g., meaning-based processing). The level-of-processing effect is a very robust phenomenon; one of the primary principles of memory is that if one wants to remember something later, he or she will do well to think hard about its meaning and importance at the time of encoding.
The generation effect is similar to the level-of-proces-sing effect in that it shows that the more meaningfully and effortfully items or events are processed, the better they will later be remembered on explicit memory tests (both recall and recognition). The way the effect is typically studied in the laboratory was popularized by Norman Slamecka and Peter Graf in the 1970s. People are either given antonyms to read (e.g., "hot-cold") or given the first word and asked to generate its antonym (e.g., "hot —''). Later, on recall and recognition tests, people remember "cold" better if they previously had to generate it themselves than if they simply read it. Consider the way children use flash cards to learn vocabulary words or the periodic table of the elements. Flash card techniques take advantage of the generation effect in that the information is generated by the user before looking at the answer. For example, the flash card user might see "mercury" on a card and try to generate its symbol (Hg) before checking the answer on the back of the card. Research has shown that even instances in which the generation attempt fails (e.g., the flash card user cannot successfully remember the item on the other side of the card), the generation effect occurs: Memory for the to-be-generated information can be facilitated relative to the condition of simply reading the information.
The picture superiority effect refers to the finding (made widely known through experiments by Allan Paivio in the late 1960s) that pictures are remembered better than words. This pattern occurs regardless of the type of explicit test—recall or recognition; it even occurs when the recognition test contains words (referring to pictures or words previously encoded). The source of this effect is thought to be that pictures access meaning more fully than words (and therefore are processed more "deeply" in level-of-processing terms); furthermore, pictures can often be accompanied by a verbal label. For example, if a picture of a fish is shown, people can easily think "fish" or "trout" to themselves when looking at the picture. Thus, pictures tend to access two types of codes (pictorial and verbal), whereas words tend to access only a single type of code (verbal). Of course, people could also form a mental image of a fish when given the word, in which case they would access both types of codes; indeed, when they do so, words are better remembered than when no imagery is invoked.
What parts of the brain contribute to our ability to remember the past? There are two primary ways of answering this question. The first and traditional way is to use unfortunate accidents of nature—naturally occurring brain lesions—to determine what cognitive processes break down when certain parts of the brain are injured due to stroke, accidents, or other insults to the brain. William Scoville and Brenda Milner described the memory impairments of a man known by his initials, H.M., who had his temporal lobes (including most of the hippocampi) surgically removed in the early 1950s in an attempt to cure intractable epilepsy (Fig. 1). The result was a profound loss of ability to remember anything that happened since the surgery (anterograde amnesia). However, H.M. was able to remember most things that occurred before the surgery and was able to converse somewhat normally. This pattern of results and similar outcomes exhibited by other patients with temporal lobe damage suggests that the temporal lobes (in or around the hippocampus) are necessary for the formation of new explicit memories. Interestingly, as will be discussed later, amnesic patients such as H.M. exhibit intact implicit memory (Fig. 2). The point to be taken from this study and other similar studies is that the medial temporal lobes play an important function in remembering the past; when they are removed or damaged, new explicit memories cannot be formed. One difficulty, however, lies in determining which stage(s) of the memory process (encoding, storage, or retrieval) this structure exhibits its effect. Does the information not enter the amnesic's brain properly? Is it encoded properly, with a breakdown in the storage or retrieval phase? Is it coded in one type of memory system (implicit) but not the other type (explicit)?
Recently, a new set of techniques has been developed that allows one to address these and similar questions. Specifically, neuroimaging techniques [e.g., event-related potentials (ERPs), functional magnetic resonance imaging (fMRI), and positron emission tomography (PET)] allow researchers to view the normal, living brain as it encodes and retrieves information (and while it processes information generally). By taking advantage of the fact that when specific brain regions are engaged in a task (e.g., a recognition memory test), these regions receive enhanced blood flow relative to their normal resting state (in the case of fMRI and PET) or the fact that electrical activity increases (in the case of ERP), these techniques can inform our understanding ofthe neural
underpinnings of memory. These approaches have shown that several regions are typically engaged during memory retrieval. Interestingly, although the hippocampus is sometimes shown to be active during
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