Retroactive and Proactive Interference

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Memory can be disrupted by events or information encountered at approximately the time of encoding. This disruption is known as an ''interference effect.''

1. Retroactive Interference

Retroactive interference (RI) refers to the disrupting effect that new learning has on the ability to recall previously learned information. For example, a student studying several text chapters for an exam will be able to recall a greater amount of information from a chapter that he or she has just finished reading than from a chapter that was read earlier in the evening. In this example, information read in the most recent chapter interferes with memory for information from previous chapters.

Examples Proactive Interference

Figure 1 Memory test profiles expected in neurologically intact individuals and patients with primary deficits of encoding, retrieval, or retention. A list of 16 words is presented over five learning trials, with memory for the list assessed immediately after each trial. Memory for the word list is again assessed after a 20-min delay by means of free recall and recognition. Compared to individuals with no memory disorder, patients with encoding problems do not recall more words with repeated exposure to information. Patients with retrieval problems show poor delayed memory when assessed by free recall but normal memory when assessed by recognition. Patients with retention problems do not benefit from recognition testing.

Figure 1 Memory test profiles expected in neurologically intact individuals and patients with primary deficits of encoding, retrieval, or retention. A list of 16 words is presented over five learning trials, with memory for the list assessed immediately after each trial. Memory for the word list is again assessed after a 20-min delay by means of free recall and recognition. Compared to individuals with no memory disorder, patients with encoding problems do not recall more words with repeated exposure to information. Patients with retrieval problems show poor delayed memory when assessed by free recall but normal memory when assessed by recognition. Patients with retention problems do not benefit from recognition testing.

The effect of RI is a function of the amount of new information encountered and the degree to which the interfering information and target information are similar. If a large amount of similar material is presented between initial learning and eventual recall of target information, recall will be poorer than if small amounts of dissimilar information had been presented during the intervening time.

2. Proactive Interference

Proactive interference (PI) occurs when information presented at an earlier time interferes with one's ability to learn and recall new information. For example, suppose one is asked to learn several lists of different words. After the first list is presented and memory for the words is assessed, a new list of words is presented and memory for the new words is assessed. As more lists are presented, memory for the new words declines because previously learned words produce interference.

As with RI, the effect of PI on recall also increases with the amount of interfering information and the degree of similarity to the target information. If one were presented five successive word lists, the effect of PI would be greater after the fifth list was presented than after the second list was presented. In addition, if the lists all contain similar words, such as animal names, memory for each new list will be worse than if each list contained a variety of different words.

One interesting aspect of PI is that presenting dissimilar interfering information can facilitate memory. Using the previous example, suppose a group of individuals are asked to learn four successive word lists, all of which contain different animal names. Now suppose a fifth trial is given. Half of the individuals are presented another list of animal names, whereas the other half are presented a list of words belonging to an unrelated category (e.g., clothing). Results of such an experiment are presented in Fig. 2. Individuals who are presented more animal names on trial 5 continue to demonstrate PI, whereas those presented with a

Release Proactive Interference

Figure 2 An illustration of proactive interference (PI). On four consecutive trials, study participants are asked to recall different lists of words from the same semantic category (e.g., animals). The effect of PI can be seen in the decline in performance over these trials. On the fifth trial, participants who are asked to recall another set of words from the same category continue to demonstrate PI, whereas those who are asked to recall words from a new semantic category (e.g., clothing) show improvement, known as ''release'' from PI.

Figure 2 An illustration of proactive interference (PI). On four consecutive trials, study participants are asked to recall different lists of words from the same semantic category (e.g., animals). The effect of PI can be seen in the decline in performance over these trials. On the fifth trial, participants who are asked to recall another set of words from the same category continue to demonstrate PI, whereas those who are asked to recall words from a new semantic category (e.g., clothing) show improvement, known as ''release'' from PI.

dissimilar list of words demonstrate improved recall. This phenomenon is commonly known as ''release'' from PI.

D. Anterograde and Retrograde Memory

Anterograde memory is the ability to learn new information and to form new memories from a given moment forward. In contrast, retrograde memory is the ability to recall or recognize information or events that occurred prior to a specific moment in time.

Anterograde and retrograde amnesias are often seen in patients with brain injuries. Patients who are injured in motor vehicle accidents, for example, may not recall the events leading up to the accident (i.e., a retrograde deficit) or the events that occurred immediately following the accident (i.e., an anterograde deficit). The degree of anterograde and retrograde amnesia following a head injury is highly correlated with the severity of brain damage sustained. Anterograde and retrograde amnesias can also occur independently. It is common for patients with certain disorders to have intact memory for past events but poor ability to lay down new memories. The reverse pattern, however, is rare. Individuals who present with complete loss of past memories, personal history, and personal identity typically are found to have psychological rather than neurological disorders.

1. Recent and Remote Memory

The temporal dimension of retrograde memory is often divided into recent versus remote time frames. Recent memory typically refers to information that has been acquired within a relatively short period of time prior to an event (i.e., injury or time of evaluation), whereas remote memory refers to information about events or experiences that occurred years or decades before.

Patients with retrograde amnesia often demonstrate a temporal gradient in which memory for more recent events is disrupted to a greater extent than memory for remote events. This gradient is illustrated by the case of patient P.Z., a distinguished scientist who became amnesic secondary to alcoholic Korsakoff syndrome. Several years prior to the onset of his amnesic syndrome, P.Z. completed an autobiography. Investigators used this information to assess P.Z.'s memory for his own past life events. A temporally graded retrograde amnesic disorder is illustrated in Fig. 3, as this patient demonstrated significantly greater

Figure 3 Temporally graded retrograde amnesia for autobiographical information as seen in patient PZ. Information from earlier decades in PZ's life was recalled better than information from recent decades.

impairment of memory for recent experiences and events compared to his memory for events from his early life.

E. Declarative versus Nondeclarative Memory

Declarative memory (also called explicit memory) refers to the acquisition of facts, experiences, and information about events. It is memory that is directly accessible to conscious awareness and thus can be "declared." In contrast, nondeclarative memory (also called implicit memory) refers to various forms of memory that are not directly accessible to consciousness. These include skill and habit learning, classical conditioning, priming, and other situations in which memory is expressed through performance or skill rather than through conscious recollection.

1. Types of Declarative Memory a. Episodic Memory Episodic memory refers to information that is linked to a particular place and time. The ability to answer questions regarding what you ordered at a restaurant the night before or what information was presented at a meeting you attended are examples of episodic memory. That is, in order to recall the target information correctly, the individual must access information regarding the time and place the information was acquired.

b. Semantic Memory Semantic memory refers to general knowledge that is not linked to a particular temporal or spatial context. For example, defining the word "restaurant" or reciting the alphabet do not require knowledge of where or when that information was originally learned. Both episodic and semantic memories are declarative, however, in that retrieval of information is carried out explicitly, on a conscious level.

2. Types of Nondeclarative Memory a. Procedural Memory Procedural memory is the process of retrieving information necessary to perform learned skills. These skills may be movement based, such as tying a shoe or riding a bicycle, or they may be perceptual in nature, such as learning to read mirror-reversed text. Although some aspects of skills can be declared, the skills are most often performed automatically, without conscious retrieval of information regarding the procedure. Amnesic patients such as H.M. can learn how to perform several complex tasks and will demonstrate normal retention of these new skills despite not being able to remember having ever learned how to do the tasks.

b. Conditioning Classical conditioning is one of the most basic forms of learning. A stimulus that naturally produces a certain response is paired with a neutral stimulus. After repeated pairings, the neutral stimulus alone will elicit the response. For example, a dog will naturally begin to salivate when presented food but not when presented with the sound of a bell ringing. If, however, a bell is rung repeatedly along with presentation of food, bell ringing alone will eventually produce salivation. As with procedural memory, patients with very poor declarative memory can demonstrate normal conditioning despite being unable to recall any of the training sessions that led to the conditioned response.

c. Priming Priming is the phenomenon by which prior exposure to information influences performance on later tasks even without conscious awareness. Priming effects are demonstrated by presenting a stimulus on one occasion and measuring its influence on performance on a subsequent occasion. Depending on the level of processing, priming can be perceptual or conceptual in nature. Perceptual priming occurs when exposure to the form of a stimulus influences later behavior. Two examples of perceptual processing are priming for picture identification and verbal information. In picture identification tasks, an individual looks at pictures or line drawings of different objects. Later, the individual is presented a series of "degraded" pictures (i.e., pictures in which much of the visual information is masked or missing) and asked to identify them. Identification of degraded pictures is faster and more accurate if the individual had previously seen the complete picture than when a novel picture is presented, indicating some form of memory for the information.

Perceptual priming for verbal information is commonly demonstrated in word-stem completion paradigms. During the first phase of the paradigm, individuals perform tasks designed to expose them to different words. For example, they may be shown a series of words and asked to judge whether each word has a "pleasant" or "unpleasant" connotation. Later, during the test phase, a new task is introduced in which the first three letters of words (i.e., word stems) are presented and the individuals are asked to complete each stem with the first word that comes to mind. Study participants are more likely to complete word stems with words seen during the exposure phase of the study, even if they are less commonly used words. For example, individuals who participate in the exposure task and see the word "motel" will be more likely to use that word to complete the word stem "mot '' than individuals who did not participate in the exposure task. When not previously exposed to the word "motel," the more common completion of this stem is the more frequently used word, "mother."

Although the previous example involves verbal stimuli, it is the perceptual aspect and not the meaning of those stimuli that is primed. When processing the meaning of a stimulus influences later behavior, conceptual priming has occurred. The exposure phase of conceptual priming experiments is similar to that of the word-stem completion paradigm. During the test phase, however, participants engage in word-association tasks or other activities that require the processing of word meanings. Suppose, for example, the word "crown" is presented during an exposure task. Individuals who were primed by exposure are more likely to respond with the word "crown" when asked for the first word that comes to mind in response to the word "king," whereas naive (i.e., "unprimed") study participants more often respond with the word "queen."

Patients with impaired episodic memory typically perform normally on perceptual priming tasks even though they have no memory of having previously seen the words or pictures that have been primed. Conceptual priming, however, may be reduced in patients with declarative memory dysfunction.

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