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Chapter 5 Chapter Summary
Explicit and Implicit Memory
Chapter Overview:
Implicit and explicit memory are distinct entities. Chapter 5 outlines the differences and then focuses on phenomena related to implicit memory.
Explicit memories are those we can bring into conscious awareness whereas we are not consciously aware of implicit memories. Consciousness, sustained mental representations of the external world, developed in species where reflexive responding to stimuli was not adaptive probably due to more complex environments. Over time, the two types of processing—conscious and unconscious—developed in the same species but parallel from each other (and in different regions of the brain).
The distinction between the two types of memory is particularly evident in individuals with anterograde amnesia, as they are unable to form new explicit memories but their ability to acquire new implicit memories remains intact. Studies have shown that people with anterograde amnesia will not be able to consciously recall a list of previously studied words (explicit), but the words will affect their word completion choices (implicit). Also, previous exposure to images in a Gollin figure test will not be consciously recalled (explicit) but will still improve performance (implicit). Likewise, previous practice on a pursuit-rotor task is not remembered (explicit) but will impact on subsequent mastery of the task (implicit).
However, some researchers support a processing account explanation to the disparate explicit and implicit task performance in people with amnesia. A levels-of-processing view is applied so that the differences in performance are explained by the different ways we process information instead of being distinct types of memory.
Mishkin and his colleagues proposed different neural circuits for explicit and implicit memory. For explicit memory, the neural circuitry involves an interactive flow of information through the frontal lobe, the thalamus, the temporal lobe, hippocampus, and entorhinal regions of the forebrain. We are conscious of explicit memories because all eventually go through the neocortex—the seat of consciousness. Explicit memory encoding is top-down so that the processing begins in the cortex and determines how subsequent information is processed. On the other hand, implicit memory circuits are unidirectional—they begin in the sensory and motor areas and are processed in the basal ganglia to the thalamus and then the premotor cortex. We are not aware of implicit messages because they are not processed in the neocortex.
To study implicit memory, researchers need to create situations where participants gain knowledge without being aware of doing so. As a result, a number of clever procedures have been devised to study implicit memory. For example, Reber (1967) created artificial grammar where participants had to memorize strings of letters. When the strings were organized based on specific rules, participants’ performance improved over time, suggesting that they implicitly learned the rules. In subsequent research, participants were able to identify which string of letters were consistent with the rules and which were not—even though they could not articulate what the actual rules were.
Likewise, in the pattern-learning paradigm, performance improved on a reaction time task when participants implicitly learned the pattern of a stimuli’s location. However, early research by Nissen and Bullemer (1987) suggested that at least some explicit learning was used to do the pattern-learning tasks because attention was used. Work by Stadler in 1995 suggested that pattern-learning did, in fact, use just implicit learning and Nissen and Bullemer’s results could be more accurately interpreted as a disruption of learning the pattern.
Another type of implicit learning is classical conditioning. It allows organisms to identify patterns in the environment and is governed by the same neural mechanisms as other types of implicit memory. Classical conditioning occurs when a neutral stimulus is repeatedly paired with a stimulus that naturally elicits a response. Eventually the previously neutral stimulus will elicit the same response. For example, in Ivan Pavlov’s classic study, he measured the amount of saliva produced when dogs were presented with food—the food was the unconditioned stimulus (US) and the salivation was the unconditioned response (UR). During conditioning, a bell sounded just before the food was given to the dogs so that the dogs eventually began to salivate whenever they heard the bell ring even if there was no food present—the bell was the conditioned stimulus (CS) and the salivation in response to the bell was the conditioned response (CR). Classical conditioning can also explain the mere exposure effect—the idea that we tend to prefer familiar stimuli. Repeated exposure to stimuli (CS) becomes paired with US that naturally elicit a pleasant feeling. In this way the familiar stimuli (e.g., people, letters, or paintings) elicit the pleasant feeling too.
Repetition priming is a type of implicit learning where previous exposure to stimuli facilitates the processing of that stimuli. The most widely used types of repetition priming are word-stem completion tasks and word-fragment completion tasks.
The word-stem completion (WSC) task uses the incidental learning paradigm (learning occurs without intent). Typically, participants are shown a list of words and are then asked something about the words (e.g., rated on pleasantness, number of vowels). At no time are participants asked to remember the words. After a delay, participants are given a list of word stems and asked to complete the words. For example, one of the word stems could be “pre__” and participants need to complete the word (e.g., “pretty,” “preview,” or “pretend”). Typically, researchers have presented words in the incidental learning phase that are not the most common answers to complete word stem. If participants complete the word stems with the words presented earlier from the incidental learning task at a higher rate than would be expected, incidental learning is said to have occurred. Interestingly, like all implicit memory, WSC effects are modality-specific so that images of words in an incidental learning task do not affect the word stem responses.
Word-fragment completion (WFC) tasks are similar to WFC except that participants are presented with a mix of letters and blanks and asked to complete them (e.g., E_E_ _A_ N_ for ELEPHANT). In WFC, there is usually only one correct answer and without the incidental learning task, many are unable to solve the task.
Because the effects of the WFC task last longer than the WSC (weeks compared to hours), some researchers thought that the WFC was a better tool in which to access implicit memory. However, thorough investigations by Rajaram and Roediger (1993) revealed that the difference in the length of the priming is more likely a function of word frequency so that more frequently-used words create shorter priming periods. They also confirmed that the same effects occur over different modalities
The implicit-association test (IAT) is a tool used in social psychology to gage a person’s stereotypical associations. In this task, participants must categorize a word or an image by quickly and accurately pressing one of two keys. In the critical trials, there are two different categories for one key. When the categories “go together” in a stereotype, reaction time is faster than when they do not fit. For example, in the test of gender and science, the words Girl, Aunt, Daughter, Wife would be categorized as female and Boy, Father, Male, Grandpa, Husband as male. The words Philosophy, Arts, History would be categorized as humanities and Biology, Physics, Chemistry as science. If participants linked males and science together, their response times should be shorter when categorizing with the same key than if females and science categories used the same key. These associations are often found in people who would explicitly deny the stereotype.
A particularly long-lasting implicit task is negative priming. In this task, participants decide whether two shapes are the same or different while ignoring a shape. If the ignored shape later appears as a target, response times increase even though participants cannot explicitly identify which shapes they had been exposed to previously. The negative priming paradigm provides evidence that implicit memories are formed even when stimuli are not attended to.
In both the illusory-truth effect and the false-fame effect, familiarity is essentially the measure of implicit memory. In the illusory-truth effect, participants are more likely to rate a statement as true if they have seen it before (but cannot recall it explicitly). Likewise, participants in the studies examining the false-fame effect tend to rate non-famous names as famous if they have seen them before compared to totally unfamiliar names.
From the mid-1900s until the end of the century, many believed that we can remember events that have occurred while under anaesthesia. For the most part, these were believed to be implicit memories. More recent research by Wang, Russell, and colleagues using the isolated forearm technique (IFT) suggested that it was more likely that the patients who could recall events while under anaesthesia actually became conscious during the procedure. In fact, the IFT is used so that surgeons are more likely to become aware if patients happen to become conscious. When using newer anaesthetics, patients do not move their hands when under anaesthetic (so don’t gain consciousness) and, more importantly, do not report memories.
Procedural memory is a type of implicit memory based on how to perform a task. You can explicitly know how to do something (like walking up the stairs) but this is different than performing the actions. Procedural memories are acquired by repeatedly performing a task. Some research suggests that thinking explicitly about a task can negatively interfere with performing the task itself.
Learning Objectives:
Having read this chapter, you will be able to do the following:
- Differentiate between implicit and explicit memory.
- Describe explicit and implicit memory capabilities in people with anterograde amnesia.
- Describe the processing account of the dissociation between explicit and implicit tests of memory.
- Describe the main components of Mortimer Mishkin’s neural circuit for implicit memory.
- Differentiate between the processes involved in encoding implicit and explicit memories.
- Describe artificial grammars and pattern-learning paradigms.
- Explain why classical conditioning is associated with implicit memory.
- Generate examples of the influence of implicit memory on personal preferences.
- Explain what studies of repetition priming have revealed about the nature of implicit memory.
- Describe the implicit-association test (IAT) and explain how the IAT can be used to measure stereotypes.
- Explain the illusory-truth effect and why this effect is attributed to implicit memory.
- Describe situations under which a person may form a memory under anaesthesia and explain why that memory is more likely to be implicit than explicit.
- Define procedural memory.