Which type of memory is consciously?

Explicit Memory.

Explicit memory, often referred to as declarative memory, can further be divided into episodic memory and semantic memory. Episodic memory refers to the ability to recollect everyday experiences.34 More specifically, episodic memory is the conscious recollection of personal events, along with the specific time and place (context) that they occurred. Episodic material includes autobiographical information, such as the birth of a child or graduation from high school, and includes personal information, such as a meal from the previous day or a recent golf game. These are memories that relate to an individual's own unique experience and include the details of “when and where” an event occurred. Most memory tests assess episodic memory and usually involve a free recall (retrieval), cued recall, and recognition trial and rely on an individual's ability to recollect the material to which he or she was previously exposed.6 Compared to younger adults, older adults typically perform better on recognition tasks as opposed to recall tasks. Recognition requires less cognitive effort because a target or cue is provided as a prompt to aid recall, as opposed to a recall task, which requires an individual to recall the material to which she or he was previously exposed, without any prompt. Overall, older adults are most disadvantaged when tests use explicit memory, in particular episodic memory, compared to younger adults.35,36

Semantic memory is an individual's knowledge about the world and includes memory of the meanings of words (vocabulary), facts, and concepts and, contrary to episodic memory, is not context-dependent. Knowledge is remembered regardless of when and where it was learned, such as word definitions or knowing the years when WWII occurred. Tests that assess semantic memory include vocabulary and word identification tests (e.g., AMNART),9 category fluency tasks (e.g., Animal Naming Test),37 and confrontational or object naming tests (e.g., Boston Naming Test).38 When most older adults report memory complaints, they are often referring to their difficulty in remembering words and names of objects and people.39

Tests that require recall of semantically unrelated material, such as the Rey Auditory-Verbal Learning Test (RAVLT)40 word lists, are seen as more difficult because they require more effortful strategies for encoding and retrieval than story recall tests, such as Wechsler's Logical Memory (WMS-IV, Logical Memory)41 or semantically related word lists, such as the California Verbal Learning Test (CVLT-II).42 When information is presented in a context, or words on a list belong to a category and are semantically related, the material presented is already organized in a meaningful way, which aids the recall processes. These memory tests include delayed recall and recognition trials to discern whether a deficit relates to the storage rather than retrieval of information.4

1.02.2.1 Explicit and Implicit Memory

Explicit memory refers to cases of conscious recollection. When we remember our trip to Paris or recognize that some words occurred in a recent list, these are instances of explicit memory. In cases of explicit retention, people respond to a direct request for information about their past, and such tests are called explicit memory tests. On the other hand, on tests of implicit memory, people are asked to perform some task, and the measure of interest is how some prior experience affects the task. For example, take the simple case of the word elephant appearing in a long list of words. If subjects are given a recognition test in which they are instructed to identify words studied in the list (and to reject nonstudied words), then their choice of elephant as a studied word would represent an instance of explicit retention. However, if a different group of subjects were given the same set of words to study and then were given a word stem completion test (with instructions to say the first word that comes to mind to the word stem ele______), then this would constitute a test of implicit memory. The relevant measure on this test is priming, the greater probability of completing the stem with elephant rather than other plausible words (element, elegant, electricity, etc.) when the word has been studied than when it has not been studied. For example, the probability of producing elephant to the word stem might be 10% if the word had not been studied in the list and 40% when it had been studied, which would constitute a 30% priming effect. One reason for believing that these two measures represent different forms of memory is that they can be dissociated by many experimental (and subject) variables.

Graf and Schacter (1985) introduced the terms explicit and implicit memory to the field. Explicit retention refers to tests in which people are instructed to remember events from their past; most typical measures of retention that psychologists have used over the years (recall, recognition, and their variations) are examples. On the other hand, implicit memory refers cases of past experience being deployed in the service of a task that seems unrelated to remembering. These implicit measures are usually types of transfer measures when people may not be aware of using memory at all (Jacoby, 1984). That is, past experience (memories) is used in the performance of some task and how much the past experience primes (or transfers to) the new task is the measure of interest. Various techniques are used to try to rule out subjects using conscious recollection (see Roediger and McDermott, 1993). Some writers prefer the terms direct and indirect memory for this contrast, because explicit tests measure memory directly, whereas implicit tests are indirect measures. Schacter (1987) offers a fine historical review of concepts related to implicit memory.

Psychological Sequelae

Awareness under general anesthesia can be a traumatic experience, with approximately one third of patients experiencing long-term psychological sequelae.22-24 However, some patients do not develop long-term psychological sequelae after intraoperative awareness25 and many patients without awareness have psychological symptoms consistent with posttraumatic stress disorder (PTSD).23 Some of the most common recalled awareness experiences include auditory sounds, feelings of paralysis, seeing lights, and feelings of helplessness, fear, or anxiety.26 Pain is less common, although it does occur in some patients, particularly those with complete neuromuscular blockade who are unable to move. Psychological sequelae of recalled memories may include flashbacks, anxiety/nervousness, loneliness, nightmares, and fear/panic attacks that vary from bothersome to distressing.22,26 Some patients develop severe, persistent symptoms that profoundly interfere with interpersonal relationships and daily activities.26

The risk factors for developing PTSD after awareness during general anesthesia are not completely known. An acute emotional reaction to the experience significantly predicted the development of long-term psychological sequelae.22 Dissociation related to surgery and perceiving that one’s life was threatened were associated with PTSD.23 Paralysis from neuromuscular blockade is particularly traumatic.27 The role of premorbid depression and other psychological conditions is unclear, but may contribute to risk of PTSD.23,28 Recurrence of trauma can trigger previous psychological symptoms.

More reporting from patients also increases the understanding of the experiences. In 2007, the ASA established the Anesthesia Awareness Registry to address the concerns of patients with regard to intraoperative awareness.29 The Registry collected patient self-reports of unintended awareness during general anesthesia to provide a patient perspective on their expectations and experiences of awareness. The Registry was designed to be consistent with “patient-centered” care and focused on patient preferences, needs, and values. Although the Registry relied on patients to volunteer to participate, and therefore had response bias, the results are valid to point out causes and possible solutions to patient dissatisfaction with unexpected recall during surgery.

One finding from the Registry is that patients may have different expectations than anesthesia providers concerning the lack of explicit recall during regional anesthesia or sedation. The Anesthesia Awareness Registry recruited patients who self-identified as having awareness during general anesthesia. However, upon review of the perioperative records, one third of patients had mistakenly believed that they received general anesthesia; instead, they actually received sedation or regional anesthesia.26 This result shows a disconnect between anesthesia providers’ and patients’ expectations concerning unconsciousness during surgery. This disconnection may be resolved by improved physician-patient communication concerning the possible recall of events during sedation as well as improved informed consent.

Explicit versus Implicit Memory

Explicit memory is traditionally measured using tests of recall and recognition, in which participants consciously refer back to the original learning or encoding of the material to complete the test. In contrast, implicit memory tests indirectly assess memory by having participants complete tests that are supposedly unrelated to the encoding condition. For example, in word stem completion, a commonly used implicit memory test, participants are provided with the first few letters of a word followed by a blank space (e.g., m a i ). Participants are more likely to use the word ‘maiden’ to complete the stem if they had previously encoded that word rather than the more commonly occurring word ‘mail.’ Implicit tests such as word stem completion are useful in investigating learning that occurs indirectly, without conscious awareness. Researchers have often found that age-related decline is minimal for these types of implicit memory tests. Although under some conditions age-related decline for implicit memory does occur, it is generally the case that implicit memory is relatively spared in older adults.

Explicit and Implicit Memory

Relative to explicit memory, implicit memory is generally much better preserved in amnesia. Patients show preserved perceptuo-motor skills, and respond appropriately to simple forms of classical conditioning (such as the eye-blink response). They also show priming effects (a more rapid response to an item on the basis of its prior presentation, or that of a similar item), suggesting that this process does not rely on diencephalic/medial temporal brain structures, but rather upon cortical regions. Cases have been described whereby perceptual priming is impaired, even in the face of preserved explicit memory; a role for occipital lobe circuits has been implicated in these cases. However, amnesic patients do show impaired priming in certain experimental conditions (e.g., in difficult tasks when baseline responding has been controlled) or in associative learning. This implicates a contribution of medial temporal/diencephalic structures to priming in these circumstances.

16.2.3 Declarative or Explicit Memory

Declarative or explicit memory is devoted to processing of names, dates, places, facts, events, and so forth. These are entities that are thought of as being encoded symbolically and that thus can be described with language. In terms of function, declarative memory is specialized for fast processing and learning. New information can be entered into the declarative memory system on the basis of a single trial or experience. In terms of rules of operation, declarative is fallible: one forgets names, dates, places, and so forth. Although there are compelling demonstrations of long-term remembering of lessons learned in high school and college (e.g., foreign language vocabulary: Bahrick, 2000), a great deal of forgetting from declarative memory occurs literally minutes, hours, and days after an experience. Declarative memory also has a specific neural substrate. Current conceptualizations suggest that the formation, maintenance, and subsequent retrieval of declarative or explicit memories depend on a multicomponent network involving cortical structures (including posterior–parietal, anterior–prefrontal, and limbic–temporal association areas) as well as medial temporal structures (including the hippocampus and entorhinal, perirhinal, and parahippocampal cortices: e.g., Eichenbaum and Cohen, 2001; Murray and Mishkin, 1998; Zola and Squire, 2000). The medial temporal structures may be considered ‘primary’ in the sense that without them, whether measured by recall or recognition, declarative memory is impaired (Moscovitch, 2000).

Declarative memory is itself subdivided into the categories of semantic and episodic memory (e.g., Schacter and Tulving, 1994), with a finer distinction between episodic and autobiographical memory. Semantic memory supports general knowledge about the world (Tulving, 1972, 1983). People are consulting semantic memory when they retrieve the facts that the capital of the United States is Washington, DC, that the United States has 50 states, and that with over 660 000 square miles, Alaska is the largest state in terms of land mass. For practical purposes, both the capacity of semantic memory and the longevity of the information stored in it seem infinite. Semantic memory also is not tied to a particular time or place. That is, people know facts and figures, names and dates, yet in most cases, they do not know when and where they learned this information. People might be able to reconstruct how old they were or what grade they were in when they learned some tidbits of information, but unless there was something unique about the experience surrounding the acquisition of this information, they carry it around without address or reference to a specific episode.

In contrast to semantic memory, episodic memory supports retention of information about unique events (Tulving, 1972, 1983), such as a specific visit to Washington, DC, or the fact that Alaska was one of the states on a list of state names studied in a memory experiment. Some episodic memories, such as whether a specific state was included in a word list, may not stay with one for very long and are not especially personally relevant or significant. Yet other episodic memories are personally significant and even self-defining. These so-called autobiographical memories are episodic memories that are infused with a sense of personal involvement or ownership (Bauer, 2007). They are the episodes on which people reflect when they consider who they are and how their previous experiences have shaped them.

3 What is declarative memory and how does it differ from nondeclarative memory?

Declarative memory (explicit memory) is flexible, requires awareness, allows conscious recollection, and is the type of memory damaged in amnesia. Nondeclarative memory (implicit or procedural memory) does not require the hippocampal circuitry, is not consciously accessible, one is unaware of it and it is inflexible, and it remains intact in amnesia. Examples are conditioning, priming, motor and cognitive skill learning, and habit learning. It requires the cerebellum, basal ganglia, and association cortices. In healthy individuals, both systems work together. In amnesia, implicit learning remains intact; this fact can be utilized for rehabilitation. Even patients with Alzheimer's disease, for example, can learn through this system—the repetition of facts, rather than single presentations, may allow their storage.