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Memory recall appears to be state-dependent, at least to some extent. Studies have shown that, when material is learned under the influence of a drug or alcohol, for example, it is subsequently recalled better when in the same drug state than when sober. Similarly, individuals tend to retrieve information more easily when it has the same emotional content as their current emotional state, and when the emotional state at the time of retrieval is similar to the emotional state at the time of encoding.

Recall or retrieval of memory refers to the subsequent re-accessing of events or information from the past, which have been previously encoded and stored in the brain. In common parlance, it is known as remembering. During recall, the brain "replays" a pattern of neural activity that was originally generated in response to a particular event, echoing the brain's perception of the real event. In fact, there is no real solid distinction between the act of remembering and the act of thinking.

These replays are not quite identical to the original, though - otherwise we would not know the difference between the genuine experience and the memory - but are mixed with an awareness of the current situation. One corollary of this is that memories are not frozen in time, and new information and suggestions may become incorporated into old memories over time. Thus, remembering can be thought of as an act of creative reimagination.

Because of the way memories are encoded and stored, memory recall is effectively an on-the-fly reconstruction of elements scattered throughout various areas of our brains. Memories are not stored in our brains like books on library shelves, or even as a collection of self-contained recordings or pictures or video clips, but may be better thought of as a kind of collage or a jigsaw puzzle, involving different elements stored in disparate parts of the brain linked together by associations and neural networks. Memory retrieval therefore requires re-visiting the nerve pathways the brain formed when encoding the memory, and the strength of those pathways determines how quickly the memory can be recalled. Recall effectively returns a memory from long-term storage to short-term or working memory, where it can be accessed, in a kind of mirror image of the encoding process. It is then re-stored back in long-term memory, thus re-consolidating and strengthening it.

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Several studies have shown that both episodic and semantic memories can be better recalled when the same language is used for both encoding and retrieval. For example, bilingual Russian immigrants to the United States can recall more autobiographical details of their early life when the questions and cues are presented in Russian than when they are questioned in English.

The efficiency of human memory recall is astounding. Most of what we remember is by direct retrieval, where items of information are linked directly a question or cue, rather than by the kind of sequential scan a computer might use (which would require a systematic search through the entire contents of memory until a match is found). Other memories are retrieved quickly and efficiently by hierarchical inference, where a specific question is linked to a class or subset of information about which certain facts are known. Also, the brain is usually able to determine in advance whether there is any point in searching memory for a particular fact (e.g. it instantly recognizes a question like “What is Socrates’ telephone number?” as absurd in that no search could ever produce an answer).

There are two main methods of accessing memory: recognition and recall. Recognition is the association of an event or physical object with one previously experienced or encountered, and involves a process of comparison of information with memory, e.g. recognizing a known face, true/false or multiple choice questions, etc. Recognition is a largely unconscious process, and the brain even has a dedicated face-recognition area, which passes information directly through the limbic areas to generate a sense of familiarity, before linking up with the cortical path, where data about the person's movements and intentions are processed. Recall involves remembering a fact, event or object that is not currently physically present (in the sense of retrieving a representation, mental image or concept), and requires the direct uncovering of information from memory, e.g. remembering the name of a recognized person, fill-in the blank questions, etc.

Recognition is usually considered to be “superior” to recall (in the sense of being more effective), in that it requires just a single process rather than two processes. Recognition requires only a simple familiarity decision, whereas a full recall of an item from memory requires a two-stage process (indeed, this is often referred to as the two-stage theory of memory) in which the search and retrieval of candidate items from memory is followed by a familiarity decision where the correct information is chosen from the candidates retrieved. Thus, recall involves actively reconstructing the information and requires the activation of all the neurons involved in the memory in question, whereas recognition only requires a relatively simple decision as to whether one thing among others has been encountered before. Sometimes, however, even if a part of an object initially activates only a part of the neural network concerned, recognition may then suffice to activate the entire network.

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Colour may have an effect on our ability to memorize something. People remember colour scenes better than black-and-white ones, although only if naturally (as opposed to falsely) coloured. In particular, warm colours, like red, yellow and orange, may help us to memorize things by increasing our level of attention (our ability to select from information available in the environment). The more attention is focused on outside stimuli, the greater the likelihood of those stimuli being stored in long-term memory.

In the 1980s, Endel Tulving proposed an alternative to the two-stage theory, which he called the theory of encoding specificity. This theory states that memory utilizes information both from the specific memory trace as well as from the environment in which it is retrieved. Because of its focus on the retrieval environment or state, encoding specificity takes into account context cues, and it also has some advantages over the two-stage theory as it accounts for the fact that, in practice, recognition is not actually always superior to recall. Typically, recall is better when the environments are similar in both the learning (encoding) and recall phases, suggesting that context cues are important. In the same way, emotional material is remembered more reliably in moods that match the emotional content of these memories (e.g. happy people will remember more happy than sad information, whereas sad people will better remember sad than happy information).

According to the levels-of-processing effect theory, another alternative theory of memory suggested by Fergus Craik and Robert Lockhart, memory recall of stimuli is also a function of the depth of mental processing, which is in turn determined by connections with pre-existing memory, time spent processing the stimulus, cognitive effort and sensory input mode. Thus, shallow processing (such as, typically, that based on sound or writing) leads to a relatively fragile memory trace that is susceptible to rapid decay, whereas deep processing (such as that based on semantics and meanings) results in a more durable memory trace. This theory suggests, then, that memory strength is continuously variable, as opposed to the earlier Atkinson-Shiffrin, or multi-store, memory model, which just involves a sequence of three discrete stages, from sensory to short-term to long-term memory.

The evidence suggests that memory retrieval is a more or less automatic process. Thus, although distraction or divided attention at the time of recall tends to slow down the retrieval process to some extent, it typically has little to no effect on the accuracy of retrieved memories. Distraction at the time of encoding, on the other hand, can severely impair subsequent retrieval success.

The efficiency of memory recall can be increased to some extent by making inferences from our personal stockpile of world knowledge, and by our use of schema (plural: schemata). A schema is an organized mental structure or framework of pre-conceived ideas about the world and how it works, which we can use to make realistic inferences and assumptions about how to interpret and process information. Thus, our everyday communication consists not just of words and their meanings, but also of what is left out and mutually understood (e.g. if someone says “it is 3 o’clock”, our knowledge of the world usually allows us to know automatically whether it is 3am or 3pm). Such schemata are also applied to recalled memories, so that we can often flesh out details of a memory from just a skeleton memory of a central event or object. However, the use of schemata may also lead to memory errors as assumed or expected associated events are added that did not actually occur.

There are three main types of recall:

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Several recent studies in the growing area of neuro-education have shown the value of the "testing effect" (or "retrieval effect"), where quizzes a short time after initial learning significantly improves subsequent retrieval of facts and ideas, as well as overall understanding of topics and the ability to solve related problems. Testing helps protect against "proactive interference" (the familiar feeling of being overwhelmed by too much information), and the studies suggest that a quick test is much more effective than en extra hour of study or re-reading.

• Free recall is the process in which a person is given a list of items to remember and then is asked to recall them in any order (hence the name “free”). This type of recall often displays evidence of either the primacy effect (when the person recalls items presented at the beginning of the list earlier and more often) or the recency effect (when the person recalls items presented at the end of the list earlier and more often), and also of the contiguity effect (the marked tendency for items from neighbouring positions in the list to be recalled successively).
• Cued recall is the process in which a person is given a list of items to remember and is then tested with the use of cues or guides. When cues are provided to a person, they tend to remember items on the list that they did not originally recall without a cue, and which were thought to be lost to memory. This can also take the form of stimulus-response recall, as when words, pictures and numbers are presented together in a pair, and the resulting associations between the two items cues the recall of the second item in the pair.
• Serial recall refers to our ability to recall items or events in the order in which they occurred, whether chronological events in our autobiographical memories, or the order of the different parts of a sentence (or phonemes in a word) in order to make sense of them. Serial recall in long-term memory appears to differ from serial recall in short-term memory, in that a sequence in long-term memory is represented in memory as a whole, rather than as a series of discrete items. Testing of serial recall by psychologists have yielded several general rules:
  • more recent events are more easily remembered in order (especially with auditory stimuli);
  • recall decreases as the length of the list or sequence increases;
  • there is a tendency to remember the correct items, but in the wrong order;
  • where errors are made, there is a tendency to respond with an item that resembles the original item in some way (e.g. “dog” instead of “fog”, or perhaps an item physically close to the original item);
  • repetition errors do occur, but they are relatively rare;
  • if an item is recalled earlier in the list than it should be, the missed item tends to be inserted immediately after it;
  • if an item from a previous trial is recalled in a current trial, it is likely to be recalled at its position from the original trial.

If we assume that the "purpose" of human memory is to use past events to guide future actions, then keeping a perfect and complete record of every past event is not necessarily a useful or efficient way of achieving this. So, in most people, some specific memories may be given up or converted into general knowledge (i.e. converted from episodic to semantic memories) as part of the ongoing recall/re-consolidation process, so that that we are able to generalize from experience.

It is also possible that false memories (or at least wrongly interpreted memories) may be created during recall, and carried forward thereafter. Research into false memory creation is particularly associated with Elizabeth Loftus' work in the 1970s. Among many other experiments in this area (see the side panel on the Psychogenic Amnesia page, for example), she showed how the precise wording of a question about memories (e.g. "the car hit" or "the car smashed into") can dramatically influence the recall and re-creation of memories, and can even permanently change those memories for future recalls - a phenomenon which is not lost on the legal profession. It is thought that it may even be possible, up to a point, to choose to forget, by blocking out unwanted memories during recall, a process achieved by frontal lobe activity, which inhibits the laying down or re-consolidation of a memory.

However, there is a rare condition called hyperthymesia (also known as hypermnesia or highly superior autobiographical memory (HSAM) or informally just "total recall") in which a few people show an extraordinary capacity to recall detailed specific events from their personal past, without relying on practised mnemonic strategies. Although only a handful of cases of hyperthymesia have ever been definitively confirmed, some of these cases are quite startling, such as a California woman who could recall every day in complete detail from the age of 14 onwards, a young English girl with an IQ of 191 who had a perfect photographic memory spanning almost 18 years, and a Russian man known simply as "S." who was only able to forget anything by a deliberate act of will. One of the most famous cases, known as “A.J.”, described it as a burden rather than a gift, but others seem to be able to organize and compartmentalize their prodigious memories and do not appear to feel that their brains are "cluttered" with excess information. Some may find it hard to forget painful or embarassing events. There is a good "60 Minutes" documentary on the subject at http://www.cbsnews.com/video/watch/?id=7166313n.

Often, such individuals (who may have had good, but perhaps not exceptional, memories when young) only start remembering almost everything after a particular emotional trigger point, often during the highly emotional teenage years. Their superior memories are usually very self-centred, and they may not be any better than average at remembering more impersonal details like lists of words or, say, a drinks order. They are also just as prone as the rest of us to the creation of false memories. Brain scans of HSAM individuals have not revealed any huge anatomical differences from regular brains, although there may be some additional "wiring" between the hippocampus and the frontal cortex as a result of the increased mental traffic. Having said that, some research indicates that such individuals tend to have larger-than-average temporal lobes and caudate nuclei, and many also exhibit mild Obsessive Compulsive Disorder-like behaviour (the caudate nucleus is also associated with OCD).

One possible explanation for the phenonemenon revolves around the tendency for HSAM subjects to be more prone to fantasies and daydreaming (so that they tend to revisit all of their memories again and again in the following days and weeks, in much the same way as regular people might do for an important emotional event like a wedding, thus strengthening the initial memory trace), and they also score very highly on absorption (so that they tend to become very immersed in any activity, and pay complete attention to sensations and experiences, which allows them to establish strong foundations for a recollection).