In psychology and neuroscience, implicit memory is one of the two main types of long-term human memory. It is acquired and used unconsciously, and can affect thoughts and behaviours. One of its most common forms is procedural memory, which allows people to perform certain tasks without conscious awareness of these previous experiences; for example, remembering how to tie one's shoes or ride a bicycle without consciously thinking about those activities.
The type of knowledge that is stored in implicit memory is called implicit knowledge, implicit memory's counterpart is known as explicit memory or declarative memory, which refers to the conscious, intentional recollection of factual information, previous experiences and concepts.
Evidence for implicit memory arises in priming, a process whereby subjects are measured by how they have improved their performance on tasks for which they have been subconsciously prepared. Implicit memory also leads to the illusory truth effect, which suggests that subjects are more likely to rate as true those statements that they have already heard, regardless of their truthfulness.
Evidence and current research
Early research Advanced studies of implicit memory began only in the 1980s. In early research, subjects were presented with words under different conditions and were given two types of tests: recognition memory tests and perceptual identification tests. These studies provided evidence that effects of memory on perceptual identification was independent of recognition memory. Jacoby & Brooks argued that perceptual identity effects reflect very rapid, context-specific learning. Unconscious influences of memory were found to alter the subjective experiences of participants. In one such study, participants judged that the white background noise was lower when they read words they had already been presented, thus misattributing their ease of perceiving the word to less noisy environment. This provided evidence for specific and long-living influences of past memory even when participants were unaware of its influence. Similar effects have been found with studies where participants made judgments about difficulty of anagrams and recognized famous names.
Priming studies The effect of implicit memory was tested employing priming procedures. Several studies confirm implicit memory as a separate entity. In one such experiment, participants were asked to listen to several songs and decide if they were familiar with the song or not. Half of the participants were presented with familiar American folk songs and the other half were presented with songs made using the tunes of the same songs from group 1 but mixed with new lyrics. Results show that participants in group 1 had a much higher chance of recalling the songs as being familiar, even though in both groups, the tunes of the songs were the same. This study shows that people are even implicitly making connections amongst their memories. Much memory study focuses on associative memory, or memories formed between two entities, linking them together in the brain. This study shows that people implicitly make a strong associative connection between a song's tune and its lyrics that they can't separate later.
Some clues as to the anatomical basis of implicit memory have emanated from recent studies comparing different forms of dementia. Patients with dementia of the Alzheimer type (DAT) have been reported to be severely impaired on both lexical and semantic priming tasks, while patients with Huntington's disease (HD) were able to demonstrate normal priming ability (Shimamura et al., 1987; Salmon et al., 1988). In contrast, HD patients evidenced little learning on a pursuit-rotor task that was easily mastered by both amnesic and DAT patients (Eslinger and Damasio, 1986; Heindel et al., 1988). This possible double dissociation involving HD and DAT patients suggests that different implicit memory tasks are mediated by distinct neural systems and that these tasks can be used to differentiate some of the so-called "cortical" (e.g., DAT) from "subcortical" (e.g., HD) dementias (Cummings and Benson, 1984).
A more recent contribution to the study of implicit memory comes from the experiments with a spatial organization computer game on amnesic patients (Stickgold et al., 2000). Damage to the bilateral temporal lobe and hippocampus had caused the loss of explicit memory. However, despite being unable to recall the game, these patients were able to dream of it at sleep onset. This observation is interesting as it shows that learning can be memorized without the contribution of explicit memory, which requires the activation of the hippocampus and of the temporal and basal cortex. In the cases observed by Stickgold et al., the explicit memory was definitely impaired, but a non-explicit and non-conscious kind of memory was left and could emerge in dreams. This observation shows that an experience can be stored in the implicit memory and can be represented symbolically in dreams.
Current research According to Daniel L. Schacter, "The question of whether implicit and explicit memory depend on a single underlying system or on multiple underlying systems is not yet resolved." The findings display such a variety of phenomena that there has not yet been a theory to account for all of the observations. Instead, two theories have been presented to explain different subsets of the data.
Modern discoveries in neuropsychology concerning the organization of memory allow us to hypothesize that some synaptical cortical and subcortical circuits form the seat of unconscious mental functions. The possibility of identifying, in the explicit and implicit memory respectively, the repressed and unrepressed unconscious opens new and stimulating perspectives for an integration of neuroscience with psychoanalysis, and for a possible anatomic localization of the functions of these two different forms of unconscious. This depends on a presupposition: that the experiences, emotions, phantasies, and defences that help organize an individual's unconscious psychic reality, from birth throughout life, are stored in the nervous structures concerning memory, both implicit and explicit. This is, after all, in line with Freud's conviction: 'latent conceptions, if we have any reason to suppose that they exist in the mind—as we had in the case of memory—let them be denoted by the term "unconscious"' (1912, p. 260).
There are usually two approaches to studying implicit memory. The first is to define a characteristic associated with explicit memory. If a person with a normal working memory can solve the task (e.g. remembering a list of words), then they are consciously recalling a memory. The second approach invokes neither a conscious nor an unconscious response. This approach is dependent on many independent variables that affect the response of a person's implicit and explicit memory.
Development Empirical evidence suggests infants are only capable of implicit memory because they are unable to intentionally draw knowledge from pre-existing memories. As people mature, they are usually capable of intentional recollection of memory, or explicit memory. But amnesic patients are usually the exception to developing memory, but are still capable of undergoing priming, to some extent. Since procedural memory is based on automatic responses to certain stimuli, amnesic patients are not affected by their disability when behaving habitually.
Childhood is when knowledge representation grows quickly. New concepts are formed from experience through poorly understood inductive processes. Word knowledge grows rapidly, perhaps at one a day. These processes are currently poorly understood, and may be unconscious.
Although the explicit–implicit distinction was introduced during the 1980s, the sort of contrast that it captures is not new; related distinctions between conscious and unconscious memories, to take just one example, have been around for more than a century (for historical considerations, see Roediger, 1990b; Schacter, 1987). The critical development during the past decade has been the systematic demonstration, exploration, and attempted explanation of dissociations between explicit and implicit memory. Some of these dissociations have been provided by experiments demonstrating that brain-damaged amnesic patients with severe impairments of explicit memory can exhibit intact implicit memory; others come from studies showing that specific experimental variables produce different and even opposite effects on explicit and implicit memory tasks.
The discovery of implicit memory was made by Warrington and Weiskrantz (1974) who studied with priming experiments patients affected by Korsakov's amnesia, in which the structures of explicit memory are damaged. Subsequently, the procedural dimension of implicit memory has been confirmed. As well as this, the emotional and affective dimension of implicit memory is of particular interest for psychoanalysis. It is linked to the earliest, most significant experiences of the infant with the mother and the surrounding environment.
Activation processing Activation processing is one of two parts in Mandler's dual processing theory. According to Mandler, there are two processes that operate on mental representations. The first is activation, where increased activity causes a memory to be more distinctive. This increases the familiarity component of the memory, which explains results from priming effects. The second is elaboration, which is a conscious memory used to encode explicit memories that involves activation, but also creating new relationships amongst existing memories.
Multiple memory system The multiple memory system theory ascribes the differences in implicit and explicit memory to the differences in the underlying structures. The theory says that explicit memories are associated with a declarative memory system responsible for the formation of new representations or data structures. In contrast, implicit memories are associated with a procedural memory system where memories are just modifications of existing procedures or processing operations.
Progress in identifying the structures and connections that make up the medial temporal lobe memory system has been paralleled by gains in understanding how this system participates in memory functions. An important step in this achievement was the insight that the hippocampal formation is important for only a particular kind of memory. The implication was that memory is not a single entity but consists of multiple processes or systems. Converging evidence about the selective role of the hippocampal formation in memory is now available from rats, monkeys, and humans. It took time for the idea of multiple memory systems to become firmly established. In 1962, the severely impaired amnesic patient H. M. was reported to be capable of day-to-day improvement in a hand–eye coordination skill, despite having no memory for the practice sessions (Milner, 1962). Nevertheless, subsequent discussions of memory in general and amnesia in particular tended to set aside motor skill learning and to focus on the unitary nature of the rest of memory. Amnesia was considered to impair memory globally, with the recognition that an exception should be made for motor skills.
Memory as tool vs. memory as object