Childhood amnesia

Childhood amnesia was first formally reported by psychologist Caroline Miles in her 1895 American Journal of Psychology article "A study of individual psychology". Five years later, Henri and Henri published a survey showing that the average age of the respondents' earliest recollections was three years and one month. In 1904 G. Stanley Hall noted the phenomenon in his book, Adolescence: Its Psychology and Its Relations to Physiology, Anthropology, Sociology, Sex, Crime, Religion and Education. In 1910, Sigmund Freud offered one of the most famous and controversial descriptions and explanations of childhood amnesia. Using psychoanalytic theory—but no science-based evidence—he postulated that early life events were repressed due to their inappropriately sexual nature. He asserted that childhood or infantile amnesia was a precursor to the "hysterical amnesia", or repression, presented by his adult patients. Freud asked his patients to recall their earliest memories and found that they had difficulty remembering events from before the age of six to eight. Freud coined the term "infantile" or "childhood amnesia" and discussed this phenomenon in his Three Essays on the Theory of Sexuality. To date, no experimental studies have found any evidence to support Freud's ideas. In 1972, Campbell and Spear published a seminal review about childhood amnesia in Psychological Sciences recapping the research conducted to understand this topic from neurological and behavioral perspectives in both human and animal models. ==Methods of recall==

The method of memory retrieval can influence what can be recalled. In its basic form, the experimenter gives the participant a word, and the participant responds with the first memory they think of associated with that word. A 2013 study by Bauer and Larkina used cued recall by asking children and adults to state a personal memory related to the word and then state the earliest time that it occurred. The researchers found that the younger children need more prompts or cues. For children and adults, the earliest memory retrieval was around three years old. ==Accessible and inaccessible memories==

The number of early childhood memories a person can recall depends on many factors, including the emotion associated with the event, their age at the time of the remembered event, and the age at the time they are asked to recall an early memory. According to a study by West and Bauer, earlier memories tend to have less emotional content than later memories, and to be less personally meaningful, unique, or intense. Earlier memories also do not tend to differ greatly in perspective. Certain life events do result in clearer and earlier memories. Adults find it easier to remember personal, rather than public, event memories from early childhood. This means a person would remember getting a dog, but not the appearance of Halley's Comet. Psychologists have debated the age of adults' earliest memories. Most modern data suggests somewhere between the ages 3 and 4 on average. Some research shows that the offset of childhood amnesia (earliest age of recall) is 2 years of age for hospitalization and sibling birth and 3 years of age for death or change in houses. Thus, some memories are available from earlier in childhood than previous research suggested. It also suggest adults can access fragment memories (isolated moments without context, often remembered as images, behaviors, or emotions) from around age 3, whereas event memories are usually recalled from slightly later. This is similar to research showing the difference between personal recollections and known events. Known memories change to more personal recollections at approximately 4 years of age. ==Fading memories==

Children can form memories at younger ages than adults can recall. While the efficiency of encoding and storage processes allows older children to remember more, younger children also have great memory capacity. Infants can remember the actions of sequences, the objects used to produce them, and the order in which the actions unfold, suggesting that they possess the precursors necessary for autobiographical memory. One explanation for this is that a person develops linguistic skills, memories that were not encoded verbally are lost. Contrary findings indicate that elementary-aged children remember more accurate details about events than they had reported at a younger age and that 6- to 9-year-old children tend to have verbally accessible memories from very early childhood. Research on animal models seems to indicate that childhood amnesia is not only due to the development of language or any other human faculty. This increased ability for children to remember their early years does not start to fade until children reach double digits. By age 11, children exhibit young-adult levels of childhood amnesia. These findings may indicate that some aspect of the adolescent brain, or the neurobiological processes of adolescence, prompts the development of childhood amnesia. == Animal models ==

The phenomenon of infantile amnesia is not specific to humans. Research in rat models found that younger rats forget a conditioned avoidance response to a shock-paired compartment faster than older rats do. These findings have been replicated in a number of other species with different learning paradigms. These studies have informed neurobiological findings about childhood amnesia and would be impossible to ethically conduct in humans. Because infantile amnesia has been observed in animals, the occurrence cannot be explained just by cognition specific to humans such as reading and writing or an understanding of self. A main criticism of animal models is that development and cognition in animals and humans are starkly different. Researchers have attempted to address this by creating timelines for animal development based on changes in learning and memory abilities, brain development, and hormones. Research on rat models indicates that failures in memory retrieval cause infantile amnesia. In a study on rats, it was found that one single "reminder" before a test was enough to reduce forgetting in infant rats. A similar study was done on infants, showing that a behavior that was typically forgotten in a few days could be remembered if the subject was exposed to the reinforcer before a test. These studies indicate that these memories being not lost, but rather not retrieved. ==Individual differences==

Many factors affect memory in humans, including gender and culture. Differences in early memory between these groups can tell us about potential causes and implications of childhood amnesia. The individual differences mean that elaborative parenting styles and emphasis of cultural history when teaching children may result in recollection of earlier childhood memories. This suggests that childhood amnesia offset is modifiable by parenting and teaching styles and is therefore not exclusively predetermined or entirely biological. Sex Generally, when a sex discrepancy is found in the age at first memories, females have earlier memories than males. Women's earlier first memories may be accounted for by the fact that mothers generally have more elaborative, evaluative, and emotional reminiscent style with daughters than with sons, Men exhibit more early memory focus on themselves. Men have been found more likely than women to mention bad memories. Ethnicity, culture, and society MacDonald et al. found that Chinese participants had later first memories than New Zealand European (Pākehā) or Māori participants. This effect was due to Chinese women, whose average age at first memory was 6.1 years. Māori adults report significantly earlier memories than either Pākehā or Chinese individuals. The traditional emphasis on the past in Māori culture may have led to an earlier understanding of the nature of time, and to the retrieval of earlier memories. Māori are also more likely than Pākehā or Chinese individuals to indicate a family story as a source for their memory. People who reveal a more detailed memory are more likely to be open in their daily lives and disclose a range of personal information to others. This evidence for delayed and impaired recall of trauma due to the trauma itself is contrary to the concept of repressed memories, wherein traumatic memories are stored intact in order to protect the subject and can be recovered with full accompanying narrative. Since priming can occur at a younger age than episodic recall, children in abusive situations may form implicit memory connections of violence even when no true episodic recall exists. One study found that participants were more able to correctly remember memories that occurred around the age of 10, while memories from before age 3 were more often confused with false images and memories. Memories from early childhood (around age three) are susceptible to false suggestion, making them less trustworthy. This phenomenon has been called imagination inflation and shows that imagining an event can make it seem more plausible that it happened. Similarly, people who are shown doctored photographs of themselves as children in an event that never occurred can create false memories of the event by imagining the event over time. This implies it is possible for false memories to be generated in and/or fed by a court case. This concern has led the American Psychiatric Association to advise caution in accepting memories of physically and sexually abusive events from before the age of two. It also recommends that these memories not be entirely discounted, due to the nature of the crimes. Studies also show that when mothers include emotional context when reminiscing with their children, the child is likely to grow up with a more secure attachment style, as well as show higher levels of emotional understanding and a stronger sense of self. These factors all contribute to the child developing stronger autobiographical memory skills. ==Proposed explanations==

Freud's trauma theory Sigmund Freud is famous for his theories of psychosexual development which suggest that people's personality traits stem from their libido (sexual appetite) which develops from early childhood experiences. Freudian theory, including his explanation for childhood amnesia, has been criticized for extensive use of anecdotal evidence rather than scientific research, and his observations that allow for multiple interpretations. Research has found that later memories in childhood have more propositional and emotional content than earlier memories and are rated as more meaningful and vivid. It has been suggested that differences in the emotions experienced by infants and adults may be a cause of childhood amnesia. Some studies have found that emotional experiences are connected with faster retrieval times, leading to the belief that emotional events have heightened accessibility in our memories. Specifically, the dentate gyrus, a section of the hippocampus, has been observed to primarily develop in the postnatal stages of development. It is theorized that this part of the hippocampus plays a large role in the ability to form and retrieve memories. The ability to store and recall memories gradually increases along with the postnatal development of the dentate gyrus. The physiological approach appears to support findings of memory loss in relation to amnesiacs and others who have experienced damage to the hippocampus. They cannot efficiently store or recall memories from past events, but still exhibit perceptual and cognitive skills and can still learn new information. While the neurological explanation does account for blanks in very young children's memories, it does not give a full explanation for childhood amnesia because it does not account for the years after the age of four. Nor does it explain why children do not show childhood amnesia. This discovery that three-year-olds can retrieve memories from earlier in their life implies that all necessary neurological structures are in place to recall episodic information over the short-term, but evidently not over the long-term into adulthood. The finding that all altricial species experience profound forgetting of episodic information formed during infancy suggests that human-centric explanations of infantile amnesia are inherently incomplete. A comprehensive understanding of infantile amnesia will require a neurobiological explanation of why infants forget. There are reasons to believe that different associations within the cerebral hemisphere have an effect on remembering events from a very early period in a person's life. Mixed-handedness and bilateral saccadic eye movements (as opposed to vertical or pursuit eye movements) have been associated with an earlier offset of childhood amnesia, leading to the conclusion that interactions between the two hemispheres correlate with increased memory for early childhood events. Neurobiology Research into the neural substrates of infantile amnesia using animal models has found that the major inhibitory neurotransmitter gamma-amino butyric acid (GABA) may be involved in the regulation of retrieval of infantile memories in adults. GABA activity is known to be higher in early childhood development than it is in adulthood, both in humans and other animals. Researchers have hypothesized that increased GABA activity in development has an effect on memory retrieval later in life. Past studies have shown that GABA aids in the forgetting of fear memories in infancy and that it may be a general mechanism for regulating infant memory retrieval. Benzodiazepines, a class of psychiatric medication which increase GABA expression, have been found to produce anterograde amnesia, or a failure to encode memories after taking the medication. Subjects taking benzodiazepines are found to perform worse on learning and memory tasks compared to drug-naïve subjects. Previously, it was assumed that neurogenesis, or the continued production of neurons, ended after development. Recent findings have shown that there are high levels of neurogenesis in the hippocampus in early childhood which taper out into adulthood, although neurogenesis continues to persist slowly. As the hippocampus is known to be vital to memory processes, there are obvious implications for childhood amnesia. Animal research has shown that the age of high neurogenesis is in the developmental period when persistent memories are least likely to be formed. It has been proposed that hippocampal neurogenesis degrades existing memories. This may be due to increased competition between the new and existing neurons, followed by the replacement of synapses in preexisting memory circuits. This theory has been supported in mouse models in which increasing neurogenesis levels also increased forgetting. Additionally, decreasing neurogenesis after new memory formation resulted in decreased forgetting. In addition, memories are consolidated via transferral from the hippocampus to the cortex. This transferral occurs preferably during periods of elevated excitability in the hippocampus i.e. during ripple oscillations. Ripple oscillations represent increased hippocampo-cortical communication. This increase in experience-associated activity does not occur up to a certain age suggesting that this might be a mechanism for infantile amnesia. By stimulating neurons used in 'forgotten' fear conditioning in infant mice, Guskjolen et al. found retrieval of the fear response was possible by optogenetic reactivation of the neuronal ensembles that encoded the memory drive. This retrieval lasted for up to three months, suggesting the infantile amnesia was underlaid by a biological failure to access, rather than encode, said memories. In addition, memories are consolidated via transferral from the hippocampus to the cortex. This transferral occurs preferably during periods of elevated activity in the hippocampus. This increase in experience-associated activity does not occur up to a certain age, suggesting that this inability to transfer information might be a mechanism for infantile amnesia. This acknowledged link of the past to the present and the concept of continuous-time and therefore a continuous self is also helped by memory talk with adults. If children lack language, they are unable to describe memories from infancy because they do not have the words and knowledge to explain them. Adults and children can often remember memories from around three or four years of age which is during a time of rapid language development. Before language develops, children often only hold preverbal memories and may use symbols to represent them. Therefore, once language develops, one can actively describe their memories with words. The context that one is in when they encode or retrieve memories is different for adults and infants because language is not present during infancy. This has implications for gender and cultural differences. Autobiographical memory begins to emerge as parents engage in memory talk with their children and encourage them to think about why a certain event happened. Memory talk allows children to develop memory systems in order to categorize generic versus unique events. An important aspect of this theory considers the difference between parents who discuss memories at length with their children in an elaborative style, and those who do not. Children of parents who discuss memories with them in an elaborative style report a greater number of memories than children who do not discuss their memories. Memories are described in greater detail. This has implications for cultural differences. == See also ==