The topic of infant memory capacity has been highly debated throughout the course of history. For many years, most individuals believed that infants had almost no capacity for any type of memory. This belief was referred to as “infantile amnesia” and stated that until around the age of toddlerhood, infants were not able to form, and later retrieve, any type of information from this period of life. Because of this, it was also believed that any early life traumas that infants endured would not be carried into childhood and adulthood; however, over the years researchers began to realize that infants were actually able to form some memories. Since then, many different studies and experiments have been conducted which explore the various aspects of memory and how infants are able to work through this process at such an early age. Through the use of such memory techniques as repetition, habituation, and deferred imitation, researchers were able to show evidence demonstrating infant memory capabilities. This is an important topic to research because early life events that occur within an infant’s life has the potential to have an impact on their later life cognitive and emotional development. There are major implications regarding how the infant is handled and simulated during infancy as well as the environments and cognitive experiences they are exposed to. During a time before individuals began to question this idea of “infantile amnesia” some of these issues were not considered major concerns as people believed they would not have any effect on the infant. At this time, it has been demonstrated that this may not be the case. Through the use of many current research studies, there is evidence that infants can form memories as well as maintain many of these memories from infancy to later adolescence and adulthood.
Although there has been this debate surrounding infant memory, it is believed that an infant’s memory is formed in the same what that an adults memory is. Previous studies have shown that their memory does not develop hierarchically as previously believed and that instead is present from a young age, possibly at birth or in-utero (Rovee-Collier). This has been shown through tasks done with young infants which tap different memory systems in the same way in which adults process this information. (Rovee-Collier). Despite the fact that these memory systems are present in infants, it has also been found that an infant’s memory increases throughout the first year or two of life so that they are able to hold longer memories and for a longer period of time (Rovee-Collier). Some of the biggest obstacles in studying memory have been due to the fact that infants are unable to give researchers subjective accounts of their experiences. Because they cannot yet speak, it has been important for researchers to determine other nonverbal ways in which infants can demonstrate that they have actually remembered something. This has been done through kicking tasks, eye tracking, and other physical traits which may demonstrate memory. Also, another issue has risen from the general inability of older children and adults who are able to speak to verbally discuss memories from their childhood; therefore, much research has been done to determine whether they are not remembering or if the context and storage of these nonverbal memories at a young age are interfering with later verbal repetition. This too has been difficult to study as it is sometimes hard to determine whether individuals are relaying their own memories or memories that have been conditioned to them through other individuals in their lives. Current studies have looked at many of these issues and developed experiments around them which demonstrate the capacity for infant memory.
Because it has been found that infant memory systems are somewhat developed and capable of forming memory at birth, researchers have questioned whether infants too can form memory while still just a fetus. In the study by Ditrix, Nijhuis, Jongsma, and Hornstra called Aspects of fetal learning and memory; the researchers looked at habituation in the fetus of pregnant women. Habituation refers to a decrease in response given to a certain stimulus that has been repeatedly stimulated in the individual’s presence .This phenomena is distinguishable from others because when the stimulus changes, there should be an immediate re-emergence of attention given to the new stimulus. Also, when the first stimulus is repeated, the individual should faster habituation to it after the initial presentation (Ditrix et al.. 2009). This study first recruited 100 pregnant women between 28 and 38 weeks gestation. There was extensive exclusion criteria for the women including absence of disorder in the women, no use of mediation, no depression, no change in eating habits, and smoking less than 6 cigarettes a day, among others. Although the study began with 100 participants there were dropouts due to other complications and therefore the final sample included 93 women. The main hypothesis of the study was that infants would habituate to vibroacoustic stimulus which would be measured at intervals depicting both short term and long term memory storage. At thirty second intervals, a vibroacoustic stimulus was applied to the woman’s abdomen near the fetal legs and movement of the fetus one second after the stimulus was given was monitored by an ultrasound machine (Ditrix et al., 2009). The maximum number of stimuli given during the test was 24, but if the fetus was still responding at the 21st stimulus, no further stimuli were given to the fetus and therefore no habituation took place. At the end of the study, the results were somewhat in line with the hypothesis. At 30 weeks gestation, almost the all of the fetuses tested demonstrated habituation. In each group, there was a significant decline in the initial stimuli versus the repeated habituation test in the first session. The results of the study demonstrated that fetuses have a short term memory for ten minutes at 30 weeks and that there is some evidence to suggest they are able to hold longer term memories for up to 4 weeks (Ditrix et al., 2009). One important aspect of these results is that it is essential to know whether the results were actually due to habituation or if they represent some type of fatigue or receptor adaptation. This is shown through the fact that when a new stimulus is presented, dishabituation should occur. This is one major limitation of the study in that although the study was able to demonstrate faster habituation to the first stimulus, they did not present another stimulus which would demonstrate dishabituation in order show that the results were in fact due to this phenomenon of habituation. Therefore, while the results may be salient, we cannot confidently conclude that they were specifically due to habituation and not another process. Another limitation of the study is that there may not have been enough participants to give the study the power it needs because unpaired statistics often need more participant cases in order to reach a higher level of significance. Therefore, in order to increase the power and validity of the statements regarding long-term memory storage, it would be important to conduct further information to confirm these results (Ditrix et al., 2009).
Although there have been many studies that demonstrate the fact that fetus have the ability to form memory in the womb such as the one done by Ditrix et al., very few studies have demonstrated the persistence of fetal memory into infant life. The study done by Gonzalez-Gonzalez et al. helps to show the link between these two life stages (2006). This is important to study because it is believed that fetal memory can actually serve some functions in terms of parental attachment to the mother and also recognition of the infant’s mother. Therefore, it is essential to know whether fetal memory stays intact after birth because it could have a potential effect on these variables. The present study consisted of forty-one pregnant women who were 38 to 40 weeks pregnant who had an absence of both medical and obstetric disorders, not toxic habits and cephalic presentation (Gonzalez et al., 2006). Through the method of the study, the fetus were stimulated using an artificial larynx repeatedly every minute for a maximum of 24 stimuli. This is the same method used in the Ditrix et al. study along with others as it has been shown to lead to habituation. This study determined habituation when the fetus stopped responding to four repeated stimuli. This test is different from the Ditrix et al. study because the researchers presented these stimulations every 48 to 72 hours until the fetus was delivered and then one or two days after delivery, the habituation assessment took place. The results of the study demonstrated that all of the fetuses showed a positive response to the stimulus presented after birth. Also, infants that were stimulated in utero habituated earlier than infants who were not previously exposed to the stimulus (Gonzalez et al., 2006).This provides some evidence that fetal memories are able to persist into infant life. Furthermore, it demonstrates that not all infants have the same capacity for memory formation. In general, the longer it took the fetus to habituate in the first part of the study, the longer it took the infant to habituate after birth. This study went more into depth than the Ditrix et al. study although both used many of the same methods. One limitation of this study and others which study habituation is the fact that there were very few participants and therefore this may decrease the power of the overall study. Previous studies also demonstrated this problem (such as the Ditrix et al. study) and many contained more participants than this study did. It is important that further research be done to rule out any errors that occurred within this study and increase the validity of the results. Another limitation is the fact that after the initial presentation, some fetus were given more presentation of the stimulus than others before they were born. This may introduce another variable of repetition or priming effects that were not accounted for within the study. One major strength of this study that was not included in the Ditrix et al. study was that the researchers used the process of dishabituation in order to rule out other effects that may cause the fetus to stop responding.
The study involving habituation by Gonzalez et al. may involve some limitation and confounding variables, but it also demonstrates other aspects of infant memory that is important to explore: repetition and priming effects. There have been many studies done which involve these variables, yet after many years of research, the exact process involved in the encoding, storage and retrieval of memories still remains largely unaccounted for (Turati, 2008). This study consisted of three separate experiments. The first one looked at the effect of an interfering stimulus on a newborn’s memory for a specific geometric pattern and the second one looked at the effect of further exposing the infant to the geometric pattern in order to reduce the loss of recognition which occurs after interference. For experiment 1, the study consisted of 65 infants, 30 boys and 35 girls, all of which were 1-3 day old healthy, full-term infants. The stimulus that was used during the study was hexagonal and X-shaped white geometric figures that had a black background. The first phase consisted of habituating the infants to a flickering LED light and then projecting the geometric shape to the infant and then recorded how long the infant remained fixed on the shape (Turati, 2008). The distracter shape was then presented and at the end of this test the results showed that the infant preferred the control more than the interference condition. These findings demonstrated that when a distracter stimulus is introduced between a habituation and test phase, this causes inhibition of the infants recognition response. In the second experiment, 31 subjects were used (17 girls and 14 boys) and the experiment had a similar set-up to that of the first. One difference was that before the test phase, they re-presented the familiar stimulus for 15 seconds. The results of this part of the study showed that when the infant’s memory was inhibited by a distracter stimulus, it could be reactivated through a reminder stimulus of the previously forgotten stimulus (Turati, 2008). This is significant because it demonstrates the importance of the repetition priming effect in memory storage which this study shows is present at birth. One limitation of this study is that the preference of the infant to the novelty stimulus may just be due to the fact that the infant has become familiarized with this stimulus. Another criticism is that the distracter may cause the infant to habituate to the stimulus and therefore they would be unable to show a reaction response. Further studies should be done in order to rule this out. Another important aspect of the study that should be further explored is whether during the repetition priming part of the study, if the infant need to be exposed to the previous stimulus in full or if a partial reminder of the stimulus would produce the same results. The importance of this study is that it shows infants have the capacity for memory storage and that this storage has the potential to be enhanced through repetition.
Another study that looks at the impact of repetition on early childhood memory involves an apparatus that is very much a part of our society: the television. The previous study looked at the effect of distracters and the impact of repetition while this study by Barr, Muentener, Garcia, Fujimoto, and Chavez effects of imitation caused by repetition. Television is a very prevalent part of our lives today and it has been shown that the amount of exposure in infants is high (Barr et al.. 2007). This study builds off previous studies by Bandura et al. (1963) which demonstrate the fact that children were more likely to show aggressive acts after they watched them on television. The authors of this study wanted to see if this effect could be seen in young infants also. This is important to look at because previous studies have not determined the effect of repetition in infant television programming as much of what they are watching are often repetitions of specific shows. Similar to the Turati et al. (2008) study, this research was also broken down into multiple experiments. The participants in the first experiment consisted of 108 full-term, healthy infants (58 girls and 50 boys). The infants were assigned to one of three groups which were a live administration given three times, a video given six times or a baseline control condition. 24 hours after the infant was exposed to the live session or recorded videos, a researcher returned and presented the stimuli presented in the demonstration stage in front of the child. The results shows that there was no significant difference between the individuals exposed to the live versus recorded sessions but that they both had much higher levels of imitation than the control group whom was not exposed to the test stimuli. The second experiment looked at only 21 month olds and seeks to determine whether there was a video deficit present in these infants who have been exposed to more recorded television that may not be in younger infants. This part of the study employed 12 infants who were 21 months-old. The results of this part of the experiment again showed that infants who were exposed to the live and recorded situations showed a significantly higher score than the control group which again demonstrates repetition effects. One interesting finding was that the infants who were shown the video three times had a significantly lower score than the infants who were shown the live demonstration three times. This shows that repeated exposure really can increase the imitation of the desired action. The last part of the study wanted to determine if infants as young as 12 months could imitate stimuli from television after deferred imitation as previous studies claim that 14 months is the youngest age at which infants can do this (Barr et al., 2007). This part of the research used 36 full term, healthy 12 month-old infants and the results again demonstrated that the infants had much higher imitation scores than the baseline with no difference between live and recorded administrations. One limitation of these experiments by Barr et al (2007) is that there may be some perceptual coding problems. For example, it may be that it is harder to transfer the 2D images on the television from the 3D images that are presented in front of the infant and therefore may explain why this video deficit is occurring. Seeing the stimuli in the 3D demonstration may make it easier for deferred imitation to occur in the same medium of 3D presentation. Further research should employ more technology in order to separate these effects from the results. Also, the presentation of the stimuli by the different researchers varied across the participants and therefore this could have had some effect on the results. It is important that future research make the live administrations more uniform across the different presentation. Despite these methodological issues, this study shows that there is an important role of technology in infant learning and memory and shows infants do have the capacity to remember the things that they see on television. This finding is also significant in terms of videos and programs designed to improve infant intelligence that may carry onto later life outcomes.
Another article by Defrancisco and Rovee-Collier (2008) explores the effects of priming on young infants and how it can lead to the re-emergence of once forgotten memories. The fact that infants can “forget” may serve to demonstrate that they were able to form the memory in the first place. This is an important concept because in order for memories in infancy to affect later life outcomes, these memories need to be remembered and retained within the individual. In order for a memory to be properly retrieved the cues needs to match the cues from the original exposure with the memory. In this study, 138 infants were used. Similar to the other studies that have been discussed, these infants were all full-term and healthy. This is an important aspect of the studies because having infants that are not healthy or properly developed may have the potential to eschew the results. The method of this study followed previous studies conducted by Rovee-Collier which use a train task and a mobile task depending on the age. The infants were trained and tested with the mobile which was hand-painted and therefore the infant had no previous exposure to it. Their ankles were connected to a string which attached to the mobile and each time the infant kicked, the mobile moved. The train task was only used on infants 6 months and over as it was not appropriate for 3 month-olds. The procedure of the study consisted of three different phases in which the infants received training sessions that were given a day apart. After a week, they received a reactivation treatment and after another day a long-term retention test. The first part of the training began with a nonreinforcement period followed by a reinforcement period and then again another nonreinforcement period. The first experiment looked at how a reactivated memory could be retrieved by a cue that was general. This answers one of the limitations presented in a previous study by Turati (2008) which did not demonstrate this in the study. For the 9 month-old infants in this study, it was found that they showed significant retention whether they were tested with a different train in the same environment or with the same train in a different environment. For the 12 month-olds, similar results were collected. For the second experiment in the study, the researchers looked at whether 9 and 12 month-old infants could be primed with a different cue in the same environment or the same cue in a different environment when they were tested with the original cue in the original environment after 24 hours. The 9 month-olds showed remembering when they were primed with a different cue in the same environment but not when they were primed with the original cue in a different environment (Defransisco & Rovee-Collier, 2008). This was the opposite for the 12 month-olds. These findings show that by the end of the first year, the environmental constraints on remembering certain stimuli are probably less than any time before. This study shows how infant memory is present but also ever changing as the infant grows and matures. Moreover, as infants grow older, the amount of time that a cue can be remembered increases with this age increase. Also, the more time that has elapsed between the first cue and subsequent reminders of the cue increases the likelihood that the cue will not be remembered. The results demonstrate the importance of repetition in strengthening infant memory. One limitation of this study is that there was not consistency in the variation between the different contexts of the study. In order to improve the accuracy and generalizability of the study, it would be important to keep this variable stable throughout the study. Also, as is true with the other studies, the sample size may be too small on some of the individual experiments due to the fact that some could only be performed with infants of a certain age.
Many studies, such as one previously discussed involving television cues have looked into the idea of deferred imitation as a way to determine memory in infants (Barr et al., 2007). Although this was part of the study they seemed to focus more on the effects that priming can have on this. A study done by Goertz, Kolling, Frahsek, Stanisch, and Knopf (2007), used the deferred imitation task in order to assess declarative memory in 12 month-old infants. In this study infants were shown a series of objects and watch, but do not touch, the demonstration of how the objects are used. They are then given these props at some point later and observed as to whether the target action is repeated. A task like this would show that the infant remembered how the objects was previously used and that they both acquired the information and chose to voluntarily retrieve and use this information (Goertz et al., 2007). For this study 24 children (9 girls and 15 boys) were included whose ages ranged from 11 months and 15 days to 12 months and 15 days. As with all previous studies, this is a severe limitation of the study in that the results may not hold the same power as a study that included more infants would. The results of this study showed that the tin can, cup and knife and drum used as the action items were spontaneously performed more often than the toy pig and mouse were. This study showed that it is possible to study declarative memory in infants toward the end of the first year of life. Because these infants are actively choosing to manipulate the object in the desired way, this demonstrates that it is in fact declarative memory that is being shown. Although the authors did not discuss this, Barr et al. demonstrated this idea too that the infants that were repeating the items viewed on the television and live performance were probably doing so using declarative memory. This is a difficult concept to grasp as most people believe that declarative memory is mostly verbal; however, this is not the case as shown by the studies done with the 12 month-olds in which speech is not adequately developed. Despite this, through their nonverbal actions they are able to demonstrate the fact that they have retained a memory through repetition of actions previously seen.
Previously, infant memory has been a highly debated topic. Many individuals believed that infants had almost no capacity for memory before the age of two. If was furthermore believed that any events that occurred in the infants life were quickly forgotten and were not able to be consciously recalled in adulthood. Current studies have shown that infants do in fact have capacity for memory, both short and long term; however, the extent to which this memory can be retained is still the topic of much research. Through the use of many current research studies, this literature review will explore how infants can form memories as well as the extent to which infants can retain these memories from an early age and how they can be maintained over the long term.