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| How reliable are memories |
| di Alberto Oliverio |
1. The biology of memory: the classical view.
If we consider the meaning of memory within an evolutionary point of view we should look at the organisms for their potential to change as the result of events that occur during an individual's lifetime. The experiences of an organism can modify the nervous system, and the organism later behaves differently because of these experiences. This abi1ity to change gives organisms the capacity for learning and memory .
The capacity for memory is a special case of the ability of neurons to exhibit plasticity. Plasticity is a general term describing the fact that neurons can grow or shrink, or otherwise change their input and output characteristics, as a function of their local histories. For example, many neurons change in response to injury or stimulation, sometimes in a long-lasting way. Thus, if the endings of one group of neurons are destroyed, neighboring neurons may increase their branching to occupy the newly created space.
Reflections about the physiological basis of memory led most ear1y biologists to consider some type of growth or change in the existing structure of neurons: thus, the idea that existing nerve cells can grow seemed a reasonable way of accounting for the persistence of memory. This basic idea was made more specific by Hebb (1949), and it has been restated several times since, making explicit the proposal that the synapse (the tiny gap between neurons) is the critical site of plastic change. The idea is that memory involves a persistent change in the relationship between neurons, either through actual structural modification or through stable biochemical events within neurons that change the strength of the signal that neurons send to their neighbors.
Experimental evidence has become avai1able only in the past few decades. Not surprisingly, some of the most direct evidence comes from animals having relatively simple neuron systems. For example, the Aplysia, a sea slug examined extensively by Erik Kandel (1976), has about 18,000 neurons distributed among nine cell groups called ganglia. This animal exhibits several simple forms of learning and memory, including habituation (the weakening of a response through repeated stimulation), sensitization (the facilitation of a response by a strong or novel stimulus), and classical conditioning (wherein a previously neutral stimulus comes to elicit a response as a result of repeated temporal association between the neutral stimulus and a second stimulus that ordinarily elicits the response).
The short-lasting varieties of habituation and sensitization have been shown to depend on changes at synapses in the nerve circuits. Specifically, there is either a decrease (in the case of habituation) or an increase (in the case of sensitization) in the amount of chemical transmitter released at these synapses. Long-lasting behavioral change appears to require new synthesis of protein within neurons, which probably reflects a stable and potentially permanent change in the way that the genetic information in these neurons is being used. Studies of Aplysia have also provided direct evidence for the idea that morphological change in neurons has a role in behavioral memory. After behavioral training, the branchy processes of neurons in the reflex circuit become longer (after sensitization training) or shorter (after habituation), thereby altering the number of synapses formed by the neurons. In this way the strength of the connections between the neurons and their neighbors is changed
Although much headway has been made by studying invertebrates with relatively simple nervous systems, important evidence about neuronal plasticity has also been obtained by studying the more complex brains of vertebrates. For example, building on earlier work, Greenough (Chang and Greenough 1984) studied rats reared in complex environments and rats given daily training in a maze. The neurons in the neocortex (the most evolved and most complex part of the brain) had more extensive branchy processes (dendrites) than the cortical neurons of comparison animals. There were also more synapses on the dendrites. Thus, experience can increase the number of synapses, presumably by forming them outright or by selectively preserving some synapses from a population that is continuously being replaced The important conclusion is that the neurons of vertebrates show considerable capacity for morphological growth and change in response to experience: these principles were proved true for humans as well.
One kind of neuronal plasticity involved in memory is long-term potentiation (LTP). LTP is a long-lasting increase in the strength of a synaptic response following stimulation of an input pathway. It occurs prominently in the hippocampus, a structure that is important for memory. LTP also involves morphological change in the structure of neurons. Lynch and Baudry, 1984). A large body of experiments points to the conclusion that memory is stored by changes in synaptic efficacy, and that these changes are brought about by common cellular events that may be found in many or all of the neurons that change. What makes memories specific must depend largely on which neuronal connections are altered, and on the architecture of the specific networks in which alterations occur.
Questions about how individual neurons change in response to experience are fundamentally important. Yet, there are many other questions that seek a more global, systems level of understanding. Where is memory stored? Is there one kind of memory or many ? What are the brain systems and connections involved in memory and what jobs do they do?
Before the recent technological developments in neuroscience, these complex issues were studied primarily by psychologists. In the recent years, a number of neurophysiological techniques, mostly brain imaging such as functional magnetic resonance (fMR) and positron emission tomography (PET) brought new knowledge on the relationships between memory and the brain. The main result is related to the lack of a specific memory “center” where an entire memory is stored. The most likely site of memory storage is the set of particular cortical processing systems that are ordinari1y engaged during the perception, processing, and analysis of the material being learned. Memory can be considered to be distributed in the sense that no single memory center exists and in the sense that many parts of the nervous system participate in the representation of a single event. Each part of the brain contributes differently to the whole representation.
Despite this holistic view of memory, specific brain structures and connections that participate in memory functions have been identified through study of the human amnesic syndrome and animal models of the syndrome. Amnesia is a severe impairment in the ability to acquire new knowledge or to recall recently acquired knowledge, which occurs when there is bilateral damage to the medial temporal lobe or the midline of the diencephalon. The point is that there are brain structures, which are not themselves repositories of permanent memory , that are important for memory functions. The use of magnetic resonance imaging has made possible to detect abnormalities in the hippocampal formation of some amnesic patients. Damage limited to the hippocampus itself is sufficient to cause amnesia. Further studies indicate that the full medial temporal memory system consists of the hippocampus and the adjacent, anatomica1ly related structures including entorhinal cortex, parahippocampal gyrus, and perirhinal cortex and that different subcortical and cortical structures and circuits are responsible for procedural and semantic memories (Mele et al., 2004)
2. Memory reconsolidation.
It is now appreciated that memory is not a unitary mental faculty but reflects the operation of many separate systems (De Leonibus et al. 2003). This raises the possibility that different cellular mechanisms may be involved. One broad classification scheme distinguishes declarative (explicit) memory from non-declarative (implicit) memory (Squire 1987). Declarative memory includes the facts, episodes, faces, and routes of everyday life and is accessible to conscious recollection. Declarative memory is affected in amnesia, and it depends on the integrity of the damaged structures and connections. Non-declarative memory includes skillful behavior, conditioned responding, and priming. Priming refers to facilitation in identifying perceptual objects, including words, that is produced by their recent presentation. Non-declarative memory affords the capacity to acquire knowledge unconsciously. In the case of skill learning, knowledge of the skill is embedded in procedures, and the skill is expressed in performance by engaging the procedures. In the case of conditioning, dispositions are gradually acquired to respond to stimuli. In the case of priming, perception and processing of stimuli are facilitated (Tulving et al. 1988). For example, if the word baby is presented, the probability is more than doubled that baby will subsequently be the word elicited by instructions to free associate to the word child. All of these examples of learning are intact in amnesic patients. Non-declarative memory is acquired, stored, and retrieved independently of the hippocampus and other structures damaged in amnesia; non-declarative memory is thought to be a philogenetically early form of memory. By contrast, the capacity for declarative memory may have appeared relatively late in evolution, reaching its greatest development in mammals with the full elaboration of medial temporal lobe structures, especially the hippocampal formation and related cortex. This capacity allows animals to record and access the particular encounters that led to behavioral change.
Apparently, if conscious declarative memory is to be acquired and stored in an enduring, retrievable form, the hippocampus and related structures must participate in neural processing at the time of learning. Coordinated activity in neural networks in the neocortex is presumed to underlie perception and representation in short-term memory. If perception is to be transformed into persistent memory , then convergent activity must occur in the connections from the neocortex to the hippocampal system. The input is processed through various stages within this system and then exits by way of widespread, divergent connections that return information to the neocortex. In this way, the hippocampal system supports the development of conscious memories in the neocortex.
This outline suggests that consolidation from the short to the long term form of memory results in stable coding, in a permanent ad reliable form of experience. However, while the stability of long term memories has been a dogma for a long time, an increasing body of knowledge suggests that long-term memory is not as stable and impermeable as it was hypothesized in the classic models of memory. In particular, it appears that recalling a memory renders it fluid and unstable - able to be changed before being re-fixed into the circuitry of the brain - and that change could include altering its meaning or even deleting it completely.
As we have previously seen, to be turned into something more permanent, the synapses that connect the arrangement of neurons that equal an experience grow, sprouting new and thicker connections to stabilize the memory trace. Proteins are produced by a range of genes to facilitate consolidation. Initially the building of this brain pattern occurs deep in the brain in places like the hippocampus but over the course of weeks and even years it moves to more general areas of the cortex. Until recently it was felt that this was the end of the story, the memory stays in the backwaters of the mind aging but essentially remaining the same. However recent data blew this idea away. To study the process of consolidation researchers interfere with the steps involved in fixing a memory in order to test their influence on long-term recall. Joseph LeDoux and Karim Nader (Nader and LeDoux, 2000) trained rats to associate a darkened box with an electric shock to their paws. The rats learn the box is to be avoided and freeze the next time they are put into it (fear conditioning). If, a few days after this conditioning, the animals were given a drug to prevent protein synthesis before being reminded of the box it made no difference to their ability to remember it as a bad place. The memory seemed fixed and safely stored in line with the classical view of memory consolidation. But if the rat had a brief reminder of the box just before the drug was administered the rat lost its fear conditioning. The memory had somehow been erased. LeDoux and Nader labeled this phenomenon as reconsolidation.
Traditional consolidation theory suggests that memories are fixed locally by protein changes within a few hours of the event and then filed to long-term storage in the cortex after a period of days or weeks. After conditioning rats in the same way they left them for 45 days, by which time the memory should have been fixed and immune to interference. As before, the rats given no reminder of the box before being injected showed their conditioned response to the box. But the rats that were given a reminder of the box before being given the drug did develop amnesia. The consolidated memory - which conventional theory said was permanent and stable - had been removed by the action of recalling it. In Nader’s words “The dogma was that once a memory trace had been consolidated, it is permanent. But here it is labile - subject to interference in exactly the same way as a brand new experience. We were showing memory to be something incredibly dynamic.”
It appears that memory moves from the hippocampus to the cortex during consolidation, but is returned to the hippocampus for reconsolidation by the act of recall. This dynamism would be unnecessary if the brain just wanted a photograph album, but it fits perfectly if memories exist to make sense of the present, and distorting some memories while generalizing about, or even deleting, others serves to improve the mind’s recognition and understanding of the world in order to prosper within it. If a memory becomes plastic every time it is recalled then it can be re-filed in a usefully updated way. The mind can make choices about whether to merge old and new, or to reinforce their differences.
In brief, then, reconsolidation theory suggests that when memories are recalled they become vulnerable to change. Therapists who investigate past experience of their clients should be aware that every time a memory is recalled it becomes unstable and capable of change. That change can be in one of several forms:
· The memory could be strengthened in its meaning – it becomes more of what it was.
· It can be weakened - it has less effect on the belief network it’s connected to.
· It can be transformed by having the meaning of it reframed – and by doing so transform the
belief that derives from it.
3. How accurate and reliable is memory?
Autobiographical memories are one of the main components of semantic memory: different episodes of our life (episodic memory) are tied together to make up a narrative of our life. How reliable is this his story? Is it dependable or doe it reflect o production not completely accurate and truthful? The accuracy of our memories is out of discussion if we consider their general meaning, the full story: on the contrary, the details may be rather uncertain and slowly change as times goes by. For example, many studies have evaluated the memory of life events recorded on diaries by students enrolled in these experiments. After a few months the experimenter read these records and asked their authors if they did remember a number of details. In some instances the experimenter deliberately altered the record (written through a word processor and therefore changeable) even in a significant way: the longer the time elapsed, the greater was the possibility that the students recognized as their “own” the false events recorded in “their” accounts.
The difficulty to catch the difference between “true” and “false” memories mostly depends on oblivion affecting autobiographical memory. Marigold Linton has clearly described this phenomenon. Starting form 1972, the psychologist noted each day on a different card three daily events. Day after day she devoted the same space, three lines, in order to give the same space to different life events, e.g. memories. Once in a month, she randomly extracted a couple of cards, she read the notes, tried to date it and evoke these events. When the events were noted and recalled to mind Linton tried to rate them in terms of their relevance, emotional characteristics, meanings etc. She was at the same time the subject and the experimental object of her research. At the end of her research, Linton assessed that memories decay at an yearly rate of about 5-6%: at this pace about 50% of our autobiographical memories should fade or disappear if they were not inserted in the general framework of our autobiographical memory concerning general facts or life periods. As a matter of fact, while the single bricks (episodes) assembling these wider containers may deteriorate, the perception of the stream of memories and of their general meaning survives. As time goes by and memories fade, the cues necessary in order to evoke a memory progressively decreases: in order to evoke an event the stimulus must almost completely match an ancient memory in order to restore it, as to say to reach a good reminiscence. This process is somewhat similar to what happens when we visit a place once familiar: initially there is a feeling of uneasiness, of not belonging but suddenly a cue that seems irrelevant may trigger a full memory. What is the reason for our decrease capacity to recollect past memories as we age? This phenomenon may be explained with the metaphor of the lock and key: At the beginning any key opens the lock of memory, as time elapses the key becomes more and more specific until it must almost match the lock.
Another characteristics of autobiographical memories concerns our ability to date them with some accuracy. In general the more efficient is our memory, the more we feel that events occur most frequently. As a consequence, recent events are –obviously- dated with more precision and they seem to occur more often while events remote in time are roughly dated and considered rarer than what actually is. Most mistakes in dating are related to the fact that our “interior time” and physical time do not coincide: if we are active and involved in some activities closer events seem distant, the contrary happens if we are inactive and not involved in something, as it happens in old people to whom facts happened a few days before seem more remote in time. One of the mechanisms responsible for dating memories in time is the association between individual and collective –social- memories. Thus, we may recollect something happened “in the moment when…”, “the day in which..”, “the year when…” something memorable happened. When these reference points are absent, our memories are inaccurate, we feel that we cannot trust them to the point that we may have doubts about the consistency of some events of our life.
Elisabeth Loftus has persuasively contributed to show that memory consolidation does not warrant the stability and dependability of memories. By studying autobiographical memory she has shown that recollections depend on a complex activity of reorganization of “fragments” and remains. In addition to that, she has also shown that depending on the age and circumstances memories may also be “implanted” and integrated in our memory framework as if they were our own memories. This fact casts some doubts on the meaning of witnesses, in particular in those instances in which some pressure has been exerted on eyewitnesses immediately after a fact: for example, if an adult insists that “that man was tall and with dark hair” while he actually was of average height and fair-haired, a child will modify his witness since he really gets to believe to the “implanted” version of the fact.
In most general terms, memory is open to different kind of conscious or unconscious manipulations and even autobiographical memories are subject to pitfalls and inconsistencies. Since the media continuously expose us to a rich imaginary, to fantasies exerting a strong hold on our mind because our brain is mostly visual and we believe in what we see, individual memory risks to become more and more blurred, less reliable than it was in the past.
If we consider the meaning of memory within an evolutionary point of view we should look at the organisms for their potential to change as the result of events that occur during an individual's lifetime. The experiences of an organism can modify the nervous system, and the organism later behaves differently because of these experiences. This abi1ity to change gives organisms the capacity for learning and memory .
The capacity for memory is a special case of the ability of neurons to exhibit plasticity. Plasticity is a general term describing the fact that neurons can grow or shrink, or otherwise change their input and output characteristics, as a function of their local histories. For example, many neurons change in response to injury or stimulation, sometimes in a long-lasting way. Thus, if the endings of one group of neurons are destroyed, neighboring neurons may increase their branching to occupy the newly created space.
Reflections about the physiological basis of memory led most ear1y biologists to consider some type of growth or change in the existing structure of neurons: thus, the idea that existing nerve cells can grow seemed a reasonable way of accounting for the persistence of memory. This basic idea was made more specific by Hebb (1949), and it has been restated several times since, making explicit the proposal that the synapse (the tiny gap between neurons) is the critical site of plastic change. The idea is that memory involves a persistent change in the relationship between neurons, either through actual structural modification or through stable biochemical events within neurons that change the strength of the signal that neurons send to their neighbors.
Experimental evidence has become avai1able only in the past few decades. Not surprisingly, some of the most direct evidence comes from animals having relatively simple neuron systems. For example, the Aplysia, a sea slug examined extensively by Erik Kandel (1976), has about 18,000 neurons distributed among nine cell groups called ganglia. This animal exhibits several simple forms of learning and memory, including habituation (the weakening of a response through repeated stimulation), sensitization (the facilitation of a response by a strong or novel stimulus), and classical conditioning (wherein a previously neutral stimulus comes to elicit a response as a result of repeated temporal association between the neutral stimulus and a second stimulus that ordinarily elicits the response).
The short-lasting varieties of habituation and sensitization have been shown to depend on changes at synapses in the nerve circuits. Specifically, there is either a decrease (in the case of habituation) or an increase (in the case of sensitization) in the amount of chemical transmitter released at these synapses. Long-lasting behavioral change appears to require new synthesis of protein within neurons, which probably reflects a stable and potentially permanent change in the way that the genetic information in these neurons is being used. Studies of Aplysia have also provided direct evidence for the idea that morphological change in neurons has a role in behavioral memory. After behavioral training, the branchy processes of neurons in the reflex circuit become longer (after sensitization training) or shorter (after habituation), thereby altering the number of synapses formed by the neurons. In this way the strength of the connections between the neurons and their neighbors is changed
Although much headway has been made by studying invertebrates with relatively simple nervous systems, important evidence about neuronal plasticity has also been obtained by studying the more complex brains of vertebrates. For example, building on earlier work, Greenough (Chang and Greenough 1984) studied rats reared in complex environments and rats given daily training in a maze. The neurons in the neocortex (the most evolved and most complex part of the brain) had more extensive branchy processes (dendrites) than the cortical neurons of comparison animals. There were also more synapses on the dendrites. Thus, experience can increase the number of synapses, presumably by forming them outright or by selectively preserving some synapses from a population that is continuously being replaced The important conclusion is that the neurons of vertebrates show considerable capacity for morphological growth and change in response to experience: these principles were proved true for humans as well.
One kind of neuronal plasticity involved in memory is long-term potentiation (LTP). LTP is a long-lasting increase in the strength of a synaptic response following stimulation of an input pathway. It occurs prominently in the hippocampus, a structure that is important for memory. LTP also involves morphological change in the structure of neurons. Lynch and Baudry, 1984). A large body of experiments points to the conclusion that memory is stored by changes in synaptic efficacy, and that these changes are brought about by common cellular events that may be found in many or all of the neurons that change. What makes memories specific must depend largely on which neuronal connections are altered, and on the architecture of the specific networks in which alterations occur.
Questions about how individual neurons change in response to experience are fundamentally important. Yet, there are many other questions that seek a more global, systems level of understanding. Where is memory stored? Is there one kind of memory or many ? What are the brain systems and connections involved in memory and what jobs do they do?
Before the recent technological developments in neuroscience, these complex issues were studied primarily by psychologists. In the recent years, a number of neurophysiological techniques, mostly brain imaging such as functional magnetic resonance (fMR) and positron emission tomography (PET) brought new knowledge on the relationships between memory and the brain. The main result is related to the lack of a specific memory “center” where an entire memory is stored. The most likely site of memory storage is the set of particular cortical processing systems that are ordinari1y engaged during the perception, processing, and analysis of the material being learned. Memory can be considered to be distributed in the sense that no single memory center exists and in the sense that many parts of the nervous system participate in the representation of a single event. Each part of the brain contributes differently to the whole representation.
Despite this holistic view of memory, specific brain structures and connections that participate in memory functions have been identified through study of the human amnesic syndrome and animal models of the syndrome. Amnesia is a severe impairment in the ability to acquire new knowledge or to recall recently acquired knowledge, which occurs when there is bilateral damage to the medial temporal lobe or the midline of the diencephalon. The point is that there are brain structures, which are not themselves repositories of permanent memory , that are important for memory functions. The use of magnetic resonance imaging has made possible to detect abnormalities in the hippocampal formation of some amnesic patients. Damage limited to the hippocampus itself is sufficient to cause amnesia. Further studies indicate that the full medial temporal memory system consists of the hippocampus and the adjacent, anatomica1ly related structures including entorhinal cortex, parahippocampal gyrus, and perirhinal cortex and that different subcortical and cortical structures and circuits are responsible for procedural and semantic memories (Mele et al., 2004)
2. Memory reconsolidation.
It is now appreciated that memory is not a unitary mental faculty but reflects the operation of many separate systems (De Leonibus et al. 2003). This raises the possibility that different cellular mechanisms may be involved. One broad classification scheme distinguishes declarative (explicit) memory from non-declarative (implicit) memory (Squire 1987). Declarative memory includes the facts, episodes, faces, and routes of everyday life and is accessible to conscious recollection. Declarative memory is affected in amnesia, and it depends on the integrity of the damaged structures and connections. Non-declarative memory includes skillful behavior, conditioned responding, and priming. Priming refers to facilitation in identifying perceptual objects, including words, that is produced by their recent presentation. Non-declarative memory affords the capacity to acquire knowledge unconsciously. In the case of skill learning, knowledge of the skill is embedded in procedures, and the skill is expressed in performance by engaging the procedures. In the case of conditioning, dispositions are gradually acquired to respond to stimuli. In the case of priming, perception and processing of stimuli are facilitated (Tulving et al. 1988). For example, if the word baby is presented, the probability is more than doubled that baby will subsequently be the word elicited by instructions to free associate to the word child. All of these examples of learning are intact in amnesic patients. Non-declarative memory is acquired, stored, and retrieved independently of the hippocampus and other structures damaged in amnesia; non-declarative memory is thought to be a philogenetically early form of memory. By contrast, the capacity for declarative memory may have appeared relatively late in evolution, reaching its greatest development in mammals with the full elaboration of medial temporal lobe structures, especially the hippocampal formation and related cortex. This capacity allows animals to record and access the particular encounters that led to behavioral change.
Apparently, if conscious declarative memory is to be acquired and stored in an enduring, retrievable form, the hippocampus and related structures must participate in neural processing at the time of learning. Coordinated activity in neural networks in the neocortex is presumed to underlie perception and representation in short-term memory. If perception is to be transformed into persistent memory , then convergent activity must occur in the connections from the neocortex to the hippocampal system. The input is processed through various stages within this system and then exits by way of widespread, divergent connections that return information to the neocortex. In this way, the hippocampal system supports the development of conscious memories in the neocortex.
This outline suggests that consolidation from the short to the long term form of memory results in stable coding, in a permanent ad reliable form of experience. However, while the stability of long term memories has been a dogma for a long time, an increasing body of knowledge suggests that long-term memory is not as stable and impermeable as it was hypothesized in the classic models of memory. In particular, it appears that recalling a memory renders it fluid and unstable - able to be changed before being re-fixed into the circuitry of the brain - and that change could include altering its meaning or even deleting it completely.
As we have previously seen, to be turned into something more permanent, the synapses that connect the arrangement of neurons that equal an experience grow, sprouting new and thicker connections to stabilize the memory trace. Proteins are produced by a range of genes to facilitate consolidation. Initially the building of this brain pattern occurs deep in the brain in places like the hippocampus but over the course of weeks and even years it moves to more general areas of the cortex. Until recently it was felt that this was the end of the story, the memory stays in the backwaters of the mind aging but essentially remaining the same. However recent data blew this idea away. To study the process of consolidation researchers interfere with the steps involved in fixing a memory in order to test their influence on long-term recall. Joseph LeDoux and Karim Nader (Nader and LeDoux, 2000) trained rats to associate a darkened box with an electric shock to their paws. The rats learn the box is to be avoided and freeze the next time they are put into it (fear conditioning). If, a few days after this conditioning, the animals were given a drug to prevent protein synthesis before being reminded of the box it made no difference to their ability to remember it as a bad place. The memory seemed fixed and safely stored in line with the classical view of memory consolidation. But if the rat had a brief reminder of the box just before the drug was administered the rat lost its fear conditioning. The memory had somehow been erased. LeDoux and Nader labeled this phenomenon as reconsolidation.
Traditional consolidation theory suggests that memories are fixed locally by protein changes within a few hours of the event and then filed to long-term storage in the cortex after a period of days or weeks. After conditioning rats in the same way they left them for 45 days, by which time the memory should have been fixed and immune to interference. As before, the rats given no reminder of the box before being injected showed their conditioned response to the box. But the rats that were given a reminder of the box before being given the drug did develop amnesia. The consolidated memory - which conventional theory said was permanent and stable - had been removed by the action of recalling it. In Nader’s words “The dogma was that once a memory trace had been consolidated, it is permanent. But here it is labile - subject to interference in exactly the same way as a brand new experience. We were showing memory to be something incredibly dynamic.”
It appears that memory moves from the hippocampus to the cortex during consolidation, but is returned to the hippocampus for reconsolidation by the act of recall. This dynamism would be unnecessary if the brain just wanted a photograph album, but it fits perfectly if memories exist to make sense of the present, and distorting some memories while generalizing about, or even deleting, others serves to improve the mind’s recognition and understanding of the world in order to prosper within it. If a memory becomes plastic every time it is recalled then it can be re-filed in a usefully updated way. The mind can make choices about whether to merge old and new, or to reinforce their differences.
In brief, then, reconsolidation theory suggests that when memories are recalled they become vulnerable to change. Therapists who investigate past experience of their clients should be aware that every time a memory is recalled it becomes unstable and capable of change. That change can be in one of several forms:
· The memory could be strengthened in its meaning – it becomes more of what it was.
· It can be weakened - it has less effect on the belief network it’s connected to.
· It can be transformed by having the meaning of it reframed – and by doing so transform the
belief that derives from it.
3. How accurate and reliable is memory?
Autobiographical memories are one of the main components of semantic memory: different episodes of our life (episodic memory) are tied together to make up a narrative of our life. How reliable is this his story? Is it dependable or doe it reflect o production not completely accurate and truthful? The accuracy of our memories is out of discussion if we consider their general meaning, the full story: on the contrary, the details may be rather uncertain and slowly change as times goes by. For example, many studies have evaluated the memory of life events recorded on diaries by students enrolled in these experiments. After a few months the experimenter read these records and asked their authors if they did remember a number of details. In some instances the experimenter deliberately altered the record (written through a word processor and therefore changeable) even in a significant way: the longer the time elapsed, the greater was the possibility that the students recognized as their “own” the false events recorded in “their” accounts.
The difficulty to catch the difference between “true” and “false” memories mostly depends on oblivion affecting autobiographical memory. Marigold Linton has clearly described this phenomenon. Starting form 1972, the psychologist noted each day on a different card three daily events. Day after day she devoted the same space, three lines, in order to give the same space to different life events, e.g. memories. Once in a month, she randomly extracted a couple of cards, she read the notes, tried to date it and evoke these events. When the events were noted and recalled to mind Linton tried to rate them in terms of their relevance, emotional characteristics, meanings etc. She was at the same time the subject and the experimental object of her research. At the end of her research, Linton assessed that memories decay at an yearly rate of about 5-6%: at this pace about 50% of our autobiographical memories should fade or disappear if they were not inserted in the general framework of our autobiographical memory concerning general facts or life periods. As a matter of fact, while the single bricks (episodes) assembling these wider containers may deteriorate, the perception of the stream of memories and of their general meaning survives. As time goes by and memories fade, the cues necessary in order to evoke a memory progressively decreases: in order to evoke an event the stimulus must almost completely match an ancient memory in order to restore it, as to say to reach a good reminiscence. This process is somewhat similar to what happens when we visit a place once familiar: initially there is a feeling of uneasiness, of not belonging but suddenly a cue that seems irrelevant may trigger a full memory. What is the reason for our decrease capacity to recollect past memories as we age? This phenomenon may be explained with the metaphor of the lock and key: At the beginning any key opens the lock of memory, as time elapses the key becomes more and more specific until it must almost match the lock.
Another characteristics of autobiographical memories concerns our ability to date them with some accuracy. In general the more efficient is our memory, the more we feel that events occur most frequently. As a consequence, recent events are –obviously- dated with more precision and they seem to occur more often while events remote in time are roughly dated and considered rarer than what actually is. Most mistakes in dating are related to the fact that our “interior time” and physical time do not coincide: if we are active and involved in some activities closer events seem distant, the contrary happens if we are inactive and not involved in something, as it happens in old people to whom facts happened a few days before seem more remote in time. One of the mechanisms responsible for dating memories in time is the association between individual and collective –social- memories. Thus, we may recollect something happened “in the moment when…”, “the day in which..”, “the year when…” something memorable happened. When these reference points are absent, our memories are inaccurate, we feel that we cannot trust them to the point that we may have doubts about the consistency of some events of our life.
Elisabeth Loftus has persuasively contributed to show that memory consolidation does not warrant the stability and dependability of memories. By studying autobiographical memory she has shown that recollections depend on a complex activity of reorganization of “fragments” and remains. In addition to that, she has also shown that depending on the age and circumstances memories may also be “implanted” and integrated in our memory framework as if they were our own memories. This fact casts some doubts on the meaning of witnesses, in particular in those instances in which some pressure has been exerted on eyewitnesses immediately after a fact: for example, if an adult insists that “that man was tall and with dark hair” while he actually was of average height and fair-haired, a child will modify his witness since he really gets to believe to the “implanted” version of the fact.
In most general terms, memory is open to different kind of conscious or unconscious manipulations and even autobiographical memories are subject to pitfalls and inconsistencies. Since the media continuously expose us to a rich imaginary, to fantasies exerting a strong hold on our mind because our brain is mostly visual and we believe in what we see, individual memory risks to become more and more blurred, less reliable than it was in the past.