Memory is the ability to retrieve learned or acquired information. This information can be of previous events, a learned skill, or factual knowledge. Memory is usually distinguished as shortterm memory, which is the recollection of recent events, and long-term memory, which is recalling the more distant past. Memory as a biological phenomenon is a record of time.
Ideas and theories about memory have changed in recent decades. Theories regarding memory, much like the theories regarding consciousness, have been profoundly influenced by research in the neurosciences and understanding the functioning of the brain (physiologically and biochemically).
The clinical assessment of memory of the human brain is specified by three categories, which can give insight into the functioning of a person’s cognition. First is immediate memory, which functions over a period of seconds. Second is recent memory, which applies over a scale of minutes to days. Third is remote memory, which typically encompasses a period of months to years. These classifications differ only slightly compared with stipulating them as short-term and long-term memory.
Memory can be further classified according to how it is utilized. Working memory is not only classified by the duration of memory retention, but also by the manner in which it is used in daily activities. For example, performing a series of simple calculations would utilize working memory. The actual process of retaining this information (in this case, numbers to be used in a calculation) for short-period use is the working memory, because it is being used at that time. However, working memory is not to be confused with short-term memory, which is memory stored for a short period of time that is not being used functionally.
In addition to classifying memory by the length of time a particular brain is able to retrieve information, it can also be described in terms of implicit (also called procedural) and explicit (also called declarative) memory. Implicit memory is defined as memory that is retrieved automatically, or without conscious involvement. For example, memory for learned skills is claimed to be largely implicit, in that it is automatic. This is in contrast to explicit memory, which requires conscious awareness and intentional recollection to recall. An example of this would be recalling events that took place several years ago, which would require an intentional recollection of that neurological data.
Ivan Pavlov: Early Experiments in Conditioning
Ivan Pavlov (1849-1936) was a psychologist, physiologist, and physician who is well known for his work done in what is known as classical conditioning. Also well known is “Pavlov’s dog,” a phrase that arose from experiments he performed on dogs. These experiments consisted of producing a stimulus (such as ringing a bell, blowing a whistle, or striking a tuning fork) prior to feeding the dog. This was done repeatedly, and eventually that same stimulus would cause the dogs to salivate even in the absence of food. This process was called conditioning.
Early experiments in conditioning were important to the understanding of memory, because the conditioned response relied on the fact that a memory of that stimulus was associated with the presentation of food. Thus, memory is a component of learned behavior.
In establishing an understanding of human memory, four basic elements of memory have been explained: encoding, storage, retrieval, and forgetting. The first element is encoding, the registration of neurological data. This is an active procedure of processing and combining information. For example, while watching a television program, one would see (visual stimulus) and hear (auditory stimulus) information that would be processed by the brain as an event, (e.g., watching the weather report).
The second element is the storage of memory in the brain, which creates a neurological record. Currently, this is understood to take place in three stages: sensory store, short-term store, and longterm store. The sensory store is the perception of the image (e.g., the meteorologist reading the weather forecast), which is thought to last only a split second or just long enough to be perceived by the brain. Short-term store is the storage of this information for only a short period of time, typically only minutes to hours; for example, if someone just entered the room, having missed the weather forecast, and then asked you what it was, your recollection would then be based on shortterm store. Long-term store is the storage of that same information hours, days, or years later. The retention of this information for longer periods of time requires rehearsal. In fact, memorization is a method of rehearsal that allows an individual to recall information verbatim.
The third element of memory is the retrieval of memory. This is the recollection of stored information, which is not a random process. In fact, it is an intentional process that is typically in response to a cue, in reaction to a stimulus, or to perform a particular activity. However, it is also thought that memories are reconstructions of the actual event, and these reconstructions can contain errors or inconsistencies in perception when recalled; for example, one might make errors in reporting what the weather forecast was.
The fourth element of memory is forgetting. This is the loss of or the inability to retrieve stored information. There are several theories on forgetting, such as pseudo-forgetting (which is held to occur due to ineffective attention in the acquisition phase), and retrieval failure (which is claimed to be an inability to retrieve information at a particular time and, consequently, the inability to be able to recall it at a later time). It is also held that memory loss happens naturally due to decay over time or because of lack of use. Another theory, known as motivated forgetting, is an individual’s intentional attempt to forget events that are unpleasant or traumatic. This phenomenon was studied extensively by Sigmund Freud (1856-1939); he called these repressed memories. He maintained that repressed memories were not lost or forgotten; rather, they were stored in the unconsciousness and are responsible for certain psychiatric conditions that he called neuroses.
For almost a century, scientists were baffled about how the neurons in the human mammalian brain were able to store memories. In 1973, the first neurological research was published by Timothy Bliss on what he called long-term potentiation (LTP), today also known as long-term enhancement. He characterized the phenomena of LTP, which was originally observed by Per Andersen in Oslo, Norway.
While conducting experiments on the hippocampus of rabbits, Timothy Bliss and his colleges discovered that a few seconds of high-frequency electrical stimulation on particular neurons would enhance synaptic transmission in the hippocampus for days and, in some studies, for weeks. This enhanced and prolonged stimulation in the hippocampus was held to be responsible for the formation of short- and long-term memory. Today, researchers of memory concur that the most current evidence supports the role of LTPs in both memory and learning.
Prior to the idea of LTP, the Hebbian theory was the accepted idea of how memory and learning occur. The Hebbian theory was named after neuropsychologist Donald O. Hebb (1904-1985), who stated that the strengthening of the neuronal synapses to one another was primarily responsible for memory and learning. This in part could still be true, and we are learning more about neuroplasticity, which is a process in which the brain changes, or, in this case, strengthens its neuronal connections.
The Neuroanatomy of Memory
Theories about what memories actually are and how memories are actually stored have changed over time. This is mostly due to decades of research in the neurosciences. It is now known that several areas of the brain are required for obtaining, storing, and retrieving memory.
In the human brain, memory is stored and retrieved from what is known as the neural network of the brain. Information from sensory organs travels through specific parts of the brain, is processed, stored, and then able to be recalled at later periods of time. The anatomic regions currently known to be critical to the formation and recollection of memory are the medial temporal lobe, certain diencephalic nuclei, and the basal forebrain.
The medial temporal lobe contains the hippocampus and the amygDalía. The hippocampal region is where electrochemical activity converts short-term memory into long-term memory by via LTP. LTP is thought to be a persistent electrochemical increase in synaptic strength following high-frequency stimulation of a chemical synapse.
The amygDalía is claimed to rate the emotional importance of a particular experience. For example, a very intense experience, such as pain or pleasure, would create a very strong memory. Conversely, a mild or indifferent stimulus, such as tying a shoelace, may be disregarded altogether and not stored as a lasting memory.
Certain diencephalic nuclei in the dorsal medial nucleus of the thalamus and the mamillary bodies are also involved in memory. This is known because, if these areas are damaged—for example, in thiamine-deficient states or alcohol impairment—then the brain has the inability to recall events. Neurological inactivity in these areas is also noted in Korsakoff’s syndrome, a medical condition in which severe impairment is noted in recalling remote memory.
The basal forebrain consists of the basal ganglia and areas called brain-stem nuclei. These structures lie deep inside the brain and consist of the caudate nuclei, lentiform nuclei, portions of the amygDalía, and claustrum. Collectively, these areas are involved in voluntary movement and nonmotor learning. It is known that damage to these areas can result in the decline and loss of memory, as well as loss of executive functioning (planning and the ability to pay attention) and loss of ability in set-shifting (the ability to alternate between two or more tasks). This is seen in Parkinson’s disease and Huntington’s disease.
Neuroplasticity: The Brain Can Change Neuroplasticity (also known as cortical plasticity) is the ability of the brain to form new neuronal connections and to reorganize itself. This can happen in response to certain types of injuries or diseases and in response to new situations and changes in the environment. The concept of neuroplasticity has challenged the previous dogmas that the brain is immutable and that, after a certain age of development, it does not change. Neuroplasticity does allow changes in the brain, and it allows the brain to be incredibly adaptive.
How does neuroplasticity work? In the neuronal network of the brain, each neuron forms several connections with other neurons. Connections that are used infrequently eventually fade away, a process called synaptic pruning. Conversely, connections that are used regularly and frequently are strengthened (as proposed by Hebb before there was knowledge of neuroplasticity). In addition to this, neurons can also form new connections to other neurons. It is thought that these new connections are involved in forming longterm memory in response to new information.
It is maintained that the earlier hominid brain was similar to but not as complex as the more modern and evolved Homo sapiens sapiens brain. The anatomical and chemical changes in complexity had to have changed over time in order to improve the process of human memory. These changes were in all likelihood induced by natural selection. As our earlier ancestors began to evolve into a hunting and gathering species, an increase in neurological demand was made because of the increased need for communication and the ability to learn and remember more information. In short, an increase in the ability to remember equals an increased chance of survival, and this was provided by neuroplasticity. Neuroplasticity is not exclusive to Homo sapiens sapiens. The ability to learn and adapt to new information is apparent in most animals and organisms possessing a nervous system. However, it is not clear how human neuroplasticity differs from the neuroplasticity in other animals.
Like the human brain, computers are able to store and retrieve memory called data. Early computers used a two-level storage system that consisted of a main memory (RAM), which consisted of magnetic cores, and a secondary (hard disk) memory that was composed of magnetic drums. The problem with this two-level system was that the main memory was very limited, and most programs had to use the much slower hard disk (secondary memory).
In 1959, a one-level storage system known as “virtual memory” was conceived. This new system utilized a special automatic set of hardware and software that kept the majority of the current programs and memory in the faster main memory and, in conjunction with secondary memory, created the illusion of unlimited available memory.
Currently, computers are able to store and retrieve massive amounts of data in seconds, but they need to be programmed to do so. The computer’s physical memory is stored using a binary code and can be stored on computer chips, disks, or electromagnetic tapes. This is slightly different from memory storage in the human brain, which is done primarily in the hippocampus using long-term potentiation. The human brain is capable of storing a large amount of memory, but modern computers can store practically unlimited amounts of data with greater accuracy than human memory.
Much research has been done on primates, in particular chimpanzees, because of their similarities to humans. The dogma has always been that human executive and cognitive functions are superior to those in the apes. Recently, it was shown in a study done at Kyoto University that young chimpanzees could grasp many numerals at a glance and recall the sequence of these numerals. In most cases, they actually performed at a higher level than mature chimps and humans. This shows that other primates, besides humans, have extraordinary working capabilities for numerical recollection. As with our early human ancestors, this is likely a result of natural selection. Primates with better memory would have an adaptive advantage, and this would increase their chances of survival and reproduction.
Several medical conditions that impair memory or cause memory loss have been mentioned already, such as Korsakoff’s syndrome, Parkinson’s disease, and Huntington’s disease. There are several other such conditions including encephalopathy, vascular dementia, stroke, vitamin deficiencies, hypothyroidism, and psychiatric conditions. However, Alzheimer’s disease is the most well-known condition that causes memory impairment and loss in humans; this disease is a progressive neurological disorder in which the loss of short-term memory is present in early stages. During the later stages, progressive memory loss will continue, and eventually long-term memory loss takes place.
Much has been learned about the pathology process of Alzheimer’s disease. Neurologically, the brain develops extracellular deposits of amyloidbeta protein, intracellular neurofibrillary tangles, and eventually loss of neuron mass. In addition, certain genes have been identified in familial forms of Alzheimer’s disease. This suggests that, in the future, perhaps gene therapy may be able to prevent or treat these forms of Alzheimer’s disease.
Current treatments can potentially halt the progression of Alzheimer’s disease. Medications known as cholinesterase inhibitors have been somewhat effective in treating Alzheimer’s patients.
There are also other classes of medications that can help halt the progression of this disease. However, all these medications are very expensive and only slow down the eventual progression of the disease.
Improving or enhancing memory is an interesting topic, because the ability to recall more information accurately and faster would provide an individual with a great advantage. Not having to look up information in a book or journal years after that information has been forgotten would be a tremendous advantage in the work place, pursuing research, completing academic projects, or learning other languages.
No current methods or medications have proven to be 100% effective in improving human memory. Certain didactic methodologies aim at improving the retention of memories (such as facts, words, and diagrams) that can help with learning and scholastics. Herbal medicines like Gingko biloba have been shown to improve the circulation in the brain. Proposals have been made that this medicine could, in theory, improve memory, but no conclusive evidence exists as of now. A more complete understanding of the genetics that may be involved in memory could in the future propose the possibility of the genetic enhancement of memory.
It has been well documented that exercise that increases circulation improves memory but does not enhance memory. This is because improved circulation increases oxygenation to the brain. It is also true that a healthy diet gives rise to a healthier brain and thus improved memory. Dietary vitamins, especially B-vitamins and omega-3 fatty acids, are known to maintain healthy memory. Again, maintaining healthy memory does not mean enhancing memory beyond its human capacity.
Our understanding of memory has changed over time, mostly due to neurological discoveries, in particular LTP and neuroplasticity. Likewise, over time, our ability to utilize memory has improved our species’ ability to survive. However, human memory is not perfect; neurological information can become distorted, lost, or in some cases repressed. Disease can also degrade the memory process. Continuing biological evolution may gradually result in improvements in human memory. In the shorter term, perhaps our own efforts to understand the neurobiology of memory more completely will lead to future improvements in human memory through new medications or gene therapy.
John K. Grandy
See also Cognition; Consciousness; Information;
Intuition; Perception; Sleep; Time, Phenomenology of
Basar, E. (2007). Memory and brain dynamics: Oscillations integrating attention, perception, learning, and memory. New York: CRC Press.
Eichenbaum, H. (2002). The cognitive neurosciences of memory: An introduction. New York: Oxford University Press.
Grandy, J. (2005). Consciousness. In H. J. Birx (Ed.), Encyclopedia of anthropology (Vol. 2, pp. 563-566.). Thousand Oaks, CA: Sage.
Kandel, E. R. (2007). In search of memory: The emergence of a new science of mind. New York: Norton.
Pinker, S. (1999). How the mind works. New York: Norton.
Shaw, C. (2001). Toward a theory of neuroplasticity. Philadelphia, PA: Psychology Press.