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| Classifying Memories by Retention Time |
Classifying Memories by Retention Time
Your memory lets you relive events in your mind that actually occurred at some time in the past. That is the whole purpose of having memories. So by its very nature, memory includes a time factor.
The division of memory into ‘immediate-term memory’, ‘short-term memory’, ‘medium-term memory’, and ‘long-term memory’ is again somewhat arbitrary.
However, everyday experiences and experiments in applied psychology suggest that at least four time scales are needed to explain the many features of human memory.
The different time scales probably correspond to specific neurological processes, but so far no one knows exactly how memories are stored.
Immediate-term memory
Immediate-term memory helps you to keep track of everything that is going on around you at this moment. Although sensory input is filtered so that only significant information reaches your conscious mind, even most of this filtered sensory input is soon forgotten. We need to know that a car is approaching, or that there is a big puddle off to the right, and we need to keep track of that information for a few moments. However, after a few seconds our focus of attention shifts to other items of concern, and the car and puddle are forgotten. The contents of immediate-term memory correspond to our stream of consciousness.
Immediate-term memory can store items for up to 30 seconds. We can laugh at the punch line of a joke because we can still remember the beginning of the joke, but are often unable to recall the whole joke a few moments later – it has already faded from immediate-term memory. Thirty seconds is still long enough to monitor our surroundings and decide if anything important is happening. It is long enough to think of something to say, compose a sentence, and speak it without forgetting what we are doing.
The 30-second limit for immediate-term memories prevents your mind from being swamped with trivial information.
Short-term memory
Short-term memory receives selective information from immediate-term memory.
Short-term memory is devoted to storing significant events and experiences that have occurred within the last few minutes. Items in short-term memory can normally be utilized for up to a day. Unless the events in this time frame are important, unique, or unusual, your memories of them start to fade in a few hours.
You can deliberately add items to short-term memory by repeating and emphasizng information. For example, there is a simple memory game in which about four dozen different articles are placed on a tabletop and covered over before participants enter the room. The cover is then removed and participants have two minutes to memorize the names of as many items as possible. Then the articles are re-covered, and the challenge is to see who can write out the longest list of items that were on the tabletop. One strategy is to repeat the names of single items several times, and then recall as many of those names as possible from short-term memory. Another strategy is to mentally group items with similar features, then memorize the group names and hope that the group names will help you remember individual items.
Why do we need short-term memory? Why don’t we just remember everything we encounter? Everyday events present a steady stream of information to our brains. Any effort to keep track of every single detail would soon overwhelm our mental capacity.
Instead, we have a fairly efficient filtering system so that events of little account are soon forgotten.
From a neurological perspective, the mechanism that creates memories is associated with the hippocampus in the midbrain complex. When this part of the brain is damaged, individuals are no longer able to create new short-term memories. The creation of longer-term memories seems to involve the formation of little bumps along the dendrites of neurons. These ‘dendritic spines’ facilitate the formation of new synapses with nearby axons. As explained by Dr. Morgan Sheng, “new synapses can be constructed and existing synapses can be eliminated in response to experience” and “dendritic spines … change their number and shape in response to … experience and in response to electrical signalling in the brain.”
Medium-term memory
Material that is deemed important, unique, or unusual in short-term memory is automatically passed on to medium-term memory and is stored for up to a month.
You have to resort to more emphasis and repetition to deliberately store information in medium-term memory. A student studying material over several days is able to store it in medium-term memory and can then make use of that material during tests and exams over the next few weeks.
Neurologically, the synapses at dendritic spines are reinforced as we repeatedly reuse the same neural pathways, and short-term memories tend to become medium-term memories.
Unless information stored in medium-term memory is used again within a month, that information begins to fade. Studies have shown that during holidays of two or more months, students typically forget about forty percent of the specific subject knowledge they had mastered before the holidays began. Fortunately, this does not mean that all the studying effort before a holiday is wasted. Information that has faded from your medium-term memory can be refreshed with only a fraction of the original learning effort. After a few days of review, students are ready to continue their studies where they left off.
Long-term memory
Long-term memory contains information that you remember for years. Information related to your professional expertise and your continuing interests in life tend to be stored in your long-term memory. One of the best techniques for deliberately storing information into long-term memory is to use that information in a variety of situations over several months. However, even information in long-term memory can become more difficult to recall if it is neglected for a number of years.
Your memory lets you relive events in your mind that actually occurred at some time in the past. That is the whole purpose of having memories. So by its very nature, memory includes a time factor.
The division of memory into ‘immediate-term memory’, ‘short-term memory’, ‘medium-term memory’, and ‘long-term memory’ is again somewhat arbitrary.
However, everyday experiences and experiments in applied psychology suggest that at least four time scales are needed to explain the many features of human memory.
The different time scales probably correspond to specific neurological processes, but so far no one knows exactly how memories are stored.
Immediate-term memory
Immediate-term memory helps you to keep track of everything that is going on around you at this moment. Although sensory input is filtered so that only significant information reaches your conscious mind, even most of this filtered sensory input is soon forgotten. We need to know that a car is approaching, or that there is a big puddle off to the right, and we need to keep track of that information for a few moments. However, after a few seconds our focus of attention shifts to other items of concern, and the car and puddle are forgotten. The contents of immediate-term memory correspond to our stream of consciousness.
Immediate-term memory can store items for up to 30 seconds. We can laugh at the punch line of a joke because we can still remember the beginning of the joke, but are often unable to recall the whole joke a few moments later – it has already faded from immediate-term memory. Thirty seconds is still long enough to monitor our surroundings and decide if anything important is happening. It is long enough to think of something to say, compose a sentence, and speak it without forgetting what we are doing.
The 30-second limit for immediate-term memories prevents your mind from being swamped with trivial information.
Short-term memory
Short-term memory receives selective information from immediate-term memory.
Short-term memory is devoted to storing significant events and experiences that have occurred within the last few minutes. Items in short-term memory can normally be utilized for up to a day. Unless the events in this time frame are important, unique, or unusual, your memories of them start to fade in a few hours.
You can deliberately add items to short-term memory by repeating and emphasizng information. For example, there is a simple memory game in which about four dozen different articles are placed on a tabletop and covered over before participants enter the room. The cover is then removed and participants have two minutes to memorize the names of as many items as possible. Then the articles are re-covered, and the challenge is to see who can write out the longest list of items that were on the tabletop. One strategy is to repeat the names of single items several times, and then recall as many of those names as possible from short-term memory. Another strategy is to mentally group items with similar features, then memorize the group names and hope that the group names will help you remember individual items.
Why do we need short-term memory? Why don’t we just remember everything we encounter? Everyday events present a steady stream of information to our brains. Any effort to keep track of every single detail would soon overwhelm our mental capacity.
Instead, we have a fairly efficient filtering system so that events of little account are soon forgotten.
From a neurological perspective, the mechanism that creates memories is associated with the hippocampus in the midbrain complex. When this part of the brain is damaged, individuals are no longer able to create new short-term memories. The creation of longer-term memories seems to involve the formation of little bumps along the dendrites of neurons. These ‘dendritic spines’ facilitate the formation of new synapses with nearby axons. As explained by Dr. Morgan Sheng, “new synapses can be constructed and existing synapses can be eliminated in response to experience” and “dendritic spines … change their number and shape in response to … experience and in response to electrical signalling in the brain.”
Medium-term memory
Material that is deemed important, unique, or unusual in short-term memory is automatically passed on to medium-term memory and is stored for up to a month.
You have to resort to more emphasis and repetition to deliberately store information in medium-term memory. A student studying material over several days is able to store it in medium-term memory and can then make use of that material during tests and exams over the next few weeks.
Neurologically, the synapses at dendritic spines are reinforced as we repeatedly reuse the same neural pathways, and short-term memories tend to become medium-term memories.
Unless information stored in medium-term memory is used again within a month, that information begins to fade. Studies have shown that during holidays of two or more months, students typically forget about forty percent of the specific subject knowledge they had mastered before the holidays began. Fortunately, this does not mean that all the studying effort before a holiday is wasted. Information that has faded from your medium-term memory can be refreshed with only a fraction of the original learning effort. After a few days of review, students are ready to continue their studies where they left off.
Long-term memory
Long-term memory contains information that you remember for years. Information related to your professional expertise and your continuing interests in life tend to be stored in your long-term memory. One of the best techniques for deliberately storing information into long-term memory is to use that information in a variety of situations over several months. However, even information in long-term memory can become more difficult to recall if it is neglected for a number of years.
