11th September 2021
Why do the memories for some events remain vivid long afterwards
Why do the memories for some events remain vivid long afterwards
Some events in our lives are easy to recall, the memories are strong and include the emotions we felt at the time. If we are involved in a wedding or a traffic accident, we are able to recall details many years later. Some events are so vivid that years later we can even recall exactly what we were doing at the time we heard about the event. On the other hand, we may recall something of what happened on our daily walk around our neighbourhood for a day or so, but a few days later most of us will no longer remember.
At a very detailed level, memories are supported by the activity of neurons. What activity differences are responsible for the differences in our ability to recall?
A cortical pyramidal neuron is directly activated by a range of complex combinations of sensory inputs. This range is called the receptive field of the neuron. Recall of a memory involves indirect activation of a population of cortical pyramidal neurons that were activated during the earlier experience but are not being directly activated by current sensory inputs.
Because neurons do not correspond with unambiguous cognitive circumstances, such indirect activations depend on correlations in past neuron activity. Memories for facts depend on indirect activation of neurons on the basis of frequent past simultaneous activity of different groups of neurons. However, for individual past events there has been no frequent past simultaneous neuron activity. Indirect activation for such events is on the basis of past simultaneous activity at a time when a lot of neurons were changing their receptive fields.
Changes to pyramidal neuron receptive fields occur when there is a degree of novelty in the circumstances experienced by the brain. These changes are slight expansions in the fields of some inactive neurons so that they are activated. These expansions ensure that at least a minimum number of neurons is activated in every experience. The selection of which neurons will expand their receptive fields is carefully managed by the hippocampal system to ensure that changes are as small as possible. Part of this management involves records of groups of neurons that tend to expand their receptive fields at the same time, and the hippocampal system also keeps somewhat similar records of groups of neurons active during past periods of receptive field expansion.
At a very detailed level, memories are supported by the activity of neurons. What activity differences are responsible for the differences in our ability to recall?
A cortical pyramidal neuron is directly activated by a range of complex combinations of sensory inputs. This range is called the receptive field of the neuron. Recall of a memory involves indirect activation of a population of cortical pyramidal neurons that were activated during the earlier experience but are not being directly activated by current sensory inputs.
Because neurons do not correspond with unambiguous cognitive circumstances, such indirect activations depend on correlations in past neuron activity. Memories for facts depend on indirect activation of neurons on the basis of frequent past simultaneous activity of different groups of neurons. However, for individual past events there has been no frequent past simultaneous neuron activity. Indirect activation for such events is on the basis of past simultaneous activity at a time when a lot of neurons were changing their receptive fields.
Changes to pyramidal neuron receptive fields occur when there is a degree of novelty in the circumstances experienced by the brain. These changes are slight expansions in the fields of some inactive neurons so that they are activated. These expansions ensure that at least a minimum number of neurons is activated in every experience. The selection of which neurons will expand their receptive fields is carefully managed by the hippocampal system to ensure that changes are as small as possible. Part of this management involves records of groups of neurons that tend to expand their receptive fields at the same time, and the hippocampal system also keeps somewhat similar records of groups of neurons active during past periods of receptive field expansion.
Probable connectivity between a column in one of the cortical areas forming part of the hippocampal system and a column in a regular cortical area. There are three populations of pyramidal neurons in the hippocampal system column. Population P1 detects the internal activity in a group of regular cortical columns. Population P2 drives receptive field expansions in that group. Population P3 drives indirect activation of neurons in the group on the basis of past simultaneous activity in a period of receptive field expansion.
There is some degree of novelty in almost every experience. The neuron population activated during an experience is driven by sensory inputs plus the internal state of the brain. The internal state of the brain includes information derived from recent experiences and any recalls of information that have occurred recently. For example, even if we have exactly the same walk on two days, the news we have heard, our mood and what we have been thinking about just before we set off will be different. The strangers we encounter in passing may be different. Hence even if two experiences are similar, there will generally be some small degree of novelty.
For recall of an unique event, a small subset of the group of pyramidal neurons active during the event must drive the indirect activation of a much larger part of the group. The small subset may be activated more or less by accident in the course of ongoing experience, activated as a result of hearing trigger words referring to the event, by visiting the location of the event, seeing someone or something present during the event, or by some other related experience.
If there is a high degree of novelty, the brain records much more substantial information about the group of neurons active at the time. As a result of these more substantial records, it is easier for a small subset to reactivate the group at a later time.
If any small subset of the neuron population active during an event would always drive a recall, we would be overwhelmed by constant recalls. A small subset only has recommendation strength in favour of an indirect activation of the group, and this strength must compete with recommendation strengths in favour of other recalls and other types of behaviour. The attempt to recall will only be initiated if the total recommendation strength in favour is larger than the strength in favour of other behaviours. Recommendation strengths often decline over time, so whether a recall is initiated is also dependent on how long ago the event occurred.
If a subset of neurons is followed by a recall of the memory of an event, and the behaviour of recalling it is rewarded, then the recommendation strengths of the subset that drove the recall are increased and future recalls are more likely. For example, if recall of a memory led to describing the memory to someone else, and that person was interested, their interest is interpreted by the brain as a reward, which increases the recommendation strengths of the initial subset of neurons in favour of the recall.
Finally, emotions correspond with strong recommendation strengths in favour of different general types of behaviour. In situations where there are such strong emotions, an appropriate behaviour is urgently needed and also there is a higher chance that information present in the current environment will be relevant for the selection of future behaviours. Hence during events in which there is strong emotion, the brain “turns up the volume” of receptive field expansions. The amygdala and hypothalamus manage emotions, and target the hippocampal system for this purpose.
For recall of an unique event, a small subset of the group of pyramidal neurons active during the event must drive the indirect activation of a much larger part of the group. The small subset may be activated more or less by accident in the course of ongoing experience, activated as a result of hearing trigger words referring to the event, by visiting the location of the event, seeing someone or something present during the event, or by some other related experience.
If there is a high degree of novelty, the brain records much more substantial information about the group of neurons active at the time. As a result of these more substantial records, it is easier for a small subset to reactivate the group at a later time.
If any small subset of the neuron population active during an event would always drive a recall, we would be overwhelmed by constant recalls. A small subset only has recommendation strength in favour of an indirect activation of the group, and this strength must compete with recommendation strengths in favour of other recalls and other types of behaviour. The attempt to recall will only be initiated if the total recommendation strength in favour is larger than the strength in favour of other behaviours. Recommendation strengths often decline over time, so whether a recall is initiated is also dependent on how long ago the event occurred.
If a subset of neurons is followed by a recall of the memory of an event, and the behaviour of recalling it is rewarded, then the recommendation strengths of the subset that drove the recall are increased and future recalls are more likely. For example, if recall of a memory led to describing the memory to someone else, and that person was interested, their interest is interpreted by the brain as a reward, which increases the recommendation strengths of the initial subset of neurons in favour of the recall.
Finally, emotions correspond with strong recommendation strengths in favour of different general types of behaviour. In situations where there are such strong emotions, an appropriate behaviour is urgently needed and also there is a higher chance that information present in the current environment will be relevant for the selection of future behaviours. Hence during events in which there is strong emotion, the brain “turns up the volume” of receptive field expansions. The amygdala and hypothalamus manage emotions, and target the hippocampal system for this purpose.
The anatomical circuitry of the hippocampal system. This system determines when and where in the cortex there will be changes to cortical pyramidal neuron receptive fields. Such changes occur in response to novelty in current experience. The greater the novelty, the easier it is for the event to be recalled later. The degree of change is ilso increased during highly emotional experiences. This increase is driven by the amygdala and hypothalamus targetting different points in the hippocampal system circuitry
Hence the probability of recall of one specific event is determined by a combination of the size of the subset of neurons that happens to be activated, the degree of novelty in the original experience, the degree of emotion that was present during the original event, the length of time since the original experience, how often it has been recalled, and whether previous recalls have been followed by reward feedback.
Some events have a high degree of novelty and involved strong emotion. Such events are often described to other people who are very interested. As a result, recalls of such events are frequent and vivid. Because recall is on the basis of activity at a time when there was significant receptive field expansion, it is not just the event that is recalled, but whatever we were doing at the time of the event.
Some events have a high degree of novelty and involved strong emotion. Such events are often described to other people who are very interested. As a result, recalls of such events are frequent and vivid. Because recall is on the basis of activity at a time when there was significant receptive field expansion, it is not just the event that is recalled, but whatever we were doing at the time of the event.