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understanding the brain |
Neuron mechanisms for accessing information about past experiences: semantic, episodic, priming, working and procedural memory
12th June 2021
Neuron mechanisms for accessing information about past experiences: semantic, episodic, priming, working and procedural memory
Neuron mechanisms for accessing information about past experiences: semantic, episodic, priming, working and procedural memory
Memory is the ability to access information derived from past experiences that is not available in current sensory inputs. Information derived from experience is captured by the receptive fields recorded by pyramidal neurons in the cortex, and by the behavioural recommendation strengths associated with receptive field detections and recorded by medium spiny neurons in the basal ganglia. Neurons don't "know" about cognitive categories or past events. The only type of information available to neurons is that specific groups of other neurons were active in the past when the neuron itself was active. How can this temporal correlation information be used to access cognitive memories.
Suppose that two groups of neurons were active at the same time in the past in response to some sensory experience. If as a result of a later sensory experience one of the groups is active, there could be behavioural value in the active group indirectly activating the other group. Even if the first group had been itself indirectly activated, there could still be value in indirect activation of the second group.
What type of temporal correlations in activity could be valuable? If two groups were active at the same time just once, there could be value in an indirect activation a short while later, but after a longer time the number of such correlations becomes huge and an indirect activation is less likely to be useful. However, if two groups have often been active in the past at the same time, indirect activation of one by the other is much more likely to be valuable. Even if the two groups were only active at the same time once during a very novel past experience, the value of a later indirect activation could be substantial. During novel experiences, many cortical neurons expand their receptive fields, and such expansions are the way to identify the groups of neurons. Finally, there could be value in prolonging the activity of a currently active group of neurons.
Hence there are four different types of indirect activation that could have behavioural value: indirect activation on the basis of recent past simultaneous activity; indirect activation on the basis of frequent past simultaneous activity; indirect activation on the basis of past simultaneous receptive field expansion; and maintenance of activity on the basis of current simultaneous activity.
A rather different way to access information recorded in the past is that if a group of neurons has acquired significant total recommendation strength in favour of some behaviour through past experience, activation of the group by sensory inputs or by indirect activation will result in that total recommendation strength being present.
Memory is often classified into five types: semantic, episodic, priming, working and procedural. Semantic memory is the ability to recall many facts about the world. One of the most common examples is that we can recall the meanings of words. Hearing the word “tree” conjures up a mental image of a typical tree, so that we could describe the visual features of a tree. In general we do not remember when or where we learned such word meanings. Episodic memory on the other hand is the ability to recall specific individual experiences. Novel events such as a wedding or a traffic accident can be recalled for many years, with recalls including emotions felt at the time. It is even possible to recall everyday events like what we had for lunch today, but such recalls are usually only available for a day or so. Priming memory is the ability to act upon information we were not aware of perceiving at the time. This type of recall only lasts for a few minutes. If there is an animal hidden in grass, we may glimpse something without being aware, but sometimes we can put together several glimpses close together and realise that an animal is there. Working memory refers to our ability to hold information about some object active for a few seconds after the object is removed. Procedural memory refers to our ability to carry out previously learned skills like riding a bicycle, automatically performing the required muscle movements with very little conscious control.
These five types of memory correspond with the four types of indirect activation plus the activation of recommendations strengths identified earlier.
SEMANTIC MEMORY
Suppose that two groups of neurons were active at the same time in the past in response to some sensory experience. If as a result of a later sensory experience one of the groups is active, there could be behavioural value in the active group indirectly activating the other group. Even if the first group had been itself indirectly activated, there could still be value in indirect activation of the second group.
What type of temporal correlations in activity could be valuable? If two groups were active at the same time just once, there could be value in an indirect activation a short while later, but after a longer time the number of such correlations becomes huge and an indirect activation is less likely to be useful. However, if two groups have often been active in the past at the same time, indirect activation of one by the other is much more likely to be valuable. Even if the two groups were only active at the same time once during a very novel past experience, the value of a later indirect activation could be substantial. During novel experiences, many cortical neurons expand their receptive fields, and such expansions are the way to identify the groups of neurons. Finally, there could be value in prolonging the activity of a currently active group of neurons.
Hence there are four different types of indirect activation that could have behavioural value: indirect activation on the basis of recent past simultaneous activity; indirect activation on the basis of frequent past simultaneous activity; indirect activation on the basis of past simultaneous receptive field expansion; and maintenance of activity on the basis of current simultaneous activity.
A rather different way to access information recorded in the past is that if a group of neurons has acquired significant total recommendation strength in favour of some behaviour through past experience, activation of the group by sensory inputs or by indirect activation will result in that total recommendation strength being present.
Memory is often classified into five types: semantic, episodic, priming, working and procedural. Semantic memory is the ability to recall many facts about the world. One of the most common examples is that we can recall the meanings of words. Hearing the word “tree” conjures up a mental image of a typical tree, so that we could describe the visual features of a tree. In general we do not remember when or where we learned such word meanings. Episodic memory on the other hand is the ability to recall specific individual experiences. Novel events such as a wedding or a traffic accident can be recalled for many years, with recalls including emotions felt at the time. It is even possible to recall everyday events like what we had for lunch today, but such recalls are usually only available for a day or so. Priming memory is the ability to act upon information we were not aware of perceiving at the time. This type of recall only lasts for a few minutes. If there is an animal hidden in grass, we may glimpse something without being aware, but sometimes we can put together several glimpses close together and realise that an animal is there. Working memory refers to our ability to hold information about some object active for a few seconds after the object is removed. Procedural memory refers to our ability to carry out previously learned skills like riding a bicycle, automatically performing the required muscle movements with very little conscious control.
These five types of memory correspond with the four types of indirect activation plus the activation of recommendations strengths identified earlier.
SEMANTIC MEMORY
When we see some type of visual object, like a tree for example, a number of cortical pyramidal neuron receptive fields are detected within the visual inputs. When we look at different trees, different groups of receptive fields will be detected each time. However, because of the visual similarities between trees, some neurons will tend to be active relatively often when looking at trees. When we hear the word “tree” a number of neuron receptive fields are detected within the auditory inputs. When we hear the word on different occasions, spoken by different people, different groups of receptive fields will be detected each time. However, because of the auditory similarities between the spoken words, some neurons will tend to be active relatively often when hearing the word. If that word is often heard when looking at different trees, then averaged over many experiences there will be a group of visual neurons often active at the same time as a group of auditory neurons. In any actual experience significant subsets of both of these groups will be detecting their receptive fields. If on some occasion the word is heard in the absence of a tree, a significant proportion of the auditory group will be detected. Those auditory neurons can indirectly activate the visual neurons on the basis of frequent past simultaneous activity. The indirectly activated visual neurons will have many of the recommendation strengths associated with actually seeing a tree, and descriptions of a tree are therefore possible.
EPISODIC MEMORY
EPISODIC MEMORY
When there is a degree of novelty in an experience, the number of pyramidal neurons detecting their receptive fields is low in some cortical areas. This lower number means that the range of behavioural recommendations available to guide behaviour selection is low. Hence some neurons expand their receptive fields slightly to expand the range of available recommendations. These neurons are selected by the hippocampal system, which records the identity of the group that expanded for use in guiding future expansions. The hippocampal system also records the larger group of all the neurons active when the expansion occurred. If later a significant subset of the larger group happens to be activated, that subset could indirectly activate a larger subset, on the basis of past simultaneous activity during a period of receptive field expansion. If a traffic accident was experienced, the novelty would result in significant numbers of neurons expanding their receptive fields. If later the words “traffic accident” were heard, a few of these neurons might be activated on the basis of the semantic mechanism described earlier. A further step of indirect activation, driven by these neurons on the basis of simultaneous activity in the same past period of receptive field expansion could lead to a larger proportion being activated, with enough recommendation strength to describe the event.
Although a few individuals can recall what happened on every day of their lives in remarkable detail, most of us can remember a lot of everyday experiences a short while later, but not after a few days. It is rarely possible to recall the exact time we started to do something commonplace, like setting off for a walk, a week later. However, such recall may be possible later on the same day. Almost any event will have some degree of novelty, the greater the novelty the easier the episodic recall. For events with less novelty, the indirect activation capability is less.
PRIMING MEMORY
Although a few individuals can recall what happened on every day of their lives in remarkable detail, most of us can remember a lot of everyday experiences a short while later, but not after a few days. It is rarely possible to recall the exact time we started to do something commonplace, like setting off for a walk, a week later. However, such recall may be possible later on the same day. Almost any event will have some degree of novelty, the greater the novelty the easier the episodic recall. For events with less novelty, the indirect activation capability is less.
PRIMING MEMORY
If a snake was briefly glimpsed in a grassy area, a group of neurons would detect their receptive field. However, because it was only a glimpse, relatively few of the neurons often active in the past when snakes were seen might detect their receptive fields. The total recommendation strength of these neurons in favour of identifying a snake would therefore be low. If there was a different glimpse shortly afterwards, a different group neurons would detect their receptive fields, but again the total recommendation strength in favour of identifying a snake would be low. If during that second glimpse, a few of the neurons active during the first glimpse were also active, those neurons could indirectly activate the other neurons active during that first glimpse. Such an activation, on the basis of recent simultaneous activity, could lead to enough recommendation strength to identify the snake. This mechanism would only be valuable for a few minutes, which is the length of time priming memory lasts.
WORKING MEMORY
WORKING MEMORY
In the case of working memory for visual objects, the activity of some visual neurons is prolongued after the stimulus within which their receptive fields were detected is no longer there. Such prolongation will generally be valuable for just a matter of seconds.
INDIRECT ACTIVATIONS MUST BE BEHAVIOURS
Huge numbers of correlations between the activity of different groups of neurons will occur over time. If indirect activations always occurred for every past correlation, the result would be an excessive and chaotic pattern of activation. Hence indirect activations must be behaviours that are recommended by cortical receptive field detections and selected or rejected by the basal ganglia. Any group of receptive field detections will have recommendation strength in favour of indirect activation of correlating groups, and there could be other receptive field detections with supporting recommendation strength. There will be a process in the basal ganglia to determine which groups have sufficient indirect activation recommendation strength. Only the activity of these groups will be released to drive an indirect activation.
INTERMEDIATE RECEPTIVE FIELDS
INDIRECT ACTIVATIONS MUST BE BEHAVIOURS
Huge numbers of correlations between the activity of different groups of neurons will occur over time. If indirect activations always occurred for every past correlation, the result would be an excessive and chaotic pattern of activation. Hence indirect activations must be behaviours that are recommended by cortical receptive field detections and selected or rejected by the basal ganglia. Any group of receptive field detections will have recommendation strength in favour of indirect activation of correlating groups, and there could be other receptive field detections with supporting recommendation strength. There will be a process in the basal ganglia to determine which groups have sufficient indirect activation recommendation strength. Only the activity of these groups will be released to drive an indirect activation.
INTERMEDIATE RECEPTIVE FIELDS
Indirect activation requires some connectivity between the neurons. For example, in the case of semantic memory for words, the auditory neurons must somehow target the visual neurons. Although direct connectivity might be a possible solution, it would involve huge amounts of connectivity and indirect activations would be difficult to manage. A more effective mechanism is that groups of neurons are connected to other groups with temporally correlated past activity via intermediate neurons. These intermediate neurons develop receptive fields corresponding with groups of neurons often active at the same time, and target neurons that are often active when they are active.
For working memory, these intermediate receptive fields only last for seconds. For priming memory, they last for minutes. For semantic and episodic memory they last for hours, and are sometimes prolongued to last indefinitely. There are different chemical mechanisms by which the connections between neurons are modified on these different timescales.
PROCEDURAL MEMORY
In the case of procedural memory, medium spiny neurons in the basal ganglia correspond with different behaviours like specific muscle movements. Pyramidal neurons in the cortex connect to these medium spiny neurons, the connection strength being the recommendation weight of the receptive field in favour of the behaviour. Other medium spiny neurons correspond with the behaviours of changing the weights of recently active inputs to recently active medium spiny neurons. Such behaviours can be labelled reward behaviours. Some pyramidal neurons have connnections on to these reward medium spiny neurons. If a motor behaviour is selected at some point in time, and shortly afterwards receptive fields recommending a reward behaviour are detected, the resultant recommendation weight changes make the motor behaviour more (or less) likely in the future in similar circumstances. Procedural memory is the use of recommendation weights established by such past experiences.
Neurons have no cognitive knowledge, and cognitive behaviours can only use information available to neurons. This information is limited to past correlations in the activity of different neurons and groups of neurons, with different types of correlations resulting in the different types of memory observed in psychology.
For working memory, these intermediate receptive fields only last for seconds. For priming memory, they last for minutes. For semantic and episodic memory they last for hours, and are sometimes prolongued to last indefinitely. There are different chemical mechanisms by which the connections between neurons are modified on these different timescales.
PROCEDURAL MEMORY
In the case of procedural memory, medium spiny neurons in the basal ganglia correspond with different behaviours like specific muscle movements. Pyramidal neurons in the cortex connect to these medium spiny neurons, the connection strength being the recommendation weight of the receptive field in favour of the behaviour. Other medium spiny neurons correspond with the behaviours of changing the weights of recently active inputs to recently active medium spiny neurons. Such behaviours can be labelled reward behaviours. Some pyramidal neurons have connnections on to these reward medium spiny neurons. If a motor behaviour is selected at some point in time, and shortly afterwards receptive fields recommending a reward behaviour are detected, the resultant recommendation weight changes make the motor behaviour more (or less) likely in the future in similar circumstances. Procedural memory is the use of recommendation weights established by such past experiences.
Neurons have no cognitive knowledge, and cognitive behaviours can only use information available to neurons. This information is limited to past correlations in the activity of different neurons and groups of neurons, with different types of correlations resulting in the different types of memory observed in psychology.