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understanding
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The thalamus, gamma band modulation, release of cortical information
and the capacity of working memory
and the capacity of working memory
1st May 2021
The thalamus, gamma band modulation, release of cortical information and the capacity of working memory
The thalamus, gamma band modulation, release of cortical information and the capacity of working memory
Many behaviours are implemented by release of information conditions detected by neurons into and/or out of different cortical areas. Attention behaviours release a selection of current sensory conditions into the primary sensory cortices. Motor behaviours release a selection of receptive field detections in the motor cortex to drive muscle movements. Cognitive behaviours release a selection of the current receptive field detections in one specific cortical area to other specific areas. These release behaviours are determined by the basal ganglia on the basis of current receptive field detections in the cortex.
However, what exactly does release mean? All the condition detections are active and all the axon connectivity is in place before any release. What changes so that this activity is released?
Release behaviour selections are communicated by the basal ganglia to the thalamus, and the thalamus implements those behaviours into the cortex. The key element in a release is that the thalamus places a gamma band frequency modulation on the activity to be released, and this frequency modulation greatly increases the effect that the modulated outputs have on their targets.
Neuron outputs are electrical voltage spikes called action potentials. These spikes travel along the axon until they reach a synapse on a target neuron. When the spike arrives at a target synapse, it injects an electrical charge called a postsynaptic potential. For pyramidal neurons in the cortex this postsynaptic potential encourages the target neuron to fire and produce an output spike.
The postsynaptic potential injected by one spike only lasts for a few milliseconds. It rises to a peak value about two milliseconds after the arrival of the spike, and decays to about half of its peak value by a little over 5 milliseconds later. One postsynaptic potential is not sufficient to cause the neuron to fire, so multiple such potentials must reinforce each other. However, if two spikes arrive more than a few milliseconds apart, the postsynaptic potential injected by the first will have largely decayed away by the time the second arrives. Hence a number of spikes must arrive within a period of about 5 – 10 milliseconds to have an effect on the target. This 5 – 10 milliseconds period is called an integration window because inputs are only integrated together to form a larger total if they arrive within that window of time.
The input spikes to any one target neuron come from different source neurons. The outputs from these source neurons are scattered in time depending on the exact moment each source neuron had enough inputs to detect its receptive field. The thalamus targets a selection of source neurons and causes their outputs to be generated bunched together in time. The frequency with which the bunches occur is about 40 Hz, or one bunch every 25 milliseconds. This frequency is observed in the electroencephalogram (EEG) of the brain and is known as the gamma band frequency. Essentially, output spikes are frequency modulated at the gamma band frequency.
However, what exactly does release mean? All the condition detections are active and all the axon connectivity is in place before any release. What changes so that this activity is released?
Release behaviour selections are communicated by the basal ganglia to the thalamus, and the thalamus implements those behaviours into the cortex. The key element in a release is that the thalamus places a gamma band frequency modulation on the activity to be released, and this frequency modulation greatly increases the effect that the modulated outputs have on their targets.
Neuron outputs are electrical voltage spikes called action potentials. These spikes travel along the axon until they reach a synapse on a target neuron. When the spike arrives at a target synapse, it injects an electrical charge called a postsynaptic potential. For pyramidal neurons in the cortex this postsynaptic potential encourages the target neuron to fire and produce an output spike.
The postsynaptic potential injected by one spike only lasts for a few milliseconds. It rises to a peak value about two milliseconds after the arrival of the spike, and decays to about half of its peak value by a little over 5 milliseconds later. One postsynaptic potential is not sufficient to cause the neuron to fire, so multiple such potentials must reinforce each other. However, if two spikes arrive more than a few milliseconds apart, the postsynaptic potential injected by the first will have largely decayed away by the time the second arrives. Hence a number of spikes must arrive within a period of about 5 – 10 milliseconds to have an effect on the target. This 5 – 10 milliseconds period is called an integration window because inputs are only integrated together to form a larger total if they arrive within that window of time.
The input spikes to any one target neuron come from different source neurons. The outputs from these source neurons are scattered in time depending on the exact moment each source neuron had enough inputs to detect its receptive field. The thalamus targets a selection of source neurons and causes their outputs to be generated bunched together in time. The frequency with which the bunches occur is about 40 Hz, or one bunch every 25 milliseconds. This frequency is observed in the electroencephalogram (EEG) of the brain and is known as the gamma band frequency. Essentially, output spikes are frequency modulated at the gamma band frequency.
The bunching of the spikes makes it much more likely that enough will arrive at a target neuron within an integration window. Hence the modulation effectively releases the outputs to their targets, because it makes those targets much more likely to generate their own outputs.
HOW THE THALAMUS FREQUENCY MODULATES CORTICAL OUTPUTS
The main part of the thalamus is divided up into a number of nuclei, sometimes called the dorsal nuclei. Each dorsal nucleus gets inputs from a small group of cortical areas, and projects back mainly to one of those areas. One dorsal nucleus manages the releases of activity from the cortical area it targets.
There are generally five layers of pyramidal neurons in the cortex, labelled II, III, IV, V and VI going deeper away from the skull. Inputs from other areas arrive in layer IV. Neurons in layer IV target neurons in layers II and III, and neurons in layers II/III target layers V and VI.
The thalamocortical projection neurons in the dorsal nuclei target pyramidal neurons in layer IV and are targetted by pyramidal neurons in layer VI. The axons in both directions between the dorsal nucleus and the cortex are excitatory and encourage firing of their targets. Hence there are positive thalamocortical feedback loops between the dorsal nucleus and its targetted cortical area. The activity in these loops is held in check by constant inhibitory input from the basal ganglia.
Another nucleus, called the thalamic reticular nucleus (the TRN), is wrapped around the outside of the dorsal nuclei. Each dorsal nucleus has an area of the TRN associated with it. All axons to and from the dorsal nucleus and the cortical area it targets pass through its area of the TRN. These axons drop side branches on to neurons in the TRN. When a TRN neuron fires, it generates a steady output of spikes at the gamma band frequency. These spikes are inhibitory, and discourage firing of their targets.
HOW THE THALAMUS FREQUENCY MODULATES CORTICAL OUTPUTS
The main part of the thalamus is divided up into a number of nuclei, sometimes called the dorsal nuclei. Each dorsal nucleus gets inputs from a small group of cortical areas, and projects back mainly to one of those areas. One dorsal nucleus manages the releases of activity from the cortical area it targets.
There are generally five layers of pyramidal neurons in the cortex, labelled II, III, IV, V and VI going deeper away from the skull. Inputs from other areas arrive in layer IV. Neurons in layer IV target neurons in layers II and III, and neurons in layers II/III target layers V and VI.
The thalamocortical projection neurons in the dorsal nuclei target pyramidal neurons in layer IV and are targetted by pyramidal neurons in layer VI. The axons in both directions between the dorsal nucleus and the cortex are excitatory and encourage firing of their targets. Hence there are positive thalamocortical feedback loops between the dorsal nucleus and its targetted cortical area. The activity in these loops is held in check by constant inhibitory input from the basal ganglia.
Another nucleus, called the thalamic reticular nucleus (the TRN), is wrapped around the outside of the dorsal nuclei. Each dorsal nucleus has an area of the TRN associated with it. All axons to and from the dorsal nucleus and the cortical area it targets pass through its area of the TRN. These axons drop side branches on to neurons in the TRN. When a TRN neuron fires, it generates a steady output of spikes at the gamma band frequency. These spikes are inhibitory, and discourage firing of their targets.
When the basal ganglia selects a release behaviour, the constant inhibition of the thalamocortical loop corresponding with the behaviour is relaxed. This allows activity to build in the loop, and this increased activity causes the TRN neurons to fire at 40 Hz. This firing is inhibitory, and blocks production of spikes. The increased activity in the thalamocortical loop is thus blocked except out of phase with the TRN spikes. A gamma band modulation is therefore imposed on the thalamic neuron targetting the cortex. The thalamic neuron imposes the same modulation on the cortex that propagates from layer IV through to layer VI. Outputs from layer VI target other cortical areas.
OVERALL RELEASE MECHANISM
Overall, the process is that outputs from cortical layer V target the basal ganglia where they are interpreted as recommendations in favour of the release of outputs from the column in which they are located to other cortical areas. Inputs to the basal ganglia from other areas could also recommend the same release behaviour. If this behaviour has the predominant recommendation strength, the basal ganglia reduces the constant inhibition of the thalamus, allowing buildup of activity and imposition of a gamma band modulation on that activity. The modulation effectively releases the outputs to other areas by increasing their effect on neurons in those areas.
WORKING MEMORY
There is a further important aspect of the gamma band modulation. The peak to peak period of a 40 Hz frequency is 25 milliseconds. The integration window is around 8 milliseconds. So there is room for three integration windows in each period. Three integration windows allows pseudosimultaneous processing of three different sources of information. Suppose that one target neuron received inputs from two different populations of source neurons. The outputs from one population are modulated at 40 Hz so that most of the spikes arrive in a burst within a window of a few milliseconds once every 25 milliseconds, near the peak of the modulation signal. The outputs from the second population are also modulated at 40 Hz, but the peak of the modulation signal is offset from the peak of the first signal. Most of the spikes from the second population will arrive in a burst once every 25 milliseconds near the peak of the second modulation signal. Because the modulation signals are offset in time, each burst from the first population will be offset in time from the burst from the second population. If the offset is about 8 milliseconds, most of the postsynaptic potential injected by the first burst will have decayed before the second burst arrives. Essentially the target neuron processes information from the two source populations separately.
OVERALL RELEASE MECHANISM
Overall, the process is that outputs from cortical layer V target the basal ganglia where they are interpreted as recommendations in favour of the release of outputs from the column in which they are located to other cortical areas. Inputs to the basal ganglia from other areas could also recommend the same release behaviour. If this behaviour has the predominant recommendation strength, the basal ganglia reduces the constant inhibition of the thalamus, allowing buildup of activity and imposition of a gamma band modulation on that activity. The modulation effectively releases the outputs to other areas by increasing their effect on neurons in those areas.
WORKING MEMORY
There is a further important aspect of the gamma band modulation. The peak to peak period of a 40 Hz frequency is 25 milliseconds. The integration window is around 8 milliseconds. So there is room for three integration windows in each period. Three integration windows allows pseudosimultaneous processing of three different sources of information. Suppose that one target neuron received inputs from two different populations of source neurons. The outputs from one population are modulated at 40 Hz so that most of the spikes arrive in a burst within a window of a few milliseconds once every 25 milliseconds, near the peak of the modulation signal. The outputs from the second population are also modulated at 40 Hz, but the peak of the modulation signal is offset from the peak of the first signal. Most of the spikes from the second population will arrive in a burst once every 25 milliseconds near the peak of the second modulation signal. Because the modulation signals are offset in time, each burst from the first population will be offset in time from the burst from the second population. If the offset is about 8 milliseconds, most of the postsynaptic potential injected by the first burst will have decayed before the second burst arrives. Essentially the target neuron processes information from the two source populations separately.
With a modulation frequency of 40 Hz it is possible to fit three separate windows into a 25 millisecond period in such a way that the interactions between bursts in different windows are small. In other words, it is possible for one neuron to process inputs from three different populations of source neurons at the same time.
It is therefore possible for one cortical area to simultaneously process three different populations of inputs at the same time. Working memory is the ability to hold a memory active in the brain. It is possible to maintain memories of multiple objects active simultaneously, but there are limits to this ability. Experiments show that if subjects are briefly shown different numbers of objects they can recall a number of them several seconds later. This number varies, but the number of objects that can always be recalled correctly no matter what the objects is three. This number reflects the number of populations of receptive field detections that can be maintained separately in the same cortical area, higher numbers are possible if multiple areas can be involved. The base working memory limit of three objects is thus determined by a combination of the gamma band frequency and the postsynaptic potential decay time.
It is therefore possible for one cortical area to simultaneously process three different populations of inputs at the same time. Working memory is the ability to hold a memory active in the brain. It is possible to maintain memories of multiple objects active simultaneously, but there are limits to this ability. Experiments show that if subjects are briefly shown different numbers of objects they can recall a number of them several seconds later. This number varies, but the number of objects that can always be recalled correctly no matter what the objects is three. This number reflects the number of populations of receptive field detections that can be maintained separately in the same cortical area, higher numbers are possible if multiple areas can be involved. The base working memory limit of three objects is thus determined by a combination of the gamma band frequency and the postsynaptic potential decay time.