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understanding the brain |
Cognitive role of cortical areas
29th October 2021
Cognitive role of cortical areas
Cognitive role of cortical areas
The cortex is a thin sheet of neurons, mainly of the pyramidal type, wrapped around the outside of the cerebral hemispheres. There are a number of layers in the sheet, each layer having pyramidal neurons with somewhat different sizes and connectivity. The sheet is divided up into areas. The layering of the neurons within an area is slightly different from other areas, and there are also chemical differences.
Each pyramidal neuron produces an output when it detects one of a group of similar circumstances within its inputs. This group of similar circumstances is called the receptive field of the neuron.
Each area gets most of its inputs from a small number of other areas, and a few areas also get inputs fairly directly from one of the senses. Pyramidal neurons in one area therefore detect receptive fields defined by complex combinations of inputs from those other areas. The receptive fields in different areas will tend to be within different ranges of complexity, where complexity can roughly be defined as the total number of raw sensory inputs that contribute to the receptive field, directly or via intermediate areas.
Receptive fields are defined heuristically. The definition process is that neurons identify combinations of inputs that tend to occur at the same time on many different occasions. However, this does not mean that receptive fields correspond with cognitively meaningful situations like perception of a cognitive category, in the sense that the field will always be detected when an instance of the category is present and never in response to a different category. Rather, receptive fields correspond with a range of similar information circumstances that often occur, but the occurrences will be in many different types of cognitive circumstances. For example, a roughly circular image on the retina might often occur in experience when perceiving objects like balls, apples, heads or globes etc. A receptive field might be defined on the basis of this frequently repeated experience. Such a receptive field would be detected when some objects of all the different types were perceived.
Although receptive fields do not correspond with cognitive circumstances, the receptive fields defined in a given range of complexity (i.e. within one area) are effective for discriminating between certain types of circumstances. For example, receptive fields in the cortical area labelled TEO are effective for discriminating between different categories of visual objects.
Each pyramidal neuron produces an output when it detects one of a group of similar circumstances within its inputs. This group of similar circumstances is called the receptive field of the neuron.
Each area gets most of its inputs from a small number of other areas, and a few areas also get inputs fairly directly from one of the senses. Pyramidal neurons in one area therefore detect receptive fields defined by complex combinations of inputs from those other areas. The receptive fields in different areas will tend to be within different ranges of complexity, where complexity can roughly be defined as the total number of raw sensory inputs that contribute to the receptive field, directly or via intermediate areas.
Receptive fields are defined heuristically. The definition process is that neurons identify combinations of inputs that tend to occur at the same time on many different occasions. However, this does not mean that receptive fields correspond with cognitively meaningful situations like perception of a cognitive category, in the sense that the field will always be detected when an instance of the category is present and never in response to a different category. Rather, receptive fields correspond with a range of similar information circumstances that often occur, but the occurrences will be in many different types of cognitive circumstances. For example, a roughly circular image on the retina might often occur in experience when perceiving objects like balls, apples, heads or globes etc. A receptive field might be defined on the basis of this frequently repeated experience. Such a receptive field would be detected when some objects of all the different types were perceived.
Although receptive fields do not correspond with cognitive circumstances, the receptive fields defined in a given range of complexity (i.e. within one area) are effective for discriminating between certain types of circumstances. For example, receptive fields in the cortical area labelled TEO are effective for discriminating between different categories of visual objects.
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Typical receptive fields detected in cortical area TEO in a macaque monkey. This area is critical for discriminating between different categories of visual objects, but no receptive field corresponds exactly with any one category
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THE MEANING OF DISCRIMINATION
In response to any cognitive circumstance, a reasonably large group of receptive fields will be detected. Even for cognitive circumstances in which the same behaviour is appropriate (e.g. identifying the category of different examples of the same category of visual object), the detected groups of receptive fields will not be exactly the same on every occasion.
Any one cortical receptive field has a range of recommendation strengths in the basal ganglia in favour of different behaviours, each recommendation having a different weight.
Effective discrimination means that the groups of fields detected in circumstances in which the same behaviour is appropriate are sufficiently similar, and the groups of fields detected when a different behaviour is appropriate are sufficiently different, that it is possible to assign recommendation strengths in such a way that the appropriate behaviour almost always has the largest total recommendation strength across all the fields in the group.
DIFFERENT CORTICAL AREAS
There are about 150 different areas in each cortical hemisphere of the human brain. The range of complexity of receptive fields in each area is different, and each area can therefore discriminate between different types of circumstances. Natural selection has resulted in a set of areas such that the cortex can discriminate between almost all cognitive circumstances in which different behaviours are appropriate.
One area may be particularly effective for discriminating between one (or several) types of cognitive circumstances. TEO has already been mentioned as being effective for discriminating between different types of visual objects. Other areas may be particularly effective for disrciminating between different groups of objects, or groups of groups, or very complex cognitive circumstances. I will use the symbol “≈” to mean “discriminates effectively between”
In some cases it may be possible to label an area by the type of cognitive circumstances it discriminates most effectively. Such labelling may be helpful for understanding how cortical processing supports cognition, but it is only approximate. One area may support discrimination of a number of types of cognitive circumstances. To give a very simple example, suppose there were three areas, one particularly effective for discriminating between different types of visual features (≈visual features), another ≈visual objects, and a third ≈groups of objects. The ≈visual objects area would support discrimination between, for example, a dog and a coyote. However, the ≈visual features area might be effective for discriminating between the presence and absence of a collar on the animal. The ≈groups of objects area might be effective for discriminating between a man walking with an animal and a man and animal with no such relationship. Hence all three areas might contribute to discriminating between a dog and a coyote.
Nevertheless, approximate cognitive labelling of cortical areas is helpful for understanding. Frontal areas often identified as contributing to “higher cognition” can be better understood as supporting discrimination between complex circumstances in which different general types of behaviours are appropriate.
In response to any cognitive circumstance, a reasonably large group of receptive fields will be detected. Even for cognitive circumstances in which the same behaviour is appropriate (e.g. identifying the category of different examples of the same category of visual object), the detected groups of receptive fields will not be exactly the same on every occasion.
Any one cortical receptive field has a range of recommendation strengths in the basal ganglia in favour of different behaviours, each recommendation having a different weight.
Effective discrimination means that the groups of fields detected in circumstances in which the same behaviour is appropriate are sufficiently similar, and the groups of fields detected when a different behaviour is appropriate are sufficiently different, that it is possible to assign recommendation strengths in such a way that the appropriate behaviour almost always has the largest total recommendation strength across all the fields in the group.
DIFFERENT CORTICAL AREAS
There are about 150 different areas in each cortical hemisphere of the human brain. The range of complexity of receptive fields in each area is different, and each area can therefore discriminate between different types of circumstances. Natural selection has resulted in a set of areas such that the cortex can discriminate between almost all cognitive circumstances in which different behaviours are appropriate.
One area may be particularly effective for discriminating between one (or several) types of cognitive circumstances. TEO has already been mentioned as being effective for discriminating between different types of visual objects. Other areas may be particularly effective for disrciminating between different groups of objects, or groups of groups, or very complex cognitive circumstances. I will use the symbol “≈” to mean “discriminates effectively between”
In some cases it may be possible to label an area by the type of cognitive circumstances it discriminates most effectively. Such labelling may be helpful for understanding how cortical processing supports cognition, but it is only approximate. One area may support discrimination of a number of types of cognitive circumstances. To give a very simple example, suppose there were three areas, one particularly effective for discriminating between different types of visual features (≈visual features), another ≈visual objects, and a third ≈groups of objects. The ≈visual objects area would support discrimination between, for example, a dog and a coyote. However, the ≈visual features area might be effective for discriminating between the presence and absence of a collar on the animal. The ≈groups of objects area might be effective for discriminating between a man walking with an animal and a man and animal with no such relationship. Hence all three areas might contribute to discriminating between a dog and a coyote.
Nevertheless, approximate cognitive labelling of cortical areas is helpful for understanding. Frontal areas often identified as contributing to “higher cognition” can be better understood as supporting discrimination between complex circumstances in which different general types of behaviours are appropriate.
A receptive field detection in the cortex has a range of recommendation strengths in favour of different behaviours. These recommendation strengths exist in the striatum of the basal ganglia. The receptive fields in some cortical areas are more effective for recommending more strategic behaviours. These areas target the ventral basal ganglia. Receptive fields in other areas are more effective for recommending tactical, specific or detailed behaviours, and target the more dorsal regions of the striatum.
GENERAL TYPES OF BEHAVIOURS
One way of classifying general types of behaviour is along a spectrum from very strategic through more tactical to fairly specific and on to very detailed. For example, imagine a player in a soccer game. A strategic behaviour selection would be to choose to play defence rather than attack. A more tactical behaviour would be to move to a particular location on the field. A more specific behaviour would be to choose to mark one particular player. A more detailed behaviour would be to tackle that player to try to take away the ball. Yet more detailed behaviours would be the muscle movements to implement the tackle.
Some cortical areas are more effective to discriminating between different circumstances in which different strategic behaviours are appropriate, and the individual receptive fields are therefore effective for recommending ranges of strategic behaviours. These areas tend to be those located in the orbital and medial prefrontal cortical regions. Areas in the dorsolateral prefrontal cortex region are effective for recommending ranges of tactical behaviours and so on.
Some cortical areas detect receptive fields that are relatively simple combinations of sensory inputs. For example, receptive fields in different visual areas are effective for discriminating between different visual features, visual objects, groups of objects, objects positions and object motions and relative motions. Receptive fields in these areas can recommend some types of behaviours (such as identifying objects categories) but also contribute to the definitions of receptive fields in the tactical and strategic areas.
The total recommendation strengths of each behaviour across all currently detected cortical receptive fields are determined in the basal ganglia, which implements any behaviours with sufficient total recommendation strengths.
IMPLEMENTATION OF BEHAVIOURS
In the case of very detailed behaviours, implementation of a selection is carried out by release of the receptive fields that most strongly recommended them out of the motor cortex to drive muscle movements.
Implementation of strategic, tactical and specific behaviour selections is implemented by release of the receptive field detections that recommended them to other cortical areas. A selected strategic behaviour is implemented by release of the detections that recommended that strategic behaviour to areas effective for recommending tactical behaviours. As a result, the most strongly recommended tactical behaviours will tend to be of the already selected strategic type. Similarly, the receptive fields with the largest recommendation strengths in favour of a selected tactical behaviour are released to cortical areas effective for recommending more specific behaviours and so on.
One way of classifying general types of behaviour is along a spectrum from very strategic through more tactical to fairly specific and on to very detailed. For example, imagine a player in a soccer game. A strategic behaviour selection would be to choose to play defence rather than attack. A more tactical behaviour would be to move to a particular location on the field. A more specific behaviour would be to choose to mark one particular player. A more detailed behaviour would be to tackle that player to try to take away the ball. Yet more detailed behaviours would be the muscle movements to implement the tackle.
Some cortical areas are more effective to discriminating between different circumstances in which different strategic behaviours are appropriate, and the individual receptive fields are therefore effective for recommending ranges of strategic behaviours. These areas tend to be those located in the orbital and medial prefrontal cortical regions. Areas in the dorsolateral prefrontal cortex region are effective for recommending ranges of tactical behaviours and so on.
Some cortical areas detect receptive fields that are relatively simple combinations of sensory inputs. For example, receptive fields in different visual areas are effective for discriminating between different visual features, visual objects, groups of objects, objects positions and object motions and relative motions. Receptive fields in these areas can recommend some types of behaviours (such as identifying objects categories) but also contribute to the definitions of receptive fields in the tactical and strategic areas.
The total recommendation strengths of each behaviour across all currently detected cortical receptive fields are determined in the basal ganglia, which implements any behaviours with sufficient total recommendation strengths.
IMPLEMENTATION OF BEHAVIOURS
In the case of very detailed behaviours, implementation of a selection is carried out by release of the receptive fields that most strongly recommended them out of the motor cortex to drive muscle movements.
Implementation of strategic, tactical and specific behaviour selections is implemented by release of the receptive field detections that recommended them to other cortical areas. A selected strategic behaviour is implemented by release of the detections that recommended that strategic behaviour to areas effective for recommending tactical behaviours. As a result, the most strongly recommended tactical behaviours will tend to be of the already selected strategic type. Similarly, the receptive fields with the largest recommendation strengths in favour of a selected tactical behaviour are released to cortical areas effective for recommending more specific behaviours and so on.
An area of the cortex can be labelled in terms of the types of discriminations made most effectively by the receptive fields defined in the area. The ≈ symbol is used to indicate the discrimination type. Some areas support discrimination between different visual features, objects and groups of objects. Other areas support discrimination between combinations of sounds on various levels of complexity. Yet other areas support discrimination between complex circumstances such as social situations. In all cases, the critical property is the ability to discriminate between situations in which the appropriate behaviour is different. All receptive fields are defined by combinations of inputs that tend to occur at similar times. In some cases, the most useful was to label an area is by the type of temporally correlated activity that is used to define receptive fields in the area. In these cases the = symbol is used. As explained in a previous blog, in these cases the receptive fields play a role in different types of memory.
COGNITIVE LABELLING OF CORTICAL AREAS
There are therefore two ways in which areas can be labelled in terms of their cognitive functions. One way is in terms of the type of cognitive circumstances they are most effective for discriminating. The other way is in terms of the type of behaviour they are most effective for recommending. Although such labelling is very useful for understanding how cognitive processing takes place, it is important to remember that any such labelling is only approximate. It is possible that any area could recommend any type of behaviour, if the receptive fields in that area were helpful for discrimination between different circumstances in which different behaviours of the type were appropriate.
In addition, sometimes approximate cognitive labelling is less useful. All receptive fields correspond with groups of receptive fields in other areas that tend to be active at similar times in the past. Some areas support the indirect activation of receptive fields in other areas on the basis of various types of correlations in past activity. The receptive fields in these areas support semantic, episodic, working and priming memory.
One type of cognitive labelling that is misleading is to try to identify the cognitive function carried out by an area. One example is assigning the role of identifying human faces to an area labelled the fusiform face area (FFA) located in the right cortical hemisphere. The issues with this identification are that many other cortical areas are involved in face identification, and the FFA is sometimes involved in other identification functions when the brain is expert in such identifications. One example is the involvement of the area in identification of bird species by bird experts. In addition, the corresponding area in the left hemisphere is involved in identification of words presented visually. The appropriate way to label this area is that it can discriminate between large numbers of very similar visual images, when such discrimination is behaviourally important.
COGNITIVE PROCESSES
A cognitive process can be understood as a sequence of releases of cortical activity between specific areas. Releases are recommended by cortical activity, and drive further receptive field detections in target areas that in turn recommend further release behaviours.
There are therefore two ways in which areas can be labelled in terms of their cognitive functions. One way is in terms of the type of cognitive circumstances they are most effective for discriminating. The other way is in terms of the type of behaviour they are most effective for recommending. Although such labelling is very useful for understanding how cognitive processing takes place, it is important to remember that any such labelling is only approximate. It is possible that any area could recommend any type of behaviour, if the receptive fields in that area were helpful for discrimination between different circumstances in which different behaviours of the type were appropriate.
In addition, sometimes approximate cognitive labelling is less useful. All receptive fields correspond with groups of receptive fields in other areas that tend to be active at similar times in the past. Some areas support the indirect activation of receptive fields in other areas on the basis of various types of correlations in past activity. The receptive fields in these areas support semantic, episodic, working and priming memory.
One type of cognitive labelling that is misleading is to try to identify the cognitive function carried out by an area. One example is assigning the role of identifying human faces to an area labelled the fusiform face area (FFA) located in the right cortical hemisphere. The issues with this identification are that many other cortical areas are involved in face identification, and the FFA is sometimes involved in other identification functions when the brain is expert in such identifications. One example is the involvement of the area in identification of bird species by bird experts. In addition, the corresponding area in the left hemisphere is involved in identification of words presented visually. The appropriate way to label this area is that it can discriminate between large numbers of very similar visual images, when such discrimination is behaviourally important.
COGNITIVE PROCESSES
A cognitive process can be understood as a sequence of releases of cortical activity between specific areas. Releases are recommended by cortical activity, and drive further receptive field detections in target areas that in turn recommend further release behaviours.