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understanding the brain |
Understanding subjective experience in terms of anatomy and physiology
– some philosophical implications
– some philosophical implications
8th October 2021
Understanding subjective experience in terms of anatomy and physiology – some philosophical implication
Understanding subjective experience in terms of anatomy and physiology – some philosophical implication
All of us have subjective experiences of objects and events. The often used example is that when we look at something coloured red, there is a subjective experience that is different from the experience when looking at the colour blue. Even experiences of the same sensory situation can have different intensities. If we glance briefly at a red area, we will immediately be able to verbally identify it as being red. However, if we focus our attention on that red area, we can become aware of a much richer experience, although it is hard to verbally describe the nature of the extra experience.
Everyday subjective experiences like this have been the focus of extensive philosophical debate. Such experiences have been labelled qualia, and it has been argued that qualia have a number of characteristics. One such characteristic suggested by philosophers is that their content cannot be communicated or understood except by the person experiencing them. Another suggested characteristic is that comparisons between the experiences of two different people are impossible. With these properties, one person’s experience of red could not be fully described to someone who has never experienced color. This is often taken to imply that qualia contain information beyond just the physical.
In this blog I will first go over some of the philosophical issues raised around the existence of qualia, then describe how to map qualia to descriptions in terms of neural activity, and finally discuss how such mapping affects the philosophical issues.
PHILOSOPHICAL APPROACHES TO QUALIA
Many philosophical questions have been raised about qualia. For example, in the earlier red perception example, some of the questions raised are: “Is it always the same experience when we look at the colour red?”; and “Is the experience the same when different people look at the colour red?”
An often argued question is whether complex behaviour would be possible in the absence of these subjective experiences. One thought experiment postulates “zombies”, or robots that are identical with humans in terms of molecular makeup and behaviour, but do not experience qualia. If a human and a zombie both have some chocolate cake, both might behave in the same way: saying it tastes good, and taking some more. However, the human has a subjective experience of the taste, while the zombie does not experiencing anything. This thought experiment is used to argue that qualia are an additional property of human experience over and above the physical mechanisms.
Another thought experiment concerns a colour scientist often called Mary. This scientist has never left a room in which everything is black and white including paint covering her own body, and all information derived from the world outside is in black and white. Mary uses the information to become expert in all the physical aspects of colour and colour vision. However, she has no personal experience of colour. In this thought experiment she does not experience colour qualia. If eventually she leaves the room and sees coloured objects for the first time, she might say “So this is what it is like to experience red, or experience blue”. The philosophical argument is that Mary has not come to know any new information about colour, so the subjective experience is something different from physical knowledge.
It seems to some philosophers that there is a gap between our experience of what it is like to experience qualia and any possible description of what is going on in terms of brain anatomy, physiology and chemistry. In other words, although we could describe the detailed processes going on when we see the colour red, that would not answer the question “Why does experiencing the red colour feel as it does and not feel something different?”
Another area of discussion is whether other mammals, birds, reptiles, fish or insects experience qualia, and how we could determine if they did (or did not). Furthermore, if they did experience qualia, how could we describe what they were subjectively experiencing in terms we could understand subjectively, in other words “What is it like to be a bat?”.
INFORMATION PROCESSES IN THE BRAIN
A brain is a control system in the sense that it detects conditions in the information available to it, and associates different combinations of detected conditions with different behaviours. In the brain, all the conditions are defined heuristically. Each condition corresponds with a group of circumstances that are similar to each other and have occurred relatively often in experience. In other words, the brain semi-randomly picks circumstances that appear fairly frequently in the course of experience. It is important to realize that this process generally does not result in conditions that precisely correspond with unambiguous cognitive circumstances. For example, in the course of experience, a brain may often encounter objects that create roughly circular images on the retina. Such objects might include apples, onions, the full moon, heads, balls etc. From this experience the brain might define a condition that is detected when some vaguely spherical object is observed. The brain needs this condition and many others to identify the appropriate response to some perceived object.
The information currently available to the brain is constantly compared with the heuristically defined conditions. Each detected condition is then interpreted as a range of recommendations in favour of different behaviours, each recommendation having an individual weight. The brain determines and implements the behaviour with the largest total weight across all currently detected conditions. Such behaviours could include speech.
If only a few conditions are being detected, there will not be a sufficiently wide range of behavioural recommendations to generate a high integrity behaviour selection. In such situations, the range of circumstances in which some of the undetected conditions are detected is slightly expanded so that they are detected, and their recommendations added. Such expansions are the primary way in which conditions are defined. Conditions detected shortly after a behaviour is performed can recommend changes to the weights in favour of that behaviour. These changes are called reward behaviours and are the primary way in which recommendation weights are defined.
NEURON LEVEL IMPLEMENTATION OF BRAIN INFORMATION PROCESSES
A pyramidal neuron defines a condition that is made up of a group of similar circumstances, and detects any occurrence of a significant subset of those circumstances. The condition is called the receptive field of the neuron. Such pyramidal neurons are located in the cortex. Medium spiny neurons in the striatum of the basal ganglia interpret inputs from pyramidal neurons as behavioural recommendations, and the basal ganglia determine and implement the most strongly recommended behaviour. Reward behaviours that change recently used recommendation weights are also recommended by pyramidal neuron receptive field detections and selected by the basal ganglia. Behaviours that expand receptive fields are also recommended by pyramidal neuron receptive field detections but such behaviours are selected by the hippocampus.
DESCRIPTION OF SUBJECTIVE SENSORY EXPERIENCE IN TERMS OF NEURON ACTIVITY
In any situation, a wide range of neurons are active in the brain. This activity corresponds with the subjective experience of the brain at the time. In information terms this subjective experience corresponds with condition detections and their associated ranges of behavioural recommendations, plus the implementations of selected behaviours. Activity in the cortex corresponds with receptive field detections of pyramidal neurons and activation of their associated ranges of behavioural recommendations. Activity in the basal ganglia corresponds with selection of the most strongly recommended behaviours. Activity in the thalamus corresponds with implementation of the selected behaviours. Activity in the cerebellum corresponds with implementation of often used sequences of behaviours. Activity in other anatomical structures supports these primary activities.
During some sensory experience, a population of pyramidal neurons is active across the cortex. Each neuron in the population has a receptive field defined by a complex combination of sensory inputs, and that receptive field is present in the current sensory inputs. Hence the population can be regarded as detecting a wide range of different aspects of the current sensory situation. Each of the active pyramidal neurons has a range of recommendation strengths in favour of different behaviours. These recommendation strengths are located in the basal ganglia, which determines the total strengths of each possible behaviour across all the currently active pyramidal neurons. The largest such total strengths are generally in favour of behaviours appropriate in response to the current sensory environment, such as naming an observed visual object.
If the experience is very similar to a past experience, many of the same neurons will be active. However, even then all of the neurons will also have been active in many different past experiences, and will have some behavioural recommendation strengths appropriate in those past experiences. For example, a neuron with a receptive field corresponding with vaguely spherical circumstances as discussed earlier could have recommendation strength in favour of a number of different speech behaviours like saying “that is a ball”, “that is an apple”, “that is an onion”, and “that is the moon”. All these recommendation strengths will be present when the neuron is activated, but only a speech behaviour strongly recommended by a significant number of active neurons has a chance of being implemented.
HOW WE THINK BEYOND CURRENT SENSORY INPUTS
Humans are able to plan for situations with little resemblance to their current sensory environment. Furthermore, in some situations the current sensory inputs are an inadequate guide to determining current appropriate behaviour. For example, in social situations the appropriate response to a group of people requires access to a lot of information about past interactions with the individuals and different groups of individuals. How can neurons with more appropriate recommendation strengths be activated when their receptive fields are not present in current sensory inputs? Such indirect activation cannot be on the basis cognitive information because such information is unavailable to physical neurons. The only basis for such indirect activation is past temporal correlations between the activity of an inactive neuron and activity of some currently active neurons. For example, a neuron could be indirectly activated on the basis of frequent past activity at the same time as a group of currently active neurons. Alternatively, a neuron could be indirectly activated on the basis that on one occasion it was active at the same time as a group of currently active neurons, and at that time a lot of the neurons were changing their receptive fields. These two mechanisms are, respectively, the basis for semantic and episodic memory retrievals.
Hence if a population of pyramidal neurons directly activated by sensory inputs does not have a predominant recommendation strength in favour of an external response, to expand the available range of recommendation strengths the population can indirectly activate a second population on the basis of correlations in past activity. This second population could then indirectly activate a tertiary population and so on, the process continuing until there is a predominant recommendation strength.
If every neuron could indirectly activate every other neuron with which it had some past temporal correlation in activity, the result would be an excessive and chaotic pattern of activation. Hence indirect activations are behaviours that are recommended by neuron activity, and only implemented if the total recommendation strength across many active neurons is substantial.
THE PHYSIOLOGICAL BASIS OF QUALIA
When we look at the colour red, receptive fields will be directly detected within the sensory inputs. Many of the fields will often have been active in the past when the colour red was seen, and a significant proportion will have recommendation strength in favour of saying “That is red”. However, focussing attention on the colour is effectively initiating a search for behaviours beyond those immediately recommended. As described earlier, in order to gain access to a wider range of behavioural recommendations, groups of receptive fields can indirectly activate other groups on the basis of temporally correlated past activity.
Everyday subjective experiences like this have been the focus of extensive philosophical debate. Such experiences have been labelled qualia, and it has been argued that qualia have a number of characteristics. One such characteristic suggested by philosophers is that their content cannot be communicated or understood except by the person experiencing them. Another suggested characteristic is that comparisons between the experiences of two different people are impossible. With these properties, one person’s experience of red could not be fully described to someone who has never experienced color. This is often taken to imply that qualia contain information beyond just the physical.
In this blog I will first go over some of the philosophical issues raised around the existence of qualia, then describe how to map qualia to descriptions in terms of neural activity, and finally discuss how such mapping affects the philosophical issues.
PHILOSOPHICAL APPROACHES TO QUALIA
Many philosophical questions have been raised about qualia. For example, in the earlier red perception example, some of the questions raised are: “Is it always the same experience when we look at the colour red?”; and “Is the experience the same when different people look at the colour red?”
An often argued question is whether complex behaviour would be possible in the absence of these subjective experiences. One thought experiment postulates “zombies”, or robots that are identical with humans in terms of molecular makeup and behaviour, but do not experience qualia. If a human and a zombie both have some chocolate cake, both might behave in the same way: saying it tastes good, and taking some more. However, the human has a subjective experience of the taste, while the zombie does not experiencing anything. This thought experiment is used to argue that qualia are an additional property of human experience over and above the physical mechanisms.
Another thought experiment concerns a colour scientist often called Mary. This scientist has never left a room in which everything is black and white including paint covering her own body, and all information derived from the world outside is in black and white. Mary uses the information to become expert in all the physical aspects of colour and colour vision. However, she has no personal experience of colour. In this thought experiment she does not experience colour qualia. If eventually she leaves the room and sees coloured objects for the first time, she might say “So this is what it is like to experience red, or experience blue”. The philosophical argument is that Mary has not come to know any new information about colour, so the subjective experience is something different from physical knowledge.
It seems to some philosophers that there is a gap between our experience of what it is like to experience qualia and any possible description of what is going on in terms of brain anatomy, physiology and chemistry. In other words, although we could describe the detailed processes going on when we see the colour red, that would not answer the question “Why does experiencing the red colour feel as it does and not feel something different?”
Another area of discussion is whether other mammals, birds, reptiles, fish or insects experience qualia, and how we could determine if they did (or did not). Furthermore, if they did experience qualia, how could we describe what they were subjectively experiencing in terms we could understand subjectively, in other words “What is it like to be a bat?”.
INFORMATION PROCESSES IN THE BRAIN
A brain is a control system in the sense that it detects conditions in the information available to it, and associates different combinations of detected conditions with different behaviours. In the brain, all the conditions are defined heuristically. Each condition corresponds with a group of circumstances that are similar to each other and have occurred relatively often in experience. In other words, the brain semi-randomly picks circumstances that appear fairly frequently in the course of experience. It is important to realize that this process generally does not result in conditions that precisely correspond with unambiguous cognitive circumstances. For example, in the course of experience, a brain may often encounter objects that create roughly circular images on the retina. Such objects might include apples, onions, the full moon, heads, balls etc. From this experience the brain might define a condition that is detected when some vaguely spherical object is observed. The brain needs this condition and many others to identify the appropriate response to some perceived object.
The information currently available to the brain is constantly compared with the heuristically defined conditions. Each detected condition is then interpreted as a range of recommendations in favour of different behaviours, each recommendation having an individual weight. The brain determines and implements the behaviour with the largest total weight across all currently detected conditions. Such behaviours could include speech.
If only a few conditions are being detected, there will not be a sufficiently wide range of behavioural recommendations to generate a high integrity behaviour selection. In such situations, the range of circumstances in which some of the undetected conditions are detected is slightly expanded so that they are detected, and their recommendations added. Such expansions are the primary way in which conditions are defined. Conditions detected shortly after a behaviour is performed can recommend changes to the weights in favour of that behaviour. These changes are called reward behaviours and are the primary way in which recommendation weights are defined.
NEURON LEVEL IMPLEMENTATION OF BRAIN INFORMATION PROCESSES
A pyramidal neuron defines a condition that is made up of a group of similar circumstances, and detects any occurrence of a significant subset of those circumstances. The condition is called the receptive field of the neuron. Such pyramidal neurons are located in the cortex. Medium spiny neurons in the striatum of the basal ganglia interpret inputs from pyramidal neurons as behavioural recommendations, and the basal ganglia determine and implement the most strongly recommended behaviour. Reward behaviours that change recently used recommendation weights are also recommended by pyramidal neuron receptive field detections and selected by the basal ganglia. Behaviours that expand receptive fields are also recommended by pyramidal neuron receptive field detections but such behaviours are selected by the hippocampus.
DESCRIPTION OF SUBJECTIVE SENSORY EXPERIENCE IN TERMS OF NEURON ACTIVITY
In any situation, a wide range of neurons are active in the brain. This activity corresponds with the subjective experience of the brain at the time. In information terms this subjective experience corresponds with condition detections and their associated ranges of behavioural recommendations, plus the implementations of selected behaviours. Activity in the cortex corresponds with receptive field detections of pyramidal neurons and activation of their associated ranges of behavioural recommendations. Activity in the basal ganglia corresponds with selection of the most strongly recommended behaviours. Activity in the thalamus corresponds with implementation of the selected behaviours. Activity in the cerebellum corresponds with implementation of often used sequences of behaviours. Activity in other anatomical structures supports these primary activities.
During some sensory experience, a population of pyramidal neurons is active across the cortex. Each neuron in the population has a receptive field defined by a complex combination of sensory inputs, and that receptive field is present in the current sensory inputs. Hence the population can be regarded as detecting a wide range of different aspects of the current sensory situation. Each of the active pyramidal neurons has a range of recommendation strengths in favour of different behaviours. These recommendation strengths are located in the basal ganglia, which determines the total strengths of each possible behaviour across all the currently active pyramidal neurons. The largest such total strengths are generally in favour of behaviours appropriate in response to the current sensory environment, such as naming an observed visual object.
If the experience is very similar to a past experience, many of the same neurons will be active. However, even then all of the neurons will also have been active in many different past experiences, and will have some behavioural recommendation strengths appropriate in those past experiences. For example, a neuron with a receptive field corresponding with vaguely spherical circumstances as discussed earlier could have recommendation strength in favour of a number of different speech behaviours like saying “that is a ball”, “that is an apple”, “that is an onion”, and “that is the moon”. All these recommendation strengths will be present when the neuron is activated, but only a speech behaviour strongly recommended by a significant number of active neurons has a chance of being implemented.
HOW WE THINK BEYOND CURRENT SENSORY INPUTS
Humans are able to plan for situations with little resemblance to their current sensory environment. Furthermore, in some situations the current sensory inputs are an inadequate guide to determining current appropriate behaviour. For example, in social situations the appropriate response to a group of people requires access to a lot of information about past interactions with the individuals and different groups of individuals. How can neurons with more appropriate recommendation strengths be activated when their receptive fields are not present in current sensory inputs? Such indirect activation cannot be on the basis cognitive information because such information is unavailable to physical neurons. The only basis for such indirect activation is past temporal correlations between the activity of an inactive neuron and activity of some currently active neurons. For example, a neuron could be indirectly activated on the basis of frequent past activity at the same time as a group of currently active neurons. Alternatively, a neuron could be indirectly activated on the basis that on one occasion it was active at the same time as a group of currently active neurons, and at that time a lot of the neurons were changing their receptive fields. These two mechanisms are, respectively, the basis for semantic and episodic memory retrievals.
Hence if a population of pyramidal neurons directly activated by sensory inputs does not have a predominant recommendation strength in favour of an external response, to expand the available range of recommendation strengths the population can indirectly activate a second population on the basis of correlations in past activity. This second population could then indirectly activate a tertiary population and so on, the process continuing until there is a predominant recommendation strength.
If every neuron could indirectly activate every other neuron with which it had some past temporal correlation in activity, the result would be an excessive and chaotic pattern of activation. Hence indirect activations are behaviours that are recommended by neuron activity, and only implemented if the total recommendation strength across many active neurons is substantial.
THE PHYSIOLOGICAL BASIS OF QUALIA
When we look at the colour red, receptive fields will be directly detected within the sensory inputs. Many of the fields will often have been active in the past when the colour red was seen, and a significant proportion will have recommendation strength in favour of saying “That is red”. However, focussing attention on the colour is effectively initiating a search for behaviours beyond those immediately recommended. As described earlier, in order to gain access to a wider range of behavioural recommendations, groups of receptive fields can indirectly activate other groups on the basis of temporally correlated past activity.
Cortical column receptive fields directly and indirectly activated in three cortical areas in response to visual perception of the colour red. Receptive fields actually present in the visual input are coloured red. The direct activation in the three areas is illustrated twice. On the left, columns indirectly activated on the basis of frequent past activity at the same time as some directly activated columns are coloured pink. On the right, columns indirectly activated on the basis of past activity at the same time as some directly activated columns when significant changes to receptive fields occurred are also coloured pink. In practice, all the coloured columns would be active simultaneously. Different groups of the activated columns are parts of the populations of columns active in the past during various different experiences. Each part will have some recommendation strength in favour of a range of behaviours such as describing the past experience. However, the only total recommendation strengths large enough to be selected and implemented are those appropriate to the perception of the colour red, such as saying "that is red". The large population of indirectly activated columns is therefore experienced as a rich experience that cannot be described verbally.
If we have often seen some type of red object like a strawberry, some of the neurons directly activated in response to the red colour will often have been active in the past when strawberries were present. If we have had novel and memorable experiences in which something red was present, such as watching a sunset when at a beach party, some of the directly activated neurons will have been active at the time of the experience. In both cases, the directly activated neurons may indirectly activate more of the neurons active during the past experience. So one way of looking at the enhanced experience of red, or the red quale, is that it is made up of fragments of many past experiences in which red featured in some way. There could be fragments of perceptions of red balloons, red birds, red berries or red flowers etc. There could be fragments of past events such as meeting a woman in a red dress, drinking red wine, or donating blood and so on. Each fragment will have some recommendation strength in favour of behaviours at the time of these past experiences, but because they are only fragments the total such strength for any one fragment will be small. In particular, no fragment will have sufficient recommendation strength to drive a verbal description of the past experience.
The net effect will therefore be a richer experience, because of the larger number of neurons activated in many past sensory experiences, but it will be impossible to verbally describe the richer experience in any detail.
IS THE EXPERIENCE OF A RED QUALE ALWAYS THE SAME?
The population of neurons activated in the same brain in response to seeing the colour red will generally be different each time. Firstly, there may be differences in the exact tone, illumination and shape of the red colour, resulting in direct activation of somewhat different neurons. Secondly, when the colour is seen, many neurons are already active as a result of experiences a little earlier. The population that drives the indirect activations will therefore be somewhat different. Furthermore, depending on our experiences between two occasions when we perceive the colour, some indirect activation recommendation strengths may have changed, also resulting in population differences.
There will be much larger differences when different people look at the colour red. Firstly, neuron receptive fields are defined, with some degree of randomness, by experience. Every individual has a different experience, hence there is little chance that any receptive field in one brain will correspond exactly with a field in another brain. Furthermore, the events experienced by each brain are different, so the indirectly activated fragments are derived from quite different sources. For both brains, the red quale is made up of a population of active neurons in which many subgroups are the same as subgroups of the populations active in past experiences. However, there are minimal correspondences between the receptive fields of the active neurons in the two brains.
The experience of red can be described on a more detailed level in terms of neuron activations, but another question sometimes asked is “Why does experiencing the red colour feel as it does and not feel something different?”. The problem with this question is that although it feels profound, as far as we know it is unanswerable in any scientific terms. It could be paraphrased as “Why is reality as it is and not something different?” which is similar to what is sometimes called the ultimate philosophical question “Why does something exist rather than nothing?”.
WHAT IS ANIMAL EXPERIENCE LIKE AND COULD THERE BE ZOMBIES?
It can be demonstrated that any system or animal which must learn a complex combination of behaviours is constrained into a specific high level architectural form. In this form, conditions are defined from past experience, and each condition is associated with a range of recommendation strengths. Any previously defined conditions that are present in current experience are activated and their ranges of recommendation strengths made available.
In humans, the condition definitions include a lot of information derived from visual inputs, quite a bit derived from somatosensory inputs from the body, and rather less information derived from auditory inputs and taste/smell. The combination of information from different sources that makes up each condition depends on individual experience organized with a degree of randomness. Compared with a human, in an animal like a bat rather more of the information that defines conditions is derived from auditory inputs, and rather less from visual. Hence the defined conditions will be much more different from human than the differences between two humans. However, the subjective experience of the bat still corresponds with the activation of a population of conditions. It is therefore possible to understand the experience of a bat in terms of neural activity. The sometimes asked question “What is it like to be a bat” can thus be answered in terms of neural activity and its similarities to and differences from neural activity in humans. However, it is not possible to subjectively experience what a bat experiences, any more than it is possible to experience what another human experiences. Subjective experience is very sensitive to the sensory systems available and the exact past experience.
Subjective experience is essentially a high level description of the neural activity, and any complex learning system must determine behaviour by ongoing activation of conditions defined in past experience. Hence the experience of any a complex learning system will be supported by activation of populations of these conditions. In physical terms, there can be no zombies in the sense of beings without this ongoing condition activation. Scientifically, subjective experience is a high level description of the detailed neuron activity.
MARY THE COLOUR SCIENTIST
As described earlier, the hypothetical colour scientist Mary knows all about colour, but has only ever seen black and white. She finally sees coloured objects for the first time. The philosophical argument is that Mary has not come to know any new information about colour, so the subjective experience is something different from physical knowledge.
However, when only exposed to black and white the conditions defined and detected in her brain did not include the combinations of sensory inputs that occur when looking at red, or blue, or other colours. It is likely that Mary at this point would not be able to discriminate visually between different colours. The first exposures to colours are novel experiences, which will result in pyramidal neuron receptive field expansions that physically define new conditions on the neurons and allow such discrimination.
When her subjective experience of colour is described at a neural level, the population of activated neurons includes additional neurons that have expanded their receptive fields to include new physical conditions. Hence that different subjective experience includes new information physical information about colour.
CONCLUSIONS
Subjective experience can be described in terms of the activation of cortical pyramidal neurons. Each neuron has a receptive field defined by complex combinations of sensory inputs that have often occurred together in the past. No complex combination corresponds a cognitive category, but is associated with a range of recommendations in favour of different behaviours. Pyramidal neurons are directly activated by the presence of one of their programmed combinations in current sensory inputs, or indirectly activated as a result of past temporal correlations between their activity and the activity of groups of neurons that are already active.
This description provides insights into some of the philosophical issues that have been raised about subjective experiences. Qualia such as the experience of the colour red correspond with activations of populations of pyramidal neurons, each neuron generations a range of recommendations in favour of different behaviours. A strong recommendation of the population as a whole could be a red appropriate behaviour such as saying “that is red”. However, such populations can be also regarded as being made up of different size fragments of many different past experiences. Each fragment will have some total recommendation strength in favour of speech describing the past experience, but very few of the fragments will have a large enough total to achieve an implemented behaviour. Hence subjectively the experience will be very complex but impossible to describe verbally in detail.
The net effect will therefore be a richer experience, because of the larger number of neurons activated in many past sensory experiences, but it will be impossible to verbally describe the richer experience in any detail.
IS THE EXPERIENCE OF A RED QUALE ALWAYS THE SAME?
The population of neurons activated in the same brain in response to seeing the colour red will generally be different each time. Firstly, there may be differences in the exact tone, illumination and shape of the red colour, resulting in direct activation of somewhat different neurons. Secondly, when the colour is seen, many neurons are already active as a result of experiences a little earlier. The population that drives the indirect activations will therefore be somewhat different. Furthermore, depending on our experiences between two occasions when we perceive the colour, some indirect activation recommendation strengths may have changed, also resulting in population differences.
There will be much larger differences when different people look at the colour red. Firstly, neuron receptive fields are defined, with some degree of randomness, by experience. Every individual has a different experience, hence there is little chance that any receptive field in one brain will correspond exactly with a field in another brain. Furthermore, the events experienced by each brain are different, so the indirectly activated fragments are derived from quite different sources. For both brains, the red quale is made up of a population of active neurons in which many subgroups are the same as subgroups of the populations active in past experiences. However, there are minimal correspondences between the receptive fields of the active neurons in the two brains.
The experience of red can be described on a more detailed level in terms of neuron activations, but another question sometimes asked is “Why does experiencing the red colour feel as it does and not feel something different?”. The problem with this question is that although it feels profound, as far as we know it is unanswerable in any scientific terms. It could be paraphrased as “Why is reality as it is and not something different?” which is similar to what is sometimes called the ultimate philosophical question “Why does something exist rather than nothing?”.
WHAT IS ANIMAL EXPERIENCE LIKE AND COULD THERE BE ZOMBIES?
It can be demonstrated that any system or animal which must learn a complex combination of behaviours is constrained into a specific high level architectural form. In this form, conditions are defined from past experience, and each condition is associated with a range of recommendation strengths. Any previously defined conditions that are present in current experience are activated and their ranges of recommendation strengths made available.
In humans, the condition definitions include a lot of information derived from visual inputs, quite a bit derived from somatosensory inputs from the body, and rather less information derived from auditory inputs and taste/smell. The combination of information from different sources that makes up each condition depends on individual experience organized with a degree of randomness. Compared with a human, in an animal like a bat rather more of the information that defines conditions is derived from auditory inputs, and rather less from visual. Hence the defined conditions will be much more different from human than the differences between two humans. However, the subjective experience of the bat still corresponds with the activation of a population of conditions. It is therefore possible to understand the experience of a bat in terms of neural activity. The sometimes asked question “What is it like to be a bat” can thus be answered in terms of neural activity and its similarities to and differences from neural activity in humans. However, it is not possible to subjectively experience what a bat experiences, any more than it is possible to experience what another human experiences. Subjective experience is very sensitive to the sensory systems available and the exact past experience.
Subjective experience is essentially a high level description of the neural activity, and any complex learning system must determine behaviour by ongoing activation of conditions defined in past experience. Hence the experience of any a complex learning system will be supported by activation of populations of these conditions. In physical terms, there can be no zombies in the sense of beings without this ongoing condition activation. Scientifically, subjective experience is a high level description of the detailed neuron activity.
MARY THE COLOUR SCIENTIST
As described earlier, the hypothetical colour scientist Mary knows all about colour, but has only ever seen black and white. She finally sees coloured objects for the first time. The philosophical argument is that Mary has not come to know any new information about colour, so the subjective experience is something different from physical knowledge.
However, when only exposed to black and white the conditions defined and detected in her brain did not include the combinations of sensory inputs that occur when looking at red, or blue, or other colours. It is likely that Mary at this point would not be able to discriminate visually between different colours. The first exposures to colours are novel experiences, which will result in pyramidal neuron receptive field expansions that physically define new conditions on the neurons and allow such discrimination.
When her subjective experience of colour is described at a neural level, the population of activated neurons includes additional neurons that have expanded their receptive fields to include new physical conditions. Hence that different subjective experience includes new information physical information about colour.
CONCLUSIONS
Subjective experience can be described in terms of the activation of cortical pyramidal neurons. Each neuron has a receptive field defined by complex combinations of sensory inputs that have often occurred together in the past. No complex combination corresponds a cognitive category, but is associated with a range of recommendations in favour of different behaviours. Pyramidal neurons are directly activated by the presence of one of their programmed combinations in current sensory inputs, or indirectly activated as a result of past temporal correlations between their activity and the activity of groups of neurons that are already active.
This description provides insights into some of the philosophical issues that have been raised about subjective experiences. Qualia such as the experience of the colour red correspond with activations of populations of pyramidal neurons, each neuron generations a range of recommendations in favour of different behaviours. A strong recommendation of the population as a whole could be a red appropriate behaviour such as saying “that is red”. However, such populations can be also regarded as being made up of different size fragments of many different past experiences. Each fragment will have some total recommendation strength in favour of speech describing the past experience, but very few of the fragments will have a large enough total to achieve an implemented behaviour. Hence subjectively the experience will be very complex but impossible to describe verbally in detail.