Even a quick glance at the adjacent picture should bring to mind for most people not only the name of this famous person, but a whole host of associated information – what he does (he’s an actor), perhaps some movies he’s been in, who he’s married to, maybe even who he’s no longer married to. Most of this information (depending on one’s level of familiarity with the particulars of the gentleman in question) will have sprung to mind automatically and effortlessly– indeed, it would be very difficult to actively stop it springing to mind, once the person is recognised. (Try thinking of an elephant without thinking of what colour it is).
For some people, however, it is anything but effortless – it is impossible. Readers with prosopagnosia, for example, may still be waiting to find out who the hell I’ve been writing about (it’s Brad Pitt). This condition, also known as face blindness, impairs the ability to recognise people by their faces – the visual stimulus of the face is not linked in the normal way to the rest of the associated information. Similarly, people with colour agnosia may be perfectly able to think of elephants without thinking of the colour grey – in fact, they might be unable to bring the appropriate colour to mind even if asked to. In this case, colour is not connected into the wider concepts of types of objects.
These networks of associations are called “schemas” – the mental representations of the various attributes of a person, object or concept. The interesting thing about them is that they link very different types of information (from the different senses, for example) – information that is processed in very different parts of the brain. For a schema to emerge that includes all the relevant information, all the relevant regions of the brain have to be talking to each other. One hypothesis to explain conditions like prosopagnosia or colour agnosia is that the face or colour regions of the brain are not wired into the wider network normally.
Schemas are built up through experience – we learn that bananas are yellow and curved and about so big and a bit mushy and smell and taste like banana. Just thinking of a banana will bring many of those attributes to mind (though, interestingly, usually not the smell or taste, even though smelling or tasting one can activate the schema). Similarly, we learn that the letter “A” looks like that, or like this: “a” or this: “a” or this: “a” and makes a sound like “ay” or “ah” or the way New Zealanders pronounce it, which cannot be written down.
This kind of learning is believed to involve strengthening connections between ensembles of neurons that represent the various attributes of an object (or type of object). If these various attributes reliably occur together, then the connections between these representations are strengthened. So much so, that activating the representation of one of the attributes of an object (like the shape of the letter A) is usually enough to cross-activate the representations of its other attributes (such as its canonical sounds).
Schemas are thus the neural substrates of knowledge – the statistical regularities and contingencies of our experience wired into our neural networks. The conditions referred to as “agnosias” – literally the lack of knowledge of something – can generally be thought of as a failure to link all the attributes of an object into a schema. These include the conditions mentioned above, but also other types of object agnosia (which are quite diverse and sometimes bizarrely specific), as well as things like congenital amusia (also known as tone or tune deafness) and dyslexia and dyscalculia.
Agnosias can be acquired – usually caused by injury to specific parts of the brain. But there is a growing appreciation that they can also be congenital, and, in some cases, are very clearly inherited in what seems like a Mendelian manner. These conditions are also not as rare as one might expect – face blindness may affect 1-2% of the population, for example. Familial inheritance of this condition, as well as congenital amusia and colour agnosia have all been documented and the high heritability of dyslexia and dyscalculia is well known.
Specific mutations may thus cause an inability to link faces to other information about people, or to link the sounds and shapes of letters, or to link colour to the rest of our knowledge about objects. This may sound like a paradox – after all, I have just been saying that the development of these schemas depends on experience. That is true, but the ability to link different attributes together depends on the wiring of the brain, which can be affected by genetic mutations. If the different regions of the brain are not connected normally then the opportunity for experience to link contingent stimuli will not arise.
There is evidence of physical differences in connectivity in the brains of people with some of these conditions, particularly between regions processing the various stimuli. These conditions may also be characterised by not only a disconnection between various elements of a network – which represent various attributes of an object – but by a further disconnection between these networks and frontal areas that mediate conscious awareness.
There is good evidence in prosopagnosia, for example, that faces of familiar people are actually “recognised” by visual brain areas (which show a different response than to strange faces), even though the person is unaware of this recognition. Similar results have been reported for congenital amusia, where discordant notes are detected by the brain, but not reported to the mind of the person.
If these disorders can be thought of as “disconnection syndromes”, the condition of synaesthesia may reflect just the opposite – hyperconnectivity between brain areas. This condition is usually thought of as a cross-sensory phenomenon – a triggering of visual percepts by sounds, for example, or the experience of tactile shapes triggered by tastes. In many cases, however, the triggers and the induced experiences are not sensory, but cognitive – the automatic association, conceptually, of some extra attribute into the schema of an object. This extra attribute may or may not be actively experienced as a percept.
For example, a common experience in synaesthesia is having a coloured alphabet, where different letters “have” different colours. These associations are highly idiosyncratic but also very stable and very definite – the colour of a particular letter is as much an integral part of its schema for that person as the shapes and sounds associated with it. Similarly, the shape of a taste or the taste of a word, the smell of a musical note or the personality of a number – these are all extra attributes integrally tied to the larger concepts of the inducing stimuli.
Like many agnosias, synaesthesia runs very strongly in families and most are characterised by an apparently simple mode of inheritance. It is quite common for different members of a family to have different types of synaesthetic experiences, however. This suggests that a general predisposition to the condition can be inherited, but that the particular form that emerges depends on additional factors, possibly including chance variation in brain development as well as experience.
One hypothesis to explain synaesthesia is that genetic differences affect the wiring of networks of brain areas, resulting in this case in the inclusion of extra areas into networks to which they do not normally belong. An initial difference in wiring may then alter the subjective experience of the person as they are learning to recognise and categorise various types of stimuli (such as letters, numbers, musical notes, flavours, etc.). If, for example, the “colour area(s)” of the brain are reliably co-activated when letters are being learned, then the experienced colour will be automatically incorporated into the schema of each letter.
We do not yet know the identity of any of the affected genes in these conditions, (with a couple exceptions for dyslexia), but it is likely they will be discovered in the near future. It is important to note that these will not be genes “for reading” or “for face processing” or “for not thinking letters have colours”. Their normal functions may be far removed from the effects when they are mutated. For example, one gene known to result in a condition characterised by dyslexia encodes a protein required for normal cell migration. Altered neuronal migration leads to groups of ectopic neurons located in the white matter of the brain – these are thought to impair communication along these nerve fibres, resulting in disconnection of areas required for linking the visual shapes of graphemes with their associated phonemes (that’s the working hypothesis at least). This is a gene for neuronal migration, not for reading.
But the identification of genes involved in these conditions will tell us a lot about an area of developmental neurobiology we still know very little about – how different areas of the cerebral cortex become, on the one hand, specialised to process specific types of information and, on the other, integrated into larger networks that allow different aspects to be associated through experience. This relies on both the initial wiring of cortical networks, driven by a genetic program, and their subsequent refinement as we learn that elephants are grey, bananas are mushy and Brad Pitt is one lucky son-of-a-bitch.
For more on this, see here.