by Sanjay Manohar, 2001A "linked list" of common theories of consciousness
Phenomena such as the effect of brain damage or drugs on conscious states, and considerations that humans are conscious where amoebae (maybe) are not, motivate many varied theories of consciousness. They all aim to explain these things and perhaps also determine some unknowns, e.g. whether intermediate animals are conscious, or whether computers can be conscious.
Although the theories come from diverse branches of knowledge, there is much to suggest that we should break down today's notion of consciousness into a few different kinds. We can divide the evidence into a groups according how much they interact in a causal way. The numbers and letters (e.g. 2b) refer to connections between theories that may be linked - they could be different facets of the same theory.
- Thalamus The thalamus is a deep nucleus of the prosencephalon which connects to most areas of the cerebral cortex. It straddles the midline, and all fibres entering the cortex synapse here first. Knowledge about this area has grown immensely in the last 10 year, but is probably still in its very early infancy.
- Brain stem The brain stem is an evolutionarily conserved area which is necessary for vital functions like breathing, projects axons widely into the cortex from the 'reticular activating system', whose activity is required for many vertebrate organisms to remain alert.
- Basal ganglia Multiple interconnected nuclei deep within each hemisphere receive fibres from the cerebral cortex, and send fibres variously to the thalamus (and thus back to the cortex), brainstem, cerebellum and spinal cord.
- Neural synchrony In the 1980s several experiments showed that brain cells receiving the same signal from two sources are more likely to pass the information on if the two inputs are synchronised, in terms of action potential timing. It has become clear that spike-timing is critical in what information is "amplified or attenuated" by a cell.
- Attention Attention is the ability to select certain features of the environment, at exclusion of others, for further processing. Further processing can mean, for example, the production or priming of an action, or storage in memory. Although attention is obviously confounded with just those things that we can observe psychologically, it correlates well with common descriptions of how conscious representations should behave.
- Linguistic capability The ability to manipulate the order of symbols to change meanings allows a potentially infinite set of token combinations, and therefore meanings. This faculty, which appears all-or-none, means that a single mental token can be put to any use, and can be involved in any semantic operation. So far it seems that humans are the only organisms that can uncontraversially do this.
- Wave function collapse According to a common interpretation of quantum mechanicals, the wave function of a system descripbes superpositions of multiple states of the world - and these all have some objective existence, with varying probabilities. When measurements are made, the wave function appears to collapse stochastically to a reduced set of states. From a subjective viewpoint, the moment that collapse occurs must be at the moment I know the result of the measurement. Consciousness therefore appears to infect every world state that can be measured, suggesting that what I know determines how physical processes proceed.
- Algorithmic complexity If a person's brain were replaced cell-by-cell by electronic components that perform the same function, it seems that their thought processes (and consenquentially consciousness) is preserved. Such arguments indicate that computers that emulate thought have this kind of equivalence with real minds. Strong artificial intelligence arguments go one step further and suggest that this equivalence is sufficient for consciousness: as long as the algorithms of the mind are being actively performed by any device, that device can be said to be conscious. The Turing test could be seen as a practical spin-off from this view.
- Topological geometrodynamics ??
- Free will The feeling of being free (to act, choose, or think) seems tightly bound to being conscious. Compatibilists often take this sense in which we are free as a datum, although psychologists treat it as a datum to be explained in a critical manner.
- Communication To be conscious may require a degree of self-representation, which can only arise by acquiring the linguistic tokens to be able to represent distinctions such as self and other.
- Time Relating the tensed perspective (looking out from the subjective present) and untensed perspective (events being before and after one another) requires a mechanism by which the subjective now is placed in relation to particular temporal events. Such a mechanism would need to explain why there is only ever a 'here and now', but would thereby go a good way to explaining the subjectivity of consciousness.
- Intentionality/functionalism The strong claim that there is no more to consciousness than how we represent ourselves as being conscious has understandably been met with some disappointment. However such an argument does answer some of a dualist's questions, in particular it may be able to provide a critical assessment of why we experience consciousness as such a peculiar thing. It is, of course, not easy to argue against functionalism, even when one's intuitions may oppose it.