You see, but you do not observe…
A Scandal in Bohemia, The Adventures of Sherlock Holmes: Arthur Conan Doyle
What is the distinction between seeing and observing? The term ‘seeing’ suggests a passive process, whereas observation clearly requires something additional; the attention to a particular detail or details within the visual scene, the extraction of salient information and perhaps the further evaluation of that information. Neuroscience has made great strides in understanding the functioning of our basic sensory mechanisms, such as those that allow seeing. This work has reached such a level that we are now coming close to being able to create ‘bionic eyes’; mechanical replicas which can mimic the workings of damaged parts of the visual system (1). However is it a much harder task to fully understand the myriad of different ‘higher order’ functions that serve to differentiate observation from merely seeing. These functions are the reason that human experience is much more than the sum of the output from our sensory systems. At the heart of this problem is the need to understand the phenomenon of consciousness. Consciousness can be difficult to define precisely, with different philosophers breaking consciousness down into different sets of features (2) producing concepts that, perhaps inevitably, tend to be somewhat vague and potentially overlapping. However the most fundamental aspect of consciousness would appear to be our ability to experience awareness of (certain) sensory information, and to impose our higher order abilities on that information. In short, given that the majority of sensory processing is performed outside of consciousness, how is it that certain information can be sectioned off and subject to processes such as attention, evaluation and reflection, and how is it that we are aware of both the selected data, and the cognitive processes we perform on it?
Brain waves and synchronisation
The simplest way of addressing the issue of consciousness is to compare the response of the brain during circumstances where the level of consciousness awareness is different. It has long been known that states of consciousness (such as wakefulness, sleep and coma) are marked by differences in the pattern of ‘brain waves’; the oscillating electrical signals that are produced by the brain. It would seem sensible therefore to assume that such changes in the pattern of brain waves reflect, at least in part, changes in the functioning of the mechanism that enables consciousness. Similar changes in brain oscillations are also seen in a wide variety of different brain areas during performance of cognitive tasks, which of course also require the conscious processing of information. In general cognitive processes appear not only to alter the power of such oscillations, but also to evoke an increase in synchronisation between these oscillations (such that the phase difference between the signals generated from the brain areas activated by the task remains constant over time). Such synchronisation is believed to allow communication between disparate brain areas; so-called ‘communication through coherence’ (3). If one takes the simple example of one neuronal population passing a signal to another, then to provide the greatest likelihood of that signal being received, the sending neurons must all fire at the same time (hence the oscillating nature of brain waves) thus maximising the signal sent to the receiving neurons. However the timing of this signal is also important. To maximise the chance of the signal being propagated, the firing of the sending neurons must be timed so that the signal arrives at a time when the receiving neurons are optimally receptive to the signal (or alternatively, if inhibition of signalling is required, at a time when the receiving neurons are optimally insensitive of the signal). Therefore when different brain areas need to communicate in order to facilitate cognitive processing their pattern of neuronal firing much achieve coherence, so they tend to synchronise with (for unidirectional, excitation signals at least) the conduction delay between the two areas being equal to the phase difference between the two oscillating signals.
Global Neuronal Workspace
As the cognitive tasks that produce neural synchrony all require conscious processing of some sort, we would expect that the experience of consciousness in general must rely on changes in synchrony between brain areas. Indeed studies that have directly compared conscious vs non conscious processing (e.g. comparing instances where the same stimulus is consciously perceived versus instances where it is not) have found an increase in synchronisation between distant cortical sites not directly related to the processing of the relevant sensory information (e.g. 4). Evidence from several MRI studies suggests that the location of these synchronising sites is consistent across different tasks, involving a specific set of areas in the frontal and parietal lobes as well as the thalamo-cortical circuits that control the flow of sensory information to and from the cortex (see 5 for a review). The relevance of this finding to consciousness is supported by evidence that the source of the altered brain response between different states of consciousness appears to be generated by a similar set of areas (6). This has led to the idea that these brain areas represent a ‘global neuronal workspace’ (GNW: 5,7) that supports consciousness. The GNW system is thought to be able to orchestrate synchronisation between different sensory processing areas in such a way as to allow certain sensory representations to be amplified and maintained, while inhibiting others. As synchronisation facilitates neuronal communication it may allow the specific information being held within different sensory areas to form a single, multi-sensory representation within the workspace, explaining how the conscious experience of perception is of a unified sensation, despite the fact that information from each sense is analysed separately (8 – the ‘perceptual binding’ problem). In addition the parietal and frontal areas of the GNW contain a large number of neurons with long axons which allow these areas to project information to a wide variety of disparate brain areas. This in turn is thought to allow them to make the representation held within the GNW available to the areas of the brain involved in higher processing functions. In effect the amplified representation that is maintained by the GNW is also broadcast to these other processing sites, thus allowing higher order processing of conscious information. It is this selection and amplification of a specific representation, and it’s subsequent global availability (to other brain areas) which we experience as consciousness. The concept of synchronous firing and a global neuronal workspace may also help explain other aspects of the conscious experience, such as metacognition (our ability to perform mental processing on the outputs of other mental processing e.g. to know what we know). Metacognition may simply be the conscious component of a much larger perceptual system that is continuously reflecting on our own activity and its likely consequences (9). The metacognition we experience consciously may therefore simply be the instances where this process reaches conscious access via the GNW and is therefore exposed to other higher order processing functions.
The consequences a neural explanation of consciousness
The study of the neural basis of consciousness is an exciting, but complex subject. It also however raises significant philosophical questions. The idea that consciousness is merely a manifestation of the firing patterns of neurons and their arrangement vis-a-vis each other is not a particularly controversial conclusion from a neuroscience perspective, as one would expect every aspect of human cognition to manifest via changes in brain physiology. However the topic is controversial in general because it suggests that if something as core to our being, to our experience of being ‘human’, as consciousness is in fact solely reliant on biological mechanisms, then concepts such as the mind, the soul and free are redundant. If there is no ‘ghost in the machine’ driving our conscious behaviour then are we really nothing more than just a collection of tissue; are we really just, in effect, extremely complex machines? The consequences of this discussion has important implications for philosophy and morality (for an interesting discussion on this topic see 10). More optimistically however, the ability to understand the biological underpinnings of consciousness can lead to greater understanding of the basis of neurological disorders that cause the loss of conscious abilities, and of psychiatric symptoms that relate to the disruption of consciousness. For example many people suffering from forms of psychosis can experience what could be termed failures of consciousness, such that patterns of conscious thought become disordered, or that they may feel that their thoughts are being read or even controlled by others. An understanding as to how the brain generates consciousness is surely an important step in identifying what has gone wrong in these situations, and potentially how they can be remedied.
Image ‘Idea and Creative Concept’ by ‘Mr Lightman’, courtesy of freedigitalphotos.net http://www.freedigitalphotos.net/images/view_photog.php?photogid=3921
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8. Varela, F., Lachaux, J.P., Rodriguez, E., and Martinerie, J. (2001). The brainweb: Phase synchronization and large-scale integration. Nat. Rev. Neurosci. 2, 229–239. http://www.nature.com/nrn/journal/v2/n4/abs/nrn0401_229a.html
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