Mind the gapUntil recently, consciousness was something of a taboo subject for scientists, most of whom seemed happy to leave such esoteric studies to philosophers and psychoanalysts. It is almost universally agreed that consciousness arises from the activity of the brain, and yet we are still waiting for a simple, satisfactory, brain-centred explanation of consciousness. An unhappy state of affairs, but one that may not be around for much longer, for scientists have finally woken from their slumber: armed with a flood of new discoveries from the neurosciences, they have been able to provide several competing models of consciousness.
One of these models, the Global Workspace Model, has recently gained support from a study by Raphael Gaillard and his colleagues in Paris, who showed that conscious visual information is rapidly and widely distributed across the brain, provoking the synchronised brain activity that is taken by the model to be the hallmark of consciousness.
The Global Workspace Model was first proposed by the cognitive scientist Bernard Baars in the late 1980s and proposes that at any given time, many modular cerebral networks are active in parallel, and process information in an unconscious manner. Regarding incoming visual information, for example, it becomes conscious if and only if the following three conditions are met:
Condition 1: information must be explicitly represented by the neuronal firing of perceptual networks located in the primary visual cortex at the rear of the brain.
Condition 2: this neuronal representation must reach a minimal threshold of duration and intensity in order to bring in a second stage of processing, distributed across the brain’s cortex, and especially involving the prefrontal cortex, which is believed to be a major centre for associating multiple kinds of information.
Condition 3: through joint bottom-up propagation (condition 1) and top-down attentional amplification (condition 2), the ensuing brain-scale neural assembly must “ignite” into a self-sustained reverberant state of coherent activity that involves many neurons distributed throughout the brain.
Why would this ignited state correspond to a conscious state? Gaillard et al. argue that the key idea behind the workspace model is that because of its massive interconnectivity, the active coherent assembly of workspace neurons can distribute its contents to a great variety of other brain processors, thus making this information globally available. The global workspace model postulates that this global availability of information is what we subjectively experience as a conscious state.
Just how did Gaillard and his colleagues set about measuring the neural signature of the conscious perception of a visual stimulus? Using patients with medically intractable epilepsy who, in preparation for surgery, had multiple shallow recording electrodes implanted within their cerebral cortices to locate seizure activity, they were able to directly record the patients’ neural activity as it happened. According to the scientists, this “unique opportunity” afforded greater spatial and temporal resolution than noninvasive methods used previously to probe the neural basis of consciousness, such as functional magnetic resonance imaging, which can only scan the brain about once every two seconds. They compared neural activity concomitant with conscious and non-conscious processing of words by using a visual masking procedure that allowed them to manipulate the conscious visibility of briefly masked words.
The scientists showed the subjects a computer screen upon which they projected a set of hash marks (which acts as the mask) for 400milliseconds (ms), then a word for 29ms, and then either a blank screen or a set of ampersands (another mask) for 71ms. The entire sequence thus took only half a second, but in both cases the word was registered at the earliest stages of visual processing, as shown by electrical activity in the primary visual cortex, thus meeting the first condition of the global workspace model.
When the subjects were exposed to a word followed by a second mask, they could only guess at the nature of the word they saw. But when subjects were exposed to a word and the second mask was absent, the word was consciously reportable and readable, so the scientists could compare masked (non-conscious) perception and unmasked (conscious) perception of briefly flashed words.
Non-conscious perception of words elicited short-lasting activity across multiple cortical areas, including parietal and visual areas. In sharp contrast, only consciously perceived words were accompanied by long-lasting effects (>200 ms) across a great variety of cortical sites, with a special involvement of the prefrontal lobes. This sustained pattern of neural activity was characterised by a specific increase of coherence between distant areas, suggesting conscious perception is broadcasted widely across the cortex.
Gaillard and his colleagues do acknowledge one shortcoming with their research, which is that “whenever a subject is conscious, he is necessarily conscious of a given mental content.” Consciousness is “intentional” in form (it is “about” a certain content), and the scientists admit it may be illusory to look for a “pure” form of consciousness independent of its particular contents and of the tasks that initiate it.
When applied to neuroscientific experiments, this implies that when imaging a brain having some conscious experience, one will necessarily observe activations corresponding to a specific conscious content. Can one really extrapolate from this to make generalisations about consciousness as a whole? Gaillard et al. believe that further experiments with other kinds of stimuli are clearly necessary, as they will reveal which late-stage, widespread brain events are common to all conscious processing, and which are specific to the experiment at hand.
Another shortcoming is perhaps more telling. Gaillard’s research joins a growing body of scientific evidence over the last few decades (from such techniques as electroencephalography, positron emission topography, and functional magnetic resonance imaging) that have helped to show how observable brain activity correlates with our inner feelings of blueness, sadness, pleasure, pain, and all the other subjective qualities that make up our conscious awareness.
But these neural correlates of consciousness, as they are known, do nothing to explain just how it is that a particular group of neurons brings about our feeling of blueness. They do nothing to close what philosophers term the explanatory gap, as Ned Block and Robert Stalnaker explain:
“Suppose that consciousness is identical to a property of the brain – say activity in the pyramidal cells of layer 5 of the cortex involving reverberatory circuits from cortical layer 6 to the thalamus and back to layers 4 and 6 – as Crick and Koch have suggested for visual consciousness. Still, that identity itself calls out for explanation!”
For all its sophistication and advancement upon earlier research, in the end the research of Galliard and his colleagues does nothing to close the explanatory gap between the scientific explanation of a mental content and the actual experiencing of the content itself.
As the philosopher Uriah Kriegel has put it “There is a persistent feeling that scientific theories of consciousness do not do much to explain phenomenal consciousness. Moreover, there is a widespread sense that there is something principled about the way in which they fail to do so.”
But it is to this phenomenal side of consciousness that scientists must attend if they are to provide a complete explanation of consciousness. Galliard and his colleagues have undoubtedly taken us a further step along the path to understanding consciousness, but there is clearly still a long way to go.
“Converging Intracranial Markers of Conscious Access” by Gaillard R, Dehaene S, Adam C, Clémenceau S, Hasboun D, et al. PLoS Biology Vol. 7, No. 3, 2009.
“Exploring the ‘Global Workspace’ of Consciousness” by Richard Robinson PLoS Biology Vol. 7, No. 3, 2009.
“Conceptual Analysis, Dualism, and the Explanatory Gap” by Ned Block and Robert Stalnaker, The Philosophical Review, Vol. 108, No. 1, January 1999.
Mathew Iredale’s Sci-Phi column appears every issue in tpm