We all know what it means to be conscious. You are, of course, conscious right now – if you were not, you would be unable to read this. And while you read, you will be conscious of the words on your computer screen; of tactile sensations originating from the mouse you are holding and the chair you are sitting in; and possibly of some background noise, even though you are not explicitly paying attention to it.
Nevertheless, the term consciousness still has no adequate definition, and the question of how it is generated by the brain is the biggest challenge to modern neuroscience. Researchers investigating this mysterious phenomenon therefore use a working definition, and focus on the subjective contents of conscious awareness. This is usually done indirectly, by using functional neuroimaging to investigate, for example, the neural correlates of visual perception.
Now a group of French researchers and clinicians report that they have measured the neural correlates of conscious awareness directly, and with higher spatial and temporal resolution that previous studies, using electrodes implanted into the brains of patients undergoing surgery for epilepsy. Their findings, which have just been published in the open access journal PLoS Biology, lend support to the global workspace model of consciousness.
The working definition of consciousness is based on the assumptions that all mental representations are derived from brain activity and, therefore, that every mental state is associated with a neural state. Thus, researchers investigating consciousness have typically used functional neuroimaging to compare the brain activity associated with visual stimuli which enter conscious awareness to the activity associated with those that do not. But neuroimaging is based on changes in blood flow around the brain, and so is not a direct measure activity. Moreover, a recent study has shown that the relationship between cerebral blood flow and neural activity is not clear.
Raphael Gaillard of the Cognitive Neuroimaging Unit at INSERM and his colleagues had a unique opportunity to record the neural correlates of conscious awareness directly , using electrodes implanted into the brains of epileptic patients who were about to undergo neurosurgry. In these patients, anti-convulsant drugs are ineffective in reducing the severity of the seizures, so surgical removal of the abnormal tissue is the only remaining option. But the tissue must first be located, and this is still done using the technique developed by Wilder Penfield in the 1930s, in which the surgeon electrically stimulates the patient’s cerebral cortex. Because the patient is awake and fully conscious, they can report what they experience in response to the stimulation. The surgeon can therefore identify the tissue from which the seizures originate, and establish its relation to the indispensible areas which must be avoided during the procedure.
The experiments were carried out during such evaluations of 10 epileptic patients, who agreed to participate in the study. While on the operating table, they were shown a computer screen onto which words were presented. In the masked condition, each word was preceded and quickly followed by a “mask” consisting of a set of hatch marks, so that it flashed onto the screen for just 29 thousandths of a second. In the unmasked condition, the second mask was removed, so the words remained on the screen longer. Thus, the unmasked words entered the patients’ conscious awareness, but the masked words did not, and the researchers were able to compare the brain activity associated with conscious and unconscious word processing.
Each of the patients performed a total of 548 such trials, in which the masked and unmasked words were presented randomly, and the brain activity associated with each type of stimulus was recorded using intracranial electrodes placed at 176 different locations on the surface of their brains. The initial responses to the masked and unmasked words were very similar – both types of stimulus evoked widespread activity recorded from electrodes in all four lobes of the brain. The masked effects were recorded predominantly from the visual cortical areas at the back of the brain, and the unmasked effects from the frontal cortex.
The responses to the masked words were found to begin earlier than those evoked by the unmasked words, but they decayed rapidly, first in the visual, and then in the frontal cortex. The initial responses to the unmasked words were longer lasting, and they were followed by synchronized electrical activity throughout most of the brain, which oscillated with a peak frequency of 20 cycles per second, and was recorded in the 200-500 ms time window. A mathematical model applied to the data showed that the early visual cortical responses to the unmasked words, but not the masked words, led to an increase in the strength of the signals recorded from the electrodes in the frontal cortex.
Not surprisingly, the authors conclude that conscious awareness has a highly complex functional architecture rather than a single marker. They also suggest that their findings support the global workspace model of consciousness. According to this model, information is initially processed in multiple modules acting in parallel, and will only enter consciouss awareness if it is represented in a sensory area, such as the visual cortex; if it persists for long enough, and is sufficiently intense, it will enter a second stage of processing in the prefrontal cortex and wider distributed network. Finally, this network activity must amplified, so that it “ignites” and broadcasts its contents in a self-sustaining pattern which reverberates throughout the whole brain, and is experienced as “consciousness”.
The results do indeed fit into this model nicely. Both the masked and unmasked words evoked widespread activity throughout the brain, but the response to the unmasked words was more intense – it was recorded from 68% of the electrodes, whereas the response to the masked words was recorded from just 24%. The initial unmasked effect also lasted longer, and was measured predominantly in the frontal cortex. The synchronized activity which followed reflects long-range exchange of information across a broad network of cortical structures. Thus, the brain’s response to the
unmasked words was quickly extinguished and attention was shifted instead to the unmasked words. The responses to these were amplified and communicated to the global workspace, where they ignited to produce a flame of conscious awareness.
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Gaillard, R. et al (2009). Converging Intracranial Markers of Conscious Access. PLoS Biology. DOI: 10.1371/journal.pbio.1000061.