30-Second Psychology is a new book that explains 50 key ideas in psychology in half a minute each.

The book was edited by Christian Jarrett and includes contributions from Vaughan Bell, Dave Munger, Tom Stafford and myself.

It is published by Icon Books, and is now available for purchase at the Guardian Bookshop.

I wrote eight of the 50 sections in the book. Here's one of them:

CONSCIOUSNESS 3-SECOND THOUGHT

A 'spotlight of attention' shines a bright beam on certain neural processes, which then enter into conscious awareness. 30-SECOND THEORY

We all know what it means to be conscious, but an adequate definition of consciousness remains elusive. The contents of consciousness consist of a narrow, dynamic stream of everything we are presently aware of—our perceptions of the external world and bodily sensations, together with our thoughts, actions, emotions, and memories. The contents of consciousness are commonly studied by using brain scanning to compare the brain's responses to stimuli which enter awareness with those that do not. A common framework for studying these phenomena is the global workspace theory, proposed by Bernard Baars in 1987, which likens consciousness to a working theater. The vast majority of neural events are unconscious processes taking place "behind the scenes," but some enter into conscious awareness—the "stage"—when they become the focus of an attentional spotlight. The spotlight is surrounded by a fringe of vaguely conscious but crucial events, and acts as a hub which both distributes important information globally and is directed by the unconscious processes taking place behind the scenes. Viewed in this way, consciousness can be thought of as a means by which the brain prioritizes, and gives us access to, the information needed for healthy functioning.



3-MINUTE ANALYSIS

Consciousness has long been the subject of debate amongst neuroscientists and philosophers. Modern brain research is just beginning to provide some understanding of it, and the global workspace theory is the most useful model for interpreting the available evidence. This approach has already provided valuable insight into disorders of consciousness such as coma and the persistent vegetative state, and some suggest that conditions such as schizophrenia involve a profound alteration of processing in the global workspace.



Below is an edited version of an article I wrote in March 2009, about a study which provides some evidence for the global workspace theory.

Igniting the flame of consciousness

Originally published on 17 March 2009

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, we still have no adequate definition of consciousness, and the question of how it is generated by the brain poses a major challenge to modern neuroscience. Researchers investigating this mysterious phenomenon therefore use a working definition, and focus on the subjective contents of conscious awareness. The definition is based on the assumptions that all mental representations are derived from brain activity and, therefore, that every mental state has an associated neural state.

Typically, researchers use functional neuroimaging (fMRI) to compare the brain activity associated with visual stimuli which enter conscious awareness to the activity associated with those that do not. Neuroimaging is, however, 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.

Several years ago, Raphaël Gaillard of the Cognitive Neuroimaging Unit at INSERM and his colleagues had a rare opportunity to record the neural correlates of conscious awareness directly from the brain, at a higher spatial and temporal resolution than ever before.

They did so during pre-surgical evaluation of patients with chronic, intractable epilepsy. These patients do not respond to anti-convulsant drugs, and surgical removal of the abnormal tissue causing the seizures is the only remaining option to alleviate their symptoms. The tissue is identified using a technique developed by the pioneering neurosurgeon Wilder Penfield in the 1930s, which involves implanting electrodes into the cerebral cortex while the patient remains fully conscious.

For the study, 10 such patients were shown a series of words on a computer screen while lying on the operating table. 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 milliseconds (ms, or 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 be 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, published in the open access journal PLoS Biology, fit the global workspace 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 masked 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.

Reference: Gaillard, R. et al (2009). Converging Intracranial Markers of Conscious Access. PLoS Biology DOI: 10.1371/journal.pbio.1000061.