Current concepts of consciousness with some implications for anesthesia

Abstract

HERE is now extensive evidence that consciousness is associated with brainwide distribution and integration of information. Psychological evidence for the hypothesis was summarized by Baars (1988Baars ( , 1997Baars ( , 2002 in a framework called Global Workspace (GW) theory, derived from the cognitive architecture tradition. The global hypothesis has implications for perception, emotion, motivation, learning, working memory, voluntary control, and self systems in the brain. Recent brain imaging evidence appears to be broadly supportive, and the hypothesis is now favoured by philosophers like Daniel C. Dennett and Ned Block. The GW hypothesis is directly testable by brain scanning experiments that treat consciousness as a variable. Implications for unconscious states, including general anesthesia, are discussed.T Th he e r re ed di is sc co ov ve er ry y o of f c co on ns sc ci io ou us sn ne es ss s Shortly after 1,900 behaviourists attempted to purge science of mentalistic concepts like consciousness, attention, memory, imagery and voluntary control. "Consciousness," wrote John B. Watson, "is nothing but the soul of theology." But as the facts accumulated over the 20th century, all the traditional ideas were found to be necessary. They were reintroduced with more testable definitions. Memory came back in the 60's; imagery in the 70's; selective attention in the last half century; and consciousness in the last decade or so.It is roughly true that what we are conscious of is what we can report with accuracy. Conscious brain events are therefore assessed mainly by way of reportability. We now know of numerous brain events that are reportable, and comparable ones that are not. This fact invites experimental testing: why are we conscious of these words at this moment, while a few seconds later they have faded, but can still be called to mind? Why is activity in visual temporal lobe neurons reportable, while apparently similar activity in parietal regions is not? Why does the thalamocortical system support conscious experiences, while the comparably large cerebellum and basal ganglia do not? These are all testable questions.The empirical key is to treat consciousness as a controlled variable. We now know of cortical neurons that respond to conscious pictures presented to one eye, while competing unconscious pictures presented to the other eye evoke firing in closely related cells. Goodale and Milner have shown that visual control of manual reaching and grasping appears to be unconscious. Rees and colleagues have demonstrated that of two streams of visual information at the very centre of gaze, one can be conscious and the other not; the conscious one activates distinct brain areas, as assessed by fMRI. The competing unconscious visual stream, which surely reaches cortex, could not be detected. A rich literature has arisen comparing conscious and unconscious brain events in sleep and waking, anesthesia, epileptic states of absence, very specific damage to visual cortex, spared implicit function after brain damage, com...