There is a recent study by Scheinin of Turku Finland, his team, and collaborators from U of California. (see citation below). They looked for the neural correlates of consciousness. This is a very interesting study.
They start with some differences in what is meant by ‘consciousness’. They are looking for the correlates of the ‘state’ of consciousness as shown by responsiveness to a verbal command.
Consciousness research can roughly be divided into two categories: (1) studies on the contents of consciousness and (2) studies on consciousness as a state. Whereas theories on the particular contents of consciousness, such as visual consciousness, argue for the importance of cortical structures, theories focusing on consciousness as a state stress the importance of subcortical or thalamocortical structures. There is limited human data on which brain structures engage to serve this foundation of consciousness. This study was designed to reveal the minimal neural correlates associated with a conscious state. General anesthesia is often defined as comprising pharmacologically induced unconsciousness (loss of the ability to feel or experience anything), amnesia (forgetfulness), analgesia (lack of pain), and immobility (not moving in response to surgical stimulation) . In this study, without surgical stimulation, the term anesthesia is used to describe pharmacologically induced loss of consciousness. Its objective behavioral criterion was unresponsiveness to a verbal command.
They are aware of some of the short-comings of using aesthesia to study consciousness and try to eliminate them by: using neuroimaging and pattern analysis to dissociate changes in consciousness from other effects of anesthesia; they use an anesthetic that allows awakening during a constant dosing of the drug; they compare arousal with two different drugs; the use of PET as well as MRI. What were the results?
(The authors) investigated what turns back on when consciousness re-emerges following anesthetic-induced unconsciousness. The structures that activated when consciousness resumed were the brainstem, the thalamus, the hypothalamus, and the ACC (anterior cingulate cotex). Although only limited frontal and parietal activity was observed during the return to consciousness, an active parietal region demonstrated greater functional connectivity with the ACC (and other frontal regions) during the conscious state. These findings reveal a functional network that activates with restored consciousness to enable arousal, the subjective awareness of stimuli, and the behavioral expression of the contents of consciousness. Arousal-induced activations were mostly localized in deep, phylogenetically old brain structures rather than in the neocortex. …suggesting that these deep brain structures form a common foundation for a conscious state. These structures activate also upon awakening from natural Stage 2 sleep, and their direct stimulation can reverse the unconsciousness of anesthesia or improve behavioral arousal following chronic brain injury. impaired consciousness or coma is known to result from paramedian thalamic and midbrain infarcts.
The activation of the ACC would fit with the test of consciousness in this case being a motor response.
One distinct cortical region activating during the recovery of consciousness was the ACC. This medial prefrontal region has been proposed to play a critical role in consciousness by integrating cognitive-emotional processing with the state of arousal and the intent-to-act. Others suggest that the ACC is a key site of self-regulation of behavior, or might even be involved with the feeling of free will. The neuroanatomy of the ACC and its connections with motor control regions suggest that this brain structure acts as the neural interface for translating intentions into actions. This idea is in agreement with studies showing that restored activity in the ACC correlates with the level of responsiveness in brain-injured patients. Our inferior parietal finding may have importance because of its functional connection with the ACC during the conscious state and because of its suggested role in movement intention and motor awareness. In work presented by Desmurget, direct electrical stimulation of the inferior parietal cortex produced in awake brain surgery patients the sensation of a will to move. They concluded that conscious intention and motor awareness arise from increased parietal activity before movement execution. This idea fits well with our findings.
Recovery of consciousness is seen as a process:
The recovery from anesthesia does not occur all at once, but rather it appears to occur in a bottom-up manner. When emerging from deep anesthesia there will first be signs of autonomic arousal, followed by a slow return of brainstem reflexes, eventually leading to reflexive or uncoordinated somatic movements that occur somewhat before subjects can willfully respond to simple commands. As shown in our results, only minimal cortical activity is necessary at this point. Thus, emergence of a conscious state, the essential foundation of consciousness, precedes the full recovery of neocortical processing required for rich conscious experiences… All of these data are in agreement with the experiences obtained from hydroanencephalic children, who are devoid of nearly the entire cerebral cortex and yet still display conscious-like behavior. Athough these children have clear deficits in experiencing the rich contents of consciousness, they undoubtedly are in a conscious state, supporting the idea that the subcortical areas identified in our study indicate that consciousness as a process involving both the conscious state and the contents of consciousness likely arises from the interaction between cortical and subcortical mechanism working through specific network connectivity within the brain.
Conscious information processing is pictured as a role of frontoparietal connectivity in the neocortex and is linked to the maintenance of the conscious state by the lower brain through the connections between the neocortex and the thalamus.
I have had the opinion that consciousness is far too complex a process and far too wide-spread through the brain to be of recent origin. In this model, the ‘state’ of consciousness could be very old while the particulars of the creation of content for say mammals could be more recent.
Langsjo, J., Alkire, M., Kaskinoro, K., Hayama, H., Maksimow, A., Kaisti, K., Aalto, S., Aantaa, R., Jaaskelainen, S., Revonsuo, A., & Scheinin, H. (2012). Returning from Oblivion: Imaging the Neural Core of Consciousness Journal of Neuroscience, 32 (14), 4935-4943 DOI: 10.1523/JNEUROSCI.4962-11.2012