This is my first post on LJ, ever. I normally write fictional stories on blogs like these, and though I will post something prose very soon, I wanted to share the geekier side of me, which includes the love of biology! In particular, it is the love of neurobiology, a large and unique part of our physiology that is decades away from being even menially understood. Warning, includes large words and anatomy jargon:
To hint at my conclusions, it has been my personal experience during the progression of this course that consciousness is much more complex and difficult to comprehend than I had initially given it credit for - and has now resembled something closer to a highly interconnected, adaptive, and somewhat undefined biological system that evolutionarily exists to allow humans to make rational (executive) choices over the “default” emotional responses. Indeed, the use of our rational brain is heavily dependent on feedback circuits like the basal ganglia and the amygdala, and serve as a modifier of consciousness.
The authors of the article Consciousness and Anesthesia
begins their review by disclaiming that we don’t understand consciousness very well, to which I agreed. A point is made that unresponsiveness can occur without unconsciousness, and a common example of that are REM dreams, which are typically accompanied by muscle paralysis due to the inhibition of alpha motor neurons. Ketamine, an anesthetic that decreases executive function, as well as akinetic mutism, are also given as supporting examples. Referred to as retrospective oblivion, some anesthetics also block working memory, as patients immediately forget what to do if instructed to do something. At certain doses, anesthetics can cause profound amnesia in patients, suggesting an inability to utilize explicit (conscious) memory. Studies that used the isolated forearm technique were able to successfully demonstrate that patients under generalized anesthesia are capable of carrying out conversations by using hand signals (while the rest of the body is paralyzed) - but when fully awake patients denied ever having communicated, which suggests a role of implicit or unconscious memory. A related study investigating the role of implicit memory during isoflurane anesthesia demonstrated that implicit memory is maintained in hypnotic states to such a degree as to be comparable to non-anesthetized subjects.
It is true that certain actions don’t require consciousness, but it raises an interesting point about patient H.M., who had a bilateral medial temporal lobectomy to cure his intractable epilepsy in the 50s. If we consider various anesthetics to render a patient “unconscious” with anesthetics that can block working memory, would we presume then that Henry Molaison was not conscious on August 25th, 1953 and onward?
The thalamus is critiqued as being the responsible section of the brain for consciousness - is it the ideal, switch-like structure for consciousness, as suggested in various rat experiments? It is, after all, ideally located with extremely efficient communication to all of the cerebral cortex, and possesses the ability to integrate information. It is shown in various experiments that there is a decreased metabolic activity and blood flow in the thalamus with the introduction of anesthetics, as well as reports of a damaged thalamus leading to VS in humans. Recovery of the reciprocal connections between the thalamus and the cingulate cortex is associated with recovery in patients with thalamic damage. Conversely, it is known that not all anesthetics reduce thalamic activity (e.g. ketamine), or cause sedation but not unconsciousness (sevoflurane). Also, the removal of the cortex from animals leads them to be immune to the effects of anesthetics (i.e. a cortical deactivation leads to a thalamic deactivation). Physically, there are as many as 10 times the amount of inputs into the thalamus from the cortex than there are inputs into the cortex from the thalamus. Alkire et al states that cortical arousal does not necessarily mean one is conscious, as shown in Kenneth Parks, who displayed a surprisingly sophisticated (and morbid) display of sleepwalking. On May 23rd, 1987, Kenneth got out of bed but did not awaken, got into his car and drove fourteen miles to his mother-in-law's home and stabbed her to death, and assaulted his father-in-law, before finally awakening and drove himself to the local police station. His defense attorneys posited that while Kenneth did commit the murders, it was a case of homicidal somnambulism. The brain is capable of creating extremely complex physical maneuvers, but it does not imply that one is executive control, or conscious.
So far as cortical effects of anesthetics, evoked responses in primary sensory cortices are unchanged during anesthesia. This means that simply because there is activity in primary sensory areas, this does not mean that there is an ability to perceive it, even though the sensory information is available (information progresses to the association areas but is then blocked from reaching the prefrontal cortex). In hypnotic doses, anesthetics like propofol deactivate not only posterior brain areas but also the frontal cortex, suggesting that the frontal cortex is not essential for consciousness. Indeed, lesions or even severe trauma to this area have demonstrated that they alone are not enough to produce unconsciousness, as in the infamous example of Phineas Gage, who suffered the destruction of much of his frontal lobe in a rock-blasting incident in 1848. Even though a tamping iron rocketed through Gage’s head, he was still able to talk and walk with little assistance shortly after the accident. Though after that point until the end of his life he lacked severe amounts of tact due to the destruction of the executive parts of his brain, he was still very much capable of the conscious experience.
Alkire et al state that unconsciousness is typically associated with the deactivation of the mesial parietal complex, posterior cingulate cortex and the precuneus. Contradictorily, the mesial complex is deactivated during REM sleep (paradoxical sleep), as well as a deactivation of the posterior mesial cortex with a dose of nitrous oxide with no loss of consciousness. Anesthetics have been shown to deactivate and disconnect the lateral temporo-parieto-occipital complex of multimodal associative areas on the inferior parietal complex.
One of the core concepts of this review paper lies with the authors’ proposed definition of what the essence of consciousness is, which lies with the capacity of information in the brain, as well as its ability to integrate this information globally. Disruption of cortical integration with anesthetics, or disrupting the ability of different parts of the brain to “talk” to each other, would lead to unconsciousness. As a patient fades into anesthetic-induced unconsciousness, there is a drop in the 20-80 Hz range (gamma) of the frontal to occipital lobes. This decrease in coherence is either due to anesthetics acting on structures for long-range cortico-cortical interactions and/or the slowing of neural responses. Since the corticothalamic system is considered a “small-world” network with few long-range connections, the disruption of these long-range connections produces a set of disconnected components.
Cortical information capacity, or as this paper restates it, discriminable activity patterns, may also be responsible for consciousness. The less information there is available for the corticothalamic system, there will be a resulting decrease in neural activity (however still global and lacking regional specificity). A burst-suppression on an EEG shows a stereotypical pattern where only 1 of 2 states are possible (on/off). Disruption of cortical information capacity also results in unconsciousness.
In this vein, a question is subtly posed in the review, which asks: in what ways is anesthesia like sleep? Both result in a deactivation of arousal systems (the reticular activating system, which is responsible for sleep-wake transitions). Sleep and anesthesia both result in slow oscillations (1 Hz) in cortical and thalamic neurons, which results in a loss of integration and information capacity, which can be demonstrated on subjects with TMS. Depending on the site of stimulation, the loss of integration can be seen when stimulating cortical areas like the PMC, and a loss of information capacity can be seen when stimulating the mesial parietal regions.
It is directly stated at this point in the review a proposed theory in the wake of evidence from anesthesia and sleep states studies: “Information and integration may be the very essence of consciousness” (1). As evidenced, conscious experiences are deeply informative (information capacity) as well as these experiences being unable to be subdivided into smaller, independent components (integration). In other words, “...consciousness of a physical system is related to a repertoire of different states that can be discriminated by the system as a whole” (1). I am quoting these phrases as I find them to be the most concise and clear, and I cannot find a better way of restating the ideas without confounding them. The corticothalamic complex is essential for consciousness as it brings together the required functional specialization while functionally integrating them at the same time. Thus, individual motor responses or local activations do not necessarily indicate consciousness, while the absence of an activation does not necessarily indicate unconsciousness.
Before concluding, what always tickled my fancy was the idea that humans are able to modify the state of consciousness through various natural circumstances that don’t include drugs or brain damage, and aren’t as dramatic as the abolishment of consciousness which happens during dreamless sleep, coma or death. There hasn’t been much in the way of hard or quality research (in my opinion) on the physiological changes in those who practice various forms of meditation, though there have been some interesting findings. Research with brain wave recordings during meditation has reported some apparent differences between meditation and regular relaxation, though these findings are debated on whether or not the physiological changes constitute a distinct state of consciousness (2). Various other altered states of consciousness were also inspected in the 1960s and 1970s, though these have not yet been validated by empirical studies (3). Largely, a comprehensive examination of meditation-altered consciousness yields indefinite and dubious conclusions, and while the concept seems somewhat “out there”, I thought it would be worth a look.
As touched upon in the beginning of this paper, the use of our rational brain is heavily dependent on feedback circuits like circuits involving the basal ganglia and the amygdala, and serve as a modifier of consciousness. Indeed, the basal ganglia is associated with control of voluntary movements, procedural learning, routine behaviors, cognition and emotion, while the amygdala is associated with memory and emotional reactions. Other feedback circuits may include the visual cortex, or the primary motor cortex, where information will be transferred through feedforward connections, conveyed to the thalamus and then relayed to various cortical regions. Different association areas and the phase synchrony of oscillations of neurons also play extremely important roles in the conscious experience. These circuits, characteristics and reciprocal connections of various parts of the brain that “pitch in” are required to generate a model and match it to expectations, which are in turn influenced by anticipation and memory, to create the reality that we individually experience. The use of anesthetics, in this way, “break down” this reality by stifling the ability to maintain reciprocal connections sufficient for integration, as well as the capacity at which we are able to take in and process information meaningfully, where the loss of either would result in the loss of consciousness.
1. Alkire, M.T., Hudetz, A.G., Tononi, G. (2008). Consciousness and Anesthesia
. Science, 322:876-880.
2. Murphy M., Donovan S., and Taylor E. (1997). The Physical and Psychological Effects of Meditation: A Review of Contemporary Research With a Comprehensive Bibliography
. Institute of Noetic Sciences, 1931–1996.
3. Tart, C. (2001). Ch. 2: The components of consciousness
. States of Consciousness, IUniverse.com. ISBN 978-0-595-15196-7. Retrieved 2011-10-05.