Anesthesia and Consciousness
For our Senior Seminar presentation, we decided to discuss issues related to the concept of consciousness. Consciousness itself is difficult to define. There are varying conceptions of consciousness: the waking state, which includes our ability to perceive, interact, and communicate with our environment; a label for subjective experience, or the feeling of being oneself, of living in the moment; and a synonym for the mind itself, the perception and processing of information in order to arrive at a final mental state (Fiset, 2003).
Specifically, one of the things we examined was the question of whether or not animals have consciousness. Human consciousness is assessed by self-report, which cannot be obtained from animals. Thus we must look for external signs of consciousness to assess whether individuals of a given species are conscious or not, or are even capable of consciousness. However, it is often difficult to agree on a set of criteria that must be met in order to be conscious. It is also possible that animals may be conscious, but in a different way than humans.
Another question we addressed was what the mechanisms of anesthetic agents can tell us about the neural correlates of consciousness. It is not certain whether or not consciousness even has a neural correlate. There are currently two broad theories about how consciousness is created by the brain. The first is called the neuronal-specificity theory, which suggests that there are structurally localizable "seats of consciousness" in the brain (Cariani, 2000). Though it appears that consciousness does not reside in one single site in the CNS, several important structures have been identified as playing a role in creating the conscious state. These include the cholinergic nuclei of the upper brain stem and basal forebrain, the noradrenergic nuclei of the locus coeruleus, the histaminergic nuclei of the hypothalamus, and the dopaminergic and serotoninergic pathways arising from the brainstem (Cariani, 2000). The thalamic nuclei also play an important role in the maintenance of consciousness and the performance of cognitive functions (Baars, 2001).
Some findings of the functional mechanisms of anesthesia correspond to this idea of structural correlates of consciousness. Morphine, halothane, fentanyl, and ketamine have been shown to decrease levels of acetylcholine in the pontine reticular formation, an effect that also occurs naturally during non-REM sleep (Fiset, 2000). Isoflurane, another common anesthetic, induces unconsciousness by hyperpolarizing neurons that connect the thalamus to the cortex, and in doing so blocks voltage-dependent sodium and calcium transmission (Fiset, 2000). These findings suggest that anesthesia works by suppressing or blocking neural signals in key areas of the brain (Cariani, 2000).
An alternative theory of the neural correlate of consciousness is referred to as the process-coherence theory, which suggests that consciousness arises out of synchronized patterns of activity between multiple neural systems (Cariani, 2000). This theory has been supported by an examination of neural activity that arises from the administration of other anesthetic drugs, which seem to disrupt the coherence of neural signals (Cariani, 2000). Studies have also shown that anesthesia seems to induce a series of direct and indirect effects on multiple systems throughout the brain, which suggests that consciousness is not localizable (Sewards and Sewards, 2001). Some suggest that it is instead the synchrony of neuronal firing between these various regions of the brain that give rise to consciousness (Sewards and Sewards, 2001).
Another topic that I raised in discussion was the occurrence of anesthetic awareness (University of Leicester, 2006), where patients find themselves conscious of events and sensations while under normal doses of anesthesia. Various monitoring techniques have been developed to closely track neuronal behaviors that have been identified as correlated with consciousness or unconsciousness. These techniques include tracking EEG patterns; tracking gamma-band activity,which measures the overall amount of synchrony across brain areas; and more comprehensive techniques that take multiple factors into consideration (Baars, 2001; Hameroff, 2001; Sewards and Sewards, 2001). Yet, even while being monitored with these techniques, some individuals claim to be conscious when their neural behavior suggests that they are not (Hameroff, 2001). Although these incidences are rare, they suggest that consciousness may not be as identifiable or consistent across individuals as we may think.
The questions that were raised at the start of our discussions of consciousness were as follows:
1) What are some synonyms of consciousness?
2) Do we need to define consciousness before we can study it?
3) How far down the animal kingdom does consciousness extend?
4) To what extent are non-human species conscious?
5) What are the major differences between consciousness in animals and consciousness in humans?
6) Is language necessary for consciousness? If not, is thinking in language a higher level of consciousness than thinking in images?
7) What possible explanations could there be for the incidence of "anesthetic awareness"?
8) Could there be more than one "level" or "type" of unconsciousness?
9) Ethical issues of anesthesia administration:
If the neural correlates of anesthetics are unknown, is it ethical to use them so widely?
Should EEG and BIS monitoring systems be required with the use of general anesthetics during operations?
The central focus of our discussion was the struggle to find a definition for the term "consciousness." We were able to identify the factors involved in being conscious, such as awareness, attention, memory, and perception of the passage of time. Yet we could not determine whether these were components of consciousness or if they were synonyms. Additionally, it was difficult to determine whether all of these things were required to label someone as conscious, or if only some of these things were necessary.
Another interesting suggestion was that consciousness involves the capacity to choose between various courses of action, rather than operating according to conditioned or instinctive behavioral patterns. If this is the case, then it seems likely that anything with a functioning neocortex should be considered conscious.
It was also pointed out, however, that an individual that is capable of consciousness is still not conscious all the time. Further, a person can be conscious of some things and not others at any given time. Should this person be considered unconscious in this scenario?
The issue of assessing consciousness in others is also a difficult one. In animals, the primary obstacle is the inability to obtain a self-reported status. In humans, this is also true in the case of persons under anesthesia or in a coma. We also discussed whether or not persons with short-term memory loss can be called conscious when they are constantly denying having been conscious in the past. Despite these issues, many of us feel an inherent sense that people with such memory loss are conscious. Many of us also feel that our pets are conscious, though no one seemed to be able to explain why. One possible explanation is the link many of us create between consciousness and the expression of emotion. We recognize patterns of behavior in our pets that are consistent with emotions that we humans feel, and so we assume that our pets are equally capable of such emotion and therefore must be conscious. However, this conclusion requires quite a few assumptions.
One of the most interesting conversation topics of the evening was the relationship between consciousness and the ability to learn. It was suggested that the classroom might be a more efficient learning environment if students were somehow forced to remain conscious of what they were learning for the duration of the class. On the other hand, it was argued that perhaps moments of unconsciousness, such as the opportunity to daydream, were necessary for more efficient processing of information. To take this idea further, it is possible that a classroom environment might instead be more efficient if students were allowed to learn unconsciously more often.
The conceptions of consciousness and unconsciousness that we talked about in the education example are different from the conscious and unconscious that we use to refer to people who are aware of and interacting with their environment, and those who are in a coma or under anesthesia. This highlights the point that there are multiple layers of consciousness between the two we most often think of. We are aware of things on different layers to a varying extent, which explains our ability to multitask. This would also explain how information can be stored in the brain and arise into our awareness at a later time, even when we are not able to retrieve the memory of first being exposed to that information.
How do we define these levels of consciousness more precisely? What are the neural differences between the brain of someone daydreaming in a classroom and someone who is under anesthesia? Is it possible to control what level of consciousness to store a given piece of information?If so, is it more possible for some types of information than others? Are we better at learning things at more conscious levels, or more unconscious levels (i.e. those outside of our awareness)? Though we do not yet know the answers to these questions, I believe they will eventually solve the puzzle of what makes a being capable of consciousness, illuminate the neural correlates of different levels of consciousness, as well as help to define the word "consciousness" itself.
Baars, Bernard J. (2001). The Brain Basis of a"Consciousness Monitor": Scientific and Medical Significance.Consciousness and Cognition 10, 159-164.
Carini, Peter (2000). Anesthesia, neural informationprocessing, and conscious awareness. Consciousness and Cognition 9, 387-395.
Fiset, Pierre (2003). Research on anesthesia,consciousness, or both? Understanding our anesthetic drugs and defining theneural substrate. Canadian Journal of Anesthesia 50(6), R1-R5.
Hameroff, Stuart (2001). Anesthesia: The "OtherSide" of Consciousness. Consciousness and Cognition 10, 217-229.
Pearce, David (2008). Utopian Surgery: Early argumentsagainst anaesthesia in surgery, dentistry, and childbirth. Retrieved April 6,2008, from http://www.general-anaesthesia.com/index.html
Sewards, T.V., & Sewards, M.A. (2001). On the correlationbetween synchronized oscillatory activities and consciousness. Consciousnessand Cognition 10, 485-495.
University of Leicester (2006). Awareness underanesthesia: psychologists investigating consciousness during operations revealtechniques to idenitify wakefulness. ScienceDaily. Retrieved April 6, 2008,from http://www.sciencedaily.com/releases/2006/060131091435.htm