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Implications of anaesthesiaImplications of anaesthesiaInto the void (on anaesthetics) Linda Geddes New Scientist, 26 November 2011 This article highlights an important change in the understanding of anaesthetics during the last part of the twentieth century. Previously it had been thought that anaesthetics worked by disrupting the lipid membranes of neurons. Later experiments showed that anaesthetics can bind to receptor proteins. The widely used anaesthetic, Propofol, binds to receptors for the inhibitory neurotransmitter, GABA. Studies by the anaesthetist, George Mashour, show in particular that feedback from the frontal to the sensory cortex is inhibited in anaesthesia. Modern studies of the influence of anaesthetics on the brain have implicated a large number of brain regions suggesting that there is no single site or region that is switched off by anaesthetics. The author links this to Bernard Baars' global workspace theory in which sensory information is first processed locally and unconsciously before becoming conscious when broadcast to the wider brain. Baars' theory as originally proposed appeared to lack any real basis in neuroscience, but seemed to 'get lucky' when neuroscience identified the workings of the local and global gamma synchronies. In respect of anaesthesia, studies show a loss of synchrony between different regions of the cortex. Under Propofol, small regions of the brain of the brain are still responsive to stimuli, but the spread of activity to other regions seen in normal brain processing is lacking. This article also raises the old question as to whether consciousness is an 'all-or-nothing' property, or as favoured here by both the writer and Mashour, more like a dimmer switch. In essence, there are three stages to anaesthesia. In the first stage, the patient is consciousness, but experiences a state similar to being drunk. At the second stage, the patient does not respond to the spoken word taken as an indication of unconsciousness, but would wake if penetrated by the surgeon's scalpel. This state is comparable to sleep. At a third stage the patient does not rouse when the operation takes place. Clearly, there is no comparable state in sleep, although sleep does become deeper, as reflected by changes in brain waves, during the course of a typical 90 minute sleep cycle. But does this analysis really stand up, or to borrow an often used 'get out of gaol free card' from mainstream consciousness studies, is it an illusion? We may become drowsy or in the case of anaesthesia drunken-feeling prior to losing consciousness, but we are still conscious in these states, while we are not conscious in the next stage of either anaesthesia or sleep. Dream sleep, which is viewed as a form of consciousness, usually happens in the later stages of the sleep cycle. Why labour this point? The reason is that recent studies in neuroscience point to an 'all-or-nothing' character for consciousness. This applies to both activity in single neurons and to the global gamma synchrony. Studies suggest that local gamma processing is unconscious, whereas large-scale activity, referred to as global gamma synchrony, such as reciprocal signalling between spatially separate neural assemblies is a correlate of consciousness. In other words the move from unconscious to conscious or vice versa is not a gentle transition but a jump to a different processing system. Studies quoted in this article show that there is no spread of activity between brain regions under full anaesthesia, but that there is under mild sedation, putting sedation, drowsiness, drunkenness into the same brain activity camp as wakefulness rather than that of full anaesthesia. Research indicates a close correlation between global gamma synchronisation and conscious processing (1. Lucia Melloni et al, 2007) (2&3. Singer, Wolf, 2010). Activity related to conscious responses is more synchronised, but not more vigorous. In human subjects, conscious processing has been related to phase-locked gamma oscillations in widely distributed cortical areas, whereas unconscious processing produces only local gamma activity. This is argued to be a so-called 'small worlds' system, where there is a co-existence between local and long range networks. In the brain it is suggested that the local networks are between neurons only a few hundred micrometers apart within layers of the cortex, while the long distance networks run mainly through the white matter, and link spatially separated areas of the cortex. It is these latter that can establish a global synchrony that is correlated to consciousness. Melloni et al suggest that masking is a good way of studying consciousness, because this allows the same stimuli to be either conscious or unconscious. In a study run by the authors, the same words could be perceived in some trials but not in others, where they were masked. Local synchronisation was similar in both cases, but with consciously perceived words there was a burst of long-distance gamma synchrony between the occipital, parietal and frontal cortices. Words processed at the unconscious level could lead to an increase in power in the gamma frequency range, but only conscious stimuli produced increases in long-distance synchronisation, and thus global synchronisation across spatially distributed regions of the cortex looks to be a requirement for consciousness. In the case of single neurons there is an even sharper distinction involving a sudden 'all-or-nothing' shift to activity correlated to consciousness. Study by Rafael Malach et al (3-5.), Quiroga et al (6.) and Kreiman et al (7.) investigated the behaviour of individual neurons. Malach studied the response of single neurons in the medial temporal lobe, while subjects looked at pictures of familiar faces or landmarks. The response of these neurons correlated with conscious perceptions reported by the subjects of the study. Neurons fired selectively for images of individual people. In some trials, the duration of stimuli was right on the boundary of the time needed for conscious recognition of an object (about 33ms), so that it was possible to compare the behaviour of the neurons when a person/object was recognised or alternatively not recognised by the subject. One finding of this study was the ‘all-or-nothing’ nature of the neuronal response. There was no spectrum involved. Either the neuron fired strongly, in correlation with the subject reporting recognition, or there was very little activity. In one trial, a single neuron was shown to respond selectively to a picture of the subject’s brother, but not to other people well known to the subject. Particularly noted is the marked difference in the firing of the neuron when the subject’s brother was recognised and not recognised. The stimulus duration of 33ms meant that half the time the image was recognised, and half the time not recognised. The neuron was nearly silent when the image was not recognised, but fired at nearly 50 Hz when there was conscious recognition, indicating an ‘all-or-nothing’ response from the neuron, correlated to subjective report of recognition. In another test, a single neuron went from baseline to 10 spikes per second when the subject recognised a picture of the World Trade Centre, but showed little response to all other images that were presented. Again the neuron fired in an ‘all-or-nothing’ fashion, depending on whether there was conscious recognition. In five trials not resulting in conscious recognition, this neuron did not fire a single spike. In yet another trial, the firing of a single neuron jumped from 0.05 Hz to 50 Hz when the subject reported recognition of an individual. The overall conclusion from these trials is that there is a significant relationship between the firing of neurons in the medial temporal region and the conscious perceptions of subjects. In particular, it is noted that with stimuli, at a duration where exactly the same image was recognised in some cases, but not in others, there was an entirely different (all-or-nothing) response from the neuron, according to whether or not the subject consciously recognised the image. These findings are stated to agree with earlier single-cell studies, including studies involving the inferior temporal cortex and the superior temporal sulcus. This work demonstrates that neural processing is completely distinct for exactly the same signal, with a duration that placed it on the boundary of being consciously recognised or not recognised, produced almost no response, if it was not consciously recognised, but a vigorous response if it was consciously recognised. There is a strong similarity between the experiments show that a correlation between global gamma synchrony and conscious experience and those that show a correlation. The problem here is to discover the link, if any, between these two correlations. The authors ask to what extent the spiking activity of individual neurons is related to the gamma local field potential. Earlier studies had shown a confusing variation in the degree of correlation between neuronal spiking and gamma activity, with some studies showing a strong correlation and others showing only a weak correlation. The authors here think that they have a resolution to the arguments that have arisen around this confusing data. Their study demonstrates that most of the variability in the data can be explained in terms of whether or not the activity of individual neurons is correlated to the activity of neighbouring neurons. A relationship with gamma synchrony is apparent where there is correlated activity in neighbouring neurons. The link between individual neurons that are associated with other active neurons and the gamma synchrony is apparent, both when the brain is receiving sensory stimulation, and when activity is more introspective. References:- 1.) Melloni, Lucia et al (2007) - Synchronisation of neural activity across cortical areas correlates with conscious perception - Journal of Neuroscience, 27 (11), pp. 2858-65 2.) Singer, Wolf (2010) - Neocortical Rhythms - In:- Coordination in the Brain – Eds. Chrisopher von der Malsburg, William Phillips & Wolf Singer – MIT Press 3,) Malach, Rafael – Weizmann Institute (2008) - Neuronal explosions: Neuronal dynamics underlying perceptual awareness in the cortex 4.) Malach, Rafael (2006) - Perception without a perceiver - Journal of Consciousness Studies, 13, no. 9. Pp. 57-66 5.) Malach, Rafael et al (2007) - Coupling between neuronal firing rate, gamma and fMRI is related to interneuronal activity 6.) Quiroga, Q., Kreiman, G., Koch, C. & Fried, I. (2008) - Sparse but not 'grandmother cell' coding in the medial temporal lobe - Trends in Cognitive Science (2008), vol. 12, no. 3 7.) Kreiman, G., Fried, I. & Koch, C. (2002) - Single neuron correlates of subjective vision in the human medial temporal lobe - PNAS, June 11 2002, vol. 9, no. 12, pp. 8378-83 P. |
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