conciousness
LECTURE SUMMARY:
At the very heart of our ideas about consciousness there is a deep paradox. The novelist Vladimir Nabokov expressed this very well in his novel Bend Sinister, published in 1947: ‘Consciousness is the only real thing in the world and the greatest mystery of all’
On the one hand, the only reality we know is that provided by our conscious mind, which ignites during infancy, flickers for the minuscule cosmic instant we call a lifeme, before vanishing forever at our death. So, in a sense consciousness is the only thing we know about.
On the other hand, how and why do conscious minds emerge from the materiality of neurons and brains in bodies? The short answer is that we just don’t know. So, there is a second sense in which our knowledge about consciousness is scant, at best.
Nabokov remained fascinated by consciousness, and in the brief excerpt from an interview shown here, he expresses his sense of wonder at the existence of consciousness ‘amidst the night of non-being’.
INTRODUCTORY EXERCISE:
What is consciousness, and where might we find it?
Having reflected on the nature of consciousness and where it might be found, during the introductory exercise you have just completed, you might be forgiven for worrying that the problem seems so difficult and apparently insoluble, that we might as well give up now, and spend our me on something more achievable, such as going fishing or paying a visit to the pub.
On the other hand, to a scientist, a unanswered questions are like unknown territories to adventurers – a positive invitation to explore.
Contemporary research on the topic of consciousness is thriving. The field is a humming hotbed of activity in which promising progress is being made on many fronts. Interesting, the most important contributions to consciousness research do not involve highly abstract, armchair-bound, beard-tugging theorising. Several areas of consciousness research address deeply practical problems (e.g. studies of anaesthesia, sleep, coma and other states of consciousness) – though it is also true that the field has its fair share of ambitious theories.
O-repeated advice for tackling a problem that seems impossibly difficult and insoluble goes something like this. Try to break the big problem down into a series of smaller ones, and make a start by working on one of the sub-problems. This strategy often works, regardless of whether the problem is technical and scientific, or personal.
In the case of trying to solve the enigma of consciousness, we can make a start by recognizing that that the term itself has several meanings. A discom that is often drawn involves acknowledging sometimes we use the word consciousness to refer to a state, while on other occasions we speak about the content of consciousness – whether we are or are not aware of something.
Our state of consciousness changes in the familiar daily cycle of being asleep at night – more on this shortly - and awake during the day. Of course, even during the day, our state can vary from feeling highly awake and alert to feeling drowsy.
States of consciousness can also be impaired through illness or injury. Disorders of consciousness include states such as coma or vegetative state..
Finally, states of consciousness can also become altered in other ways, perhaps through drugs or meditative practices.
As well as using consciousness to describe a state of mind and brain, we speak of being conscious of something. For example - becoming of alternative interpretations of ambiguous visual figures; or paying attention to your feet and suddenly becoming aware of sensations coming from your soles.
Though distinct, the two senses of consciousness are also inter-dependent
A final example might be familiar. Can you think of an occasion when you have been sing quietly, perhaps reading or working. Suddenly, a noise in the background – air conditioning, a fridge, distant roadworks – disappears. You were engrossed in your work or really enjoying your book, and were not aware of the background noise – unl it disappeared! In this case, the noise was arriving at your ears and being processed by a range of brain areas specialized for auditory information – but this processing was not represented in consciousness.
Although these two senses of the term consciousness are distinct, they are also related and inter-dependent. One cannot become conscious of something unless one also is conscious, in the sense of being awake and alert.
To open our examination of consciousness as a state, we will take a look at the changes we all experience every day in the period between falling asleep at night and waking in the morning.
Electroencephalography is the technique of recording ny electrical changes at the scalp, which are related to underlying brain activity. The electrical changes recorded by EEG are indeed quite ny and they provide a relatively gross and indirect measure of neural activity. The electrodes that make the recording sit on the surface of the scalp, so they are separated from the neurons of the brain by several intervening layers – hair, skin, the bone of the scalp, and the layers of slue called meninges that cover and protect the brain. Sophisticated electronics are required to amplify the ny voltages picked up by EEG scalp electrodes, so that a useful signal can be recorded.
Although EEG recording was first developed in the late 1920s, it was not used to study brain activity during sleep only the 1950s. Relevel low-cost systems are now widely available in clinics and research laboratories across the world.
Research using this technique has shown that the ny electrical changes picked up by EEG electrodes oscillate rapidly, in a repeating wave pattern. These waves can be seen clearly in the illustration shown above.
Moreover, it turns out that characterises of these EEG waves are linked quite gently with changes in consciousness. More specifically, the frequency of EEG waves varies in different states of consciousness.
Four different frequency bands are commonly distinguished.
Beta waves are relatively fast, with a frequency of around 13-24 cycles per second or Hertz (Hz). EEG waves at this frequency are characterise of normal waking, when a person is feeling alert.
Alpha waves are somewhat slower (8-12 Hz). These are seen in the EEG record when a person is feeling drowsy and relaxed. They are also seen during meditation. Closing the eyes will increase the amount of alpha activity apparent in EEG.
Theta waves are slower sll, with a frequency of 4-7 Hz. Theta activity is seen when a person enters a period of light sleep.
Delta waves are even slower – less than 4 Hz. These are seen during deep sleep, also known as slow-wave sleep.
Every night, after falling asleep we typically progress through five different stages of sleep, although two of them are sometimes grouped into one stage.
These are known as Stages 1-4, plus a fifth stage called REM sleep, that you have probably heard about before.
In the next few slides we will take a look at each of these stages, examining their characterises, including signature features apparent in EEG recordings.
As mentioned earlier, being awake and alert is characterised by the presence of relatively rapid beta waves (13-24 Hz).
As you will no doubt have moved, if you have ever sat next to someone who has dropped off during a boring lecture or tedious TV programme, breathing becomes deeper and more regular in the transition from being awake into light sleep.
If your companion happened to be wearing an array of scalp electrodes, the EEG record would show the presence of theta waves (4-7Hz).
This stage of sleep lasts around five minutes, and is also known as hypnogogic sleep.
During this stage of sleep, it is very easy awaken someone. In fact, when awakened many people may deny having been asleep at all, despite evidence to the contrary from eye closing as well as EEG changes.
The sudden, jerky movements, sometimes associated with a feeling of falling, that sometimes occur as one is dropping off, occur during this stage of sleep. These are known as hypnic jerks.
Image: htps://upload.wikimedia.org/wikipedia/commons/thumb/1/18/Domenico_Fe_-
SleepingGirl_-_WGA7863.jpg/640px-Domenico_Fe_-_Sleeping_Girl_-_WGA7863.jpg
The third row of this image illustrates a typical EEG record as a person enters Stage One sleep.
The next stage of sleep (row 4 in the earlier slide) lasts a little longer. While Stage One lasts around five minutes, the duration of Stage Two is about 20 minutes.
During this stage of relatively light sleep it is all quite easy to awaken someone. When awakened from this stage, people are generally aware that they have indeed been asleep.
A characterise of Stage Two sleep that is apparent in EEG recordings is the presence of so called ‘sleep spindles’. These are brief bursts activity in the 9-15 Hz range. The amplitude of these waves increases then decreases, so the EEG record resembles a spindle.
Sleep spindles may play an important role in consolidating memories formed during the day.
Assuming the usual trajectory of sleep is not interrupted, a person will progress from Stage 2 into the third and fourth stages of sleep. Thes stages are sometimes grouped together under the umbrella term ‘Slow wave sleep’ – see the fish in the earlier slide.
As this term implies, slow-wave sleep is characterised by the presence of the slowest category of EEG waves – delta waves, which have a frequency slower than 4 Hz. In addition to having a slow frequency, these waves also have a high amplitude – i.e. they are large and slow.
The difference between Stage 3 sleep and Stage 4 is quantile rather than qualitative. In Stage 3 sleep, EEG includes a substantial amount of delta activity; in Stage 4, more than 50% of the record comprises delta waves.
During slow-wave sleep it is considerably more difficult to waken someone. When woken from this stage, a person is likely to feel temporarily disoriented and that they were deeply asleep.
During this stage of sleep, a wide range of bodily changes occur. Heart rate and breathing slow down, while body temperature decreases.
Around 60-90 minutes after falling asleep something very strange happens in the pattern of waves apparent in EEG recordings.
After a period of slow-wave sleep, the EEG record changes, indicating a reduction in delta activity ((slow, high-amplitude waves) and a return to earlier, lighter stages of sleep. Indeed the EEG record soon begins to resemble the pattern observed when someone is awake, with prominent beta activity (rapid, low-amplitude waves).
Although the EEG pattern resembles that seen when someone is awake, the person is all clearly asleep. Indeed it is quite difficult to wake them up. The researchers who first made these observations were extremely surprised and suggested the term ‘paradoxical sleep’ for this stage, to acknowledge the strange disjunction between an awake-like EEG record and other signs indicating that the person was asleep. Fairly soon afterwards, this was superseded by the term REM sleep, which is now the preferred label for this sleep stage.
As you probably known REM is an acronym for Rapid Eye Movement. In addition to an EEG signal that resembles that seen when a person is awake, a signature feature of this sleep stage is the presence of rapid, jerky eye movements. You may have seen evidence of these flickering movements behind the closed eyelids of a sleeping partner, or perhaps in your pet dog. As noted earlier, other mammals go through sequence of sleep stages similar to humans, including the REM sleep stage. In research laboratories rapid eye movements can be recorded with some additional electrodes placed near the eyes. Contraction of the eye muscles generates a clear electrical signal that can easily be detected.
The sixth row of the earlier slide shows a typical EEG record from a period of REM sleep.
Neurophysiological characterises of REM sleep are quite different from all the other sleep stages. For this reason, authors often draw a broad distance between REM and non-REM (NREM) sleep, with the later category including Stages 1-4 described above.
At around the same me that researchers were making ground-breaking discoveries concerning sleep stages, using EEG and eye electrodes it was also discovered that if you awaken someone from a period of REM sleep they will often report being in the middle of a dream when woken. About 80 - 90% of awakenings from REM sleep are accompanied by dream reports. Even individuals who report that they dream rarely if ever, are likely to report dreaming when awakened from REM sleep. REM sleep appears to be both universal and necessary. In early studies, participants were selectively deprived of REM sleep by waking them every me the recording apparatus indicated the start of a REM period. The consequence of this was that rather than entering REM 60-90 minutes after falling asleep, participants entered REM sooner and sooner after going back to sleep. Eventually participants were being woken every few minutes, due to entering REM almost immediately after entering sleep. This suggests that not only do we need our sleep, we need our REM sleep.
Although there is a strong association between REM sleep and dreaming, the correspondence is not perfect. When awakened from other sleep stages, participants also often report dreaming. However, the dreams reported from non-REM sleep tend to be less dramatic and visual. Non-REM dreams are perhaps more like “I was thinking about something” and less like experiencing an unfolding drama. REM and NREM dreams tend to be rather different.
Although the muscles of the eyes become active, the other muscles of the body become relaxed. While breathing and heart rate slow down during the slow-wave stage, in REM sleep they become more rapid and irregular. In addition, in the REM stage there are signs of genital arousal in both men and women. Interestingly, these signs appear unrelated to the content of dreams that the sleeper may be experiencing. Signs of arousal are likely to be present, even if you are dreaming about the bus meetable.
During a typical night’s sleep you are likely cycle through the stages just described. During a sleep cycle there is a progression from light sleep to the deeper stages of slow-wave sleep, then a return through lighter stages before entering a REM period. The cycle then begins again, as the EEG record indicates a progression back towards slow wave sleep.
Each of these cycles takes around 90 minutes, meaning that most adults go through around 4-6 cycles per night. The illustration here shows a typical series of cycles during a night’s sleep. As you can see the cycles are not uniform. They change in an interesting way as the night progresses. In particular, the length of both the REM periods and periods of slow wave sleep changes. During the first part of the night, periods of slow wave sleep are longer than in the early hours of the morning. In fact, in the later part of the night, slow wave sleep may disappear entirely. Conversely, during the first part of the night periods of REM sleep are shorter than in the hours before you wake in the morning. If you are a person who tends to remember your dreams, your memories are likely to be of dreams from this later period of the night.
This change in the relative length of slow-wave and REM sleep periods across the night is shown in the illustration above.
As is well known, infants need a LOT of sleep. (Ironically, this is also a period when parents may experience significant lack of sleep.)
In addition to needing lots of sleep – as much as 16 hours per day – infants also spend a relatively high proportion of that me in REM sleep.
During the later decades of life, another change occurs. Among older adults the proportion of sleeping me spent in slow wave sleep tends to decrease. This shift begins from around the age of 40. In addition, the overall amount of me spent asleep may decrease. Many older adults report sleeping for shorter periods and experience their sleep as less satisfying.
These dramatic changes in patterns of sleeping across the lifespan are illustrated here. As you can see, the number of hours spent asleep, and the proportion of me spent in REM sleep are both at their highest in infancy.
The X-axis of the graph shown here has a logarithmic scale, to illustrate the rapid changes that occur during the childhood and teenage years.
A feature change that is not captured here is that the green portion of the graph labelled non-REM includes both light sleep (Stages 1 & 2) and slow-wave sleep (Stages 3 & 4). In the later decades of life, the contribution of slow wave sleep to the green portion of the graph decreases.
Summing Up
± In many ways, the science of consciousness is in its infancy
± Without a strong theory of consciousness questions about consciousness in other animals or in machines remain uncertain
± It is important to distinguish between being conscious of something, and the idea of consciousness as a state
± Strong advances have been made in our understanding of sleep, including
° Sleep cycles and the nature of sleep stages
° Changes in sleep across the lifespan