I thought that it was about time to start writing about my scientific progress again. (Besides the fact, that Amsterdam does not inspire me enough to write about other things). I am very looking forward to my internship in Eus van Someren’s sleep lab. In one of my classes I had to write an integration essay on two topics we had discussed earlier. So, nothing seemed to be more obvious than to write about sleep. Since I am very interested in consciousness as well, I happened to combine these issues ;]. I’d like to present it here for the interested reader. I’m not sure to what extent the background theories are understandable, ’cause the word limit was rather low ;]. Hence, I’m convinced there will be some clarification questions :]. To answer the first: No, I’m not going to investigate this topic in my internship (let alone within the next few years).
During a whole night of sleep we are undergoing different stages of sleep depth that iterate every 90 minutes. Most relevant are the neuronal as well as functional differences between SWS (slow wave sleep) and REM (rapid eye movements) sleep. SWS is characterized by high delta band activity, i.e. occurrence of slow waves with high amplitudes and is nowadays known to play a role in the consolidation of declarative memory. During this stage people are closest to unconsciousness in terms of their ability to react to external as well as internal stimuli. As the night progresses, the duration of SWS decreases while REM sleep increases. REM sleep is associated with rapid eye movements that are mostly due to the occurrence of dreams. Neuronal activity is very different from SWS and shows great similarity to wakefulness. In addition, the appearance of dreams led researchers to the conclusion that this stage might be involved in repeating past experiences, which could help improving our coping skills and reactions to certain situations. However, the only function of REM sleep supported by experimental evidence seems to be the consolidation of procedural memory (at least in adult humans [1]) [2,3].
On the other hand, the difference between those two sleep stages should not be hastily reduced to the processing of two subtypes of memory. Intuitively, one stage is experienced as unconsciousness (SWS) whereas the other stage seems to be full of conscious experiences, even though we might not have full control of the course of events. On top of that, we apparently have an innate ability for short periods of dream control, known as lucid dreaming1. To conclude, during a night of sleep we are undergoing different levels of awareness which are not yet conclusively associated with any of the known functions of sleep. In order to gain knowledge in the latter issue we first have to investigate the question of which states of consciousness we are really experiencing on a scientific level.
The most promising attempt to start with is the recurrent process theory. Proponents of this theory found evidence in the field of vision research that consciousness might correlate with feedback processes. These run in a top-down fashion back from higher to lower cortical areas after a feedforward sweep pre-processed visual stimuli in a bottom-up manner [4]. If these feedback connections are interrupted by e.g. TMS pulses the respective stimulus is not processed consciously [7].
Using experimental designs from those visual recurrent process studies and applying them to sleeping subjects might gain deeper insight into the stages of consciousness we are passing during sleep. According to the recurrent process theory’s definition of consciousness, I expect no feedback processes during SWS, since we experience SWS as unconsciousness. If we only take visual processing into account2, this would already be predicted due to the absence of visual stimuli: Whenever we see nothing, there should be neither feedforward nor feedback processes. However, when passing over to REM sleep and to near-conscious awareness of dream vision, feedforward processes could become active. The most interesting question is whether recurrent processes already occur in REM sleep, too – or whether we encounter feedback not before the even higher stage of awareness during lucid dreaming.
If the latter is the case, feedback processes could define the difference between ‘normal’ dreaming and lucid dreaming. If the former is the case and there are no differences between REM sleep and lucid dreaming with regard to recurrent processes, the two stages of consciousness we experience could be called ‘consciousness with and without knowing’ according to the recurrent process theory.
References
[1] Wilhelm, I., Diekelmann, S., and Born, J. (2008). Sleep in children improves memory performance on declarative but not procedural tasks. Learning & Memory, Vol. 15, 373-377.
[2] Walker, M.P., and Stickgold, R. (2004). Sleep-Dependent Learning and Memory Consolidation. Neuron, Vol. 44, 121-133.
[3] Plihal, W., and Born, J. (1997). Effects of early and late nocturnal sleep on declarative and procedural memory. Journal of Cognitive Neuroscience, Vol. 9, 534-547.
[4] Lamme, V.A.F. (2006). Towards a neural stance on consciousness. TRENDS in Cognitive Science, Vol. 10, 494-579.
[5] Lamme, V.A.F. (2003). Why visual attention and awareness are different. TRENDS in Cognitive Sciences, Vol.7, 12-18.
[6] Supèr, H., Spekreijse, H., Lamme, V.A.F. (2001). Two distinct modes of sensory
processing observed in monkey primary visual cortex (V1). Nature Neuroscience, Vol. 4, 304-310.
[7] Lamme, V.A.F., and Roelfsema, P.R. (2000). The distinct modes of vision offered by feedforward and recurrent processing. Trends Neuroscience, Vol. 23, 571-579.
1 There are people who even claim to be lucid in every single dream they have for the whole dream duration. Often it takes them some time to recognize that this kind of skill is not widespread in the population.
2 The recurrent process theory has not yet been investigated beyond visual processing. Thus, the extension into sleep research at hand would at first have to focus on visual processing as well.
Many thanks to my hon for helping me with the appropriate language and clarity.