Dreaming and Dream Incorporations

How is "dream" defined?

There is no single definition of what a dream is. One definition which shall be used here considers a dream as a recollection of the mental activity which has occurred during sleep (Schredl, 1999).

In which sleep stage do we dream?

Research shows that dreams occur in all different sleep stages, but that the probability of dreaming varies with the sleep stage: Subjects waken up from REM sleep report in 70-95% of the awakenings from dreams, whereas only 5-10% of NREM sleep awakenings produce similar results (Dement & Kleitman 1957a; Dement & Kleitman 1957b; Hobson, 1988). Moreover, dreams occurring during REM and NREM sleep differ regarding the characteristics of the dream: NREM dreams mostly deal with thought-like experiences, REM dreams are more visual, image-like, more bizarre and are stronger related to the person of the dreamer – they are the “classical” dream (Foulkes, 1962; Antrobus, 1991).

Can external stimuli get transported into a dream?

In daily life, nearly everybody has experienced that stimuli from the wake world sometimes are built into (“incorporated”) a dream – the famous alarm clock is only one example for this. Several studies have been conducted in order to examine how well different wake world stimuli are incorporated into dreams. Table shows an overview on different stimuli and their incorporation rate.

Study

Stimulus

Number of stimuli

Incorporation rate

Dement and Wolpert (1958)

Sinus tone (1000Hz)

35

9%

Dement and Wolpert (1958)

Light flashes

30

23%

Dement and Wolpert (1958)

Water sprayed onto the skin

33

42%

Berger (1963)

Subject’s name

45

40%

Koulack (1969)

Electric shocks at the thumb

99

56%

Hoelscher et al. (1981)

Neutral words

59

11%

Hoelscher et al. (1981)

Meaningful words

59

34%

Trotter et al. (1988)

Odor stimuli

79

19%

Nielsen et al. (1993)

Small pain stimuli

42

31%

Nielsen (1993)

Blood pressure cuff around the leg

28

87%

Leslie and Ogilvie (1996)

Rocking the hammock

45

25%

Table : Stimuli incorporations into dreams2

Are some stimuli better suited for being incorporated than others?

From the table it can be seen that the results are mixed. Some stimuli show a high incorporation rate, as for example the blood pressure cuff around the leg by Nielsen (1993), some show only a low incorporation rate, e.g. the sinus tone in Dement and Wolpert (1958). However, one has to keep in mind that there are a lot of parameters involved and that this overview can thus only give a small hint on how well stimuli are incorporated, not an exact and precise comparison. Methodological parameters, as for example in which case to consider a stimulus as being incorporated, and confounding variables such as the choice of the subjects, lead to different study results, as well as parameters concerning the stimulus. For example, a change in the loudness of the sinus tone can lead to different incorporation rates, changing the brightness of light flashes as well, the amount of water sprayed onto the skin, the intensity of odor stimuli etc.

However, from studies such as Hoelscher et al. (1981) it can be seen, that similar but different stimuli such as neutral and meaningful words lead to clearly different incorporation rates, and that thus some stimuli seem to be better suited for being incorporated into dreams than others, and also, that there seems to be some processing of the stimuli during sleep by the brain.

Moreover, if one aggregates the numbers, tactile stimuli seem to be incorporated more often than others. Schredl (2006) suggests that such tactile stimuli might be treated differently by the brain than other stimuli since they might be phylogenetically more dangerous and important to the organism than others.

In what way can a stimulus be built into a dream?

Another distinction regarding incorporation of stimuli into the dream world can be made between direct and indirect incorporations. If for example a sinus tone is played as a stimulus, it can be built into the dream directly, i.e. the tone as it is in the wake world appears in the dream but is not connected to the dream scenery, or indirectly, i.e. the tone is transformed into another sound fitting the dream environment better. One example from the experiments conducted for this thesis is a real world beeping tone which got (for the same subject) incorporated into one dream directly as is, into another dream indirectly as a beeping music in a discotheque.