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Why Do We Forget Dreams? Part I

By: , Posted on: October 8, 2014

The Biology of ThoughtOne of the most intriguing features of the brain is the phenomenon of sleep and dreams. What are dreams? What purpose do they serve? Why do we forget most of the dreams, but remember some? We do not know. Whatever is the mechanism of dreams, one thing is certain – dreams are the surreal manifestations of memory stored in the brain – which simply means that stored memory is the basic prerequisite for the formation of dreams. Putting it in another way: Sensory thoughts collected during the awaken state are obviously essential for the recollection of memory – either from the conscious mind as in day-to-day activities; or from the unconscious mind as in dreams.

Now, we can say that to study dreams in any meaningful way, we must know the fundamental mechanism by which the brain stores vast amounts of information in it as memory – i.e. an understanding of the mechanism of forgetfulness/remembrance would definitely lead us into a better analysis of dreams!

The molecular-grid model (as proposed in The Biology of Thought) unfolds a new molecular mechanism by which thoughts may be generated by the neurons by converting external stimuli into internal thoughts (Chapters 4 through 7). This model also proposes a mechanism by which memory is stored in the brain for variable periods as short-term memories and long-term memories (Chapter 8, and see below). We will now employ this model to see how it answers these questions: “Why are dreams random, and why do we forget dreams?”

A Brief Note on Dreams

Dreams are successions of bizarre and meaningless ideas and emotions that occur during certain stages of sleep, which are produced involuntarily without obvious external sensory inputs. While sleep itself is a puzzling phenomenon of the brain, dreaming is no less enigmatic!  There are various theories about their origin – Sigmund Freud famously postulated that they are the manifestations of our repressed desires and anxieties, while Carl Jung, his student, said they represent not only pent-up anxieties but dreams are meant to offer some solutions to problems. Some researchers took a more physiological explanation (in contrast to the above psychological explanations) and said that dreams are merely the manifestations of random stimulation of memory traces (see below) in the brain resulting in disorderly thoughts! Yet another theory, called the activation-synthesis hypothesis (by Hobson and McCarley – perhaps a theory much in vogue today), clubs both of these theories and says that the brain is randomly activated during dream state but the resultant irrational ideas are interpreted by the analytical human brain as some rational sequences and hence understood as purposeful “visitations”. However the disagreement continues!

Human sleep occurs in several cycles during the night, each cycle consisting of 5 stages of sleep. The first 4 stages of sleep are of varying depths ranging from light sleep to very deep sleep, and these stages correspond well with a gradual decrease in the brain activity (as demonstrated by the slowing of electrical wave rhythms in EEG). All these stages put together are referred to as non-REM sleep (or slow-wave sleep). The last stage of sleep is paradoxical in the sense that there is an increased activity of the brain (almost akin to the awaken state), and is associated with roving eye movements – hence called REM (rapid eye movement) sleep. Dreams typically occur during REM sleep.

Characteristically, the dreams experienced during sleep are not remembered after waking up and frequently they are unpleasant and ominous – though some are pleasant and welcome! Dreams are usually claimed to be associated with consolidation of memory and learning (see below) – even they are said to induce creative ideas and spur some people on new innovations. It is believed that some Renaissance painters have employed a technique of waking up while they were dreaming and depicting their dreams in their art!

A Brief Note on Neural Circuits in Sleep and Dreams

Slow, rhythmic and synchronized activity of neuronal circuits between thalamus and cortex generate oscillatory waves (the thalamo-cortical loop) and this is thought to be crucial for the development of sleep in humans. During waking periods this neural loop generates high-frequency oscillations (30-80 Hz gamma rhythm), which is suggested to be responsible for the binding of individual sensations into complex thoughts. However, in sleep primary cortical areas are shown to be shut off thus forming a “closed neural system”. Especially in REM/dream state the cholinergic neurons of the pons are activated, which then pass signals to the lateral geniculate body (in thalamus) thence to the visual cortex (ponto-geniculo-occipital spikes) (Barrett et al, 2012, pp. 272-273). It is also noted that pons sends signals to the frontal lobes which are especially activated in dreaming. Thus the integrity of brainstem is supposed to be essential for REM/dream state. Dreaming is a dopaminergic process that occurs in the limbic areas involving cortex through the Papez circuit (adding a significant emotional content to dreams).

It is thought that the characteristic inability to remember many dreams is a consequence of the dynamics of neural circuits and their neuromodulatory systems – but the following discussion reveals a different dimension to this problem!

Original Artwork by KG Dharani
Original Artwork by KG Dharani

Biological Explanation of Dreams

By the above discussion it becomes clear that there are two problems that needs explanation – one, dreams are random and disorderly, and two, we tend to forget dreams upon waking. These two outstanding features of the dreams can be accounted for by using the molecular-grid model, as shown below.

The Orderly Human Memory

Human thought is exceedingly complex, but at the same time is highly orderly. The multitude of sensory inputs we receive each moment are arranged in a sequential order to enable a meaningful idea of the external world we perceive. For example, when an image of a rose flower impinges on the retina, several signals of color are sent to the neurons in the brain where they are converted into several primary thoughts of color (details in the book!). Only when the perceptions of these colors and shades are arranged in a sequential order, we can get a meaningful idea of the rose we see – if they were to be disorderly the image we appreciate would no longer be that of a rose! Consider a more complex situation – when we pick up a rose and smell it, we are utilizing at least three modalities of sensation all at once. We look at the flower, we feel the texture of it while picking and we smell the odor of it – only when all these sensory inputs are arranged in a sequential and organized manner, it gives us the knowledge of a real rose (i.e. not a paper-rose!). When our thoughts are disorderly (as for example in an inebriated state or in insanity) the information becomes disorderly and hence the inference routs into irrational ideas (perhaps the rose is perceived as a paper rose or a blob of red waste-tissue).

What causes our conscious thoughts to be arranged in an orderly fashion, what are the mechanisms involved? And why are dreams disorderly and ephemeral?

We will answer these questions in my next blog post by looking at five questions in my next blog post. See them now!

About the Author

KG DharaniKrishnagopal Dharani is a medical doctor practicing at Adoni, a large town in South India. He has graduated in medicine from Kurnool Medical College in Andhra Pradesh, and did his general surgery from Kasturba Medical College, Manipal, South Canara. He took his post-doctoral specialization in vascular surgery at the Nizam’s Institute of Medical Sciences, Hyderabad. He is presently holding the post of Specialist Civil Surgeon in AP Medical Services, and despite having a large surgical practice, he manages to split his time between his profession and his academic pursuits in science.  The author can be contacted at kgdharanidr@gmail.com

 

 

References

1. Barrett KE, Barman SM, Boitano S, Brooks HL (eds.) (2012) Ganong’s Review of Medical Physiology, 24th Edition, New Delhi: Tata McGraw Hill

2. Guyton AC, Hall JE (eds.) (2008) Textbook of Medical Physiology, 11th Edition, Noida: Elsevier-Saunders

3. Nakanishil H, Sun Y, Nakamura RK, Moril K, Ito M, Suda S, Nambal H, Storch F, Dangl T, Mendelson W, Mishkin, Kennedyi C. Positive correlations between cerebral protein synthesis rates and deep sleep in Macaca mulaffa. European Journal of Neuroscience 1997;(9)271-279

4. Ramm P and Smith CT. Rates of cerebral protein synthesis are linked to slow wave sleep in the rat. Physiology and Behavior 1990;(48)749-753

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