How and Why Does the Brain Create Dreams?

Dreams were once thought to be messages from the gods or omens of the future; we now understand that they are neither. In 1900, Sigmund Freud believed that the contents of dreams cannot be taken in the original sense literally. He speculated that dreams have “hidden meanings” and “repressed desires.” Unfortunately, his thoughts became firmly established in popular folk psychology. Today, there is little experimental evidence to support his explanations. The neural mechanisms underlying the need to dream, and their content, remain a mystery. We don’t know why we only dream while sleeping or why the normal healthy brain requires that it experience a 90- to 120-minute cycle of deep sleep before initiating the first dream phase (unless you’re taking an afternoon nap).

Dreams from the brainstem up

Probably one of the best explanations for how dreams form is the activation-synthesis model that was first proposed in 1977 (Hobson and McCarley). The theory has been subsequently modified and refined in the light of new findings; however, the general idea is that dreaming is a two-step process: activation initiated within the brainstem followed by the synthesis of a (mostly visual) story. Dreaming is usually accompanied by rapid eye movements (REM) which are due to increased activity in the brain stem [near the neurons that control eye muscles, these movements have nothing to do with the content of the dreams] that sends activity upward into the brain and ultimately into the areas of cortex that process visual information. Some of this electrical activity also stimulates the amygdala and other regions of the limbic system, structures that control emotional expression and our fear response.

The neural activity originating in the brainstem that is fed into the higher brain systems is highly random. The brain then does with this incoming random stimulation what it always does with incoming neural activity, it attempts to process it into a coherent story that we experience as a dream. During dreaming sensory input from the external world is actively blocked, in addition, motor outputs (except to our eye muscles) are also blocked so that we do not act out our dreams. Very similar neural activity patterns have been documented in mice, rats, and monkeys, suggesting that they dream similarly to humans.

Dreams from the cortex down

Another recently proposed explanation of dream generation is that dreaming and REM sleep are dissociable states that can occur independently of each other. REM is controlled by the brainstem while dreaming is controlled by dopamine release inside the frontal lobes. This concept is supported by the fact that dreaming can be initiated by drugs that mimic dopamine and stopped by drugs that block dopamine without producing any changes in the frequency or duration of REM. Furthermore, lesions of the forebrain alone have been shown to obliterate dreams without any effects on the presence of REM.

Taken together, these discoveries suggest that the projections of dopamine neurons into the forebrain are responsible for dream generation. The brainstem systems that control REM are probably the trigger for the activation of this forebrain dopamine system. This dopamine projection is known as the mesocortical-mesolimbic dopamine system. Given what is known about the function of this dopamine system, it is proposed that emotional drive and motivation are involved in the generation of the content of dreams.

Brain regions associated with dreaming

Bilateral deep occipital strokes can completely prevent the ability to dream. The occipital lobe is responsible for processing visual imagery. Deep bilateral lesions of the frontal lobes also cause the complete cessation of dreaming. These lesions had no effect on the presence of REM sleep, consistent with the idea that dreaming and REM sleep are generated by different brain regions. We also dream during non-REM sleep; electrophysiological studies suggest that activity within the posterior cortex plays an important role in dreaming during NREM sleep. Dreams during non-REM sleep are usually much shorter in duration, with a distinctly bland storyline than dreams during REM sleep.

The purpose of dreaming

The authors of the activation-synthesis model claim that dreams have no meaning. Many studies have shown that while sleeping is important in the formation and eventual consolidation of memories, we don’t know whether dreaming per se plays an equally crucial role. Some recent studies have provided direct evidence that the brainstem wave generators that produce REM are crucial for sleep-dependent memory processing.

The threat simulation hypothesis states that dreams are a simulation of threatening events and a rehearsal of threat perception and threat avoidance. While there is some evidence to support this idea, 50 to 60 percent of recurrent dreams do not appear to have a direct association with the threat simulation theory. If dreams of threatening events are simulations, they should be realistic, but most dreams are unrealistic. The activation of the amygdala during REM sleep likely explains the intense emotions, particularly fear and anxiety, in dreams. One 40-year-old theory proposed that the function of dreaming is to eliminate unwanted memories. Dreams are not simply forgotten, they are actively unlearned. Some animal studies support this theory.

Countless anecdotal reports suggest that dreaming enhances creativity and problem-solving abilities. One study demonstrated that compared with quiet rest or non-REM sleep, REM sleep facilitates the integration of initially uncorrelated information, leading to more creative problem-solving. The dopaminergic system is thought to promote creativity, such as associative thinking, innovative insights, and cognitive flexibility (Wenk, 2017). Creativity during dreaming may be because dreams during REM sleep are narrative and bizarre, and enable the dreamer to visualize novel solutions.

Dreaming Essential Reads

To Dream, Perchance to Forget?

5 Common Recurring Dreams and What They Might Mean

One of the most challenging aspects of dream research is that scientists must rely on verbal reports. Narratives describing mental experiences during sleep are all generated during the awake state, and there is likely to be confusion between dreaming with awakening and only dreams that can be recalled have been studied.

Clear gender differences in dream structure exist, for example, females see more and brighter colors; they also report seeing the faces and hands of their sexual partners. Males report seeing fewer colors and do not usually know the identity of their sexual partners. The objects and people in dreams are familiar. For example, no one reading this post has ever seen me in a dream. Dreams emphasize perception and emotions at the expense of logic or reason. The content reflects a dreamer's waking conceptions; if you’re a Democrat when you’re awake, you do not become a Republican in your dreams. Deciphering the significance of dreams remains a dream to this date.