My Dad has always said that at the end of each of his dreams, he sees closing credits, as if he were watching a movie. Each character is acknowledged for taking part in the production of the dream as their names scroll down the black screen. As a young child, I hoped that when I grew older my dreams, too, would play movie credits. Although my stubborn (and equally humorous) Dad still refuses to admit to it, I now know that he’s just been joking with me my whole life. Obviously, our dreams are not low-budget films and do not exist purely as a form of nightly entertainment.
The reason that dreams exist is, in fact, unknown and highly debated. Why was I playing with penguins in my dorm room last night? Why did I dream about my close family friends running a preschool in the basement of their new house? While some dreams are more mundane than others (mine often being humdrum), dreaming has historically been a curious subject. It was once commonly believed that during the nighttime, an individual’s soul left their body to explore the dream world. More recently, Freud, a well-known 19th century psychologist, saw dreams as a portal into the unconscious mind. Perhaps Freud would have found some unwitting desire of mine to return to my childhood in my dream about the preschool in our basement. Despite the popularity of these theories and others alike, more recent theories focus less on dreams themselves as the objective and more as a sort of byproduct of some fundamental neurological process.
One such theory suggests that dreams are a collective reflection of stimuli encountered throughout one’s life that are being consolidated simultaneously. Maybe this is why people, conversations, sounds, objects etc. encountered during the previous day make their way into your dreams. Research suggests that procedural memories, or non-declarative memories, are especially strengthened during REM sleep (Dickelmann et al., 2010); these are the memories that you can’t necessarily put into words, but allow you to do tasks, for example ride a bike. REM is the stage of sleep characterized by rapid-eye movement (hence the name REM), lack of muscle tone and the production of a bizarre mingling of imagery. It’s the stage at which you spot your second grade soccer coach on a date with your college roommate at Applebee’s or frantically arrive later to your first day of middle school, even though you’re 30 years old.
So, how do we see such vivid images at night without direct input from our eyes? When waking, superior visual regions responsible for advanced visual processing are only triggered through direct visual activation of the primary visual cortices (V1). The primary regions receive contrast in the form of light from your optic nerve. For example, when you look at the following picture on the top, specialized V1 cells in your brain are able to distinguish where the black ends and the white starts and vice versa (Martin, personal communication). In order to see more complex images, such as the Zebras on the bottom, thousands of these interpretations through the specialized cells are combined and more superior (downstream) regions are also involved to form the image. During REM sleep, activation of these downstream visual cortices has been shown without V1 activation (Braun et al., 1998). Again, activation of superior regions alone does not occur in waking hours, and is unique to REM sleep. Lack of visual input can be interpreted as an absence of supervision or management from V1 cells. This lack of inhibition allows the advanced visual systems to act freely. This distinctive closed circuit can be argued to produce the bizarre patterns that we construe as dreams upon waking.
Dreams are not limited, however, to visual stimuli. Tactile and auditory stimuli are also frequent. I was curious how dreaming differs for blind people, including those born blind. Are their downstream visual cortices also active during REM sleep? Do they have visual imagery in their dreams? I found an interesting study that shed some light on this topic. This study compared recalled dream components from congenitally blind, late blind, and sighted participants. The data suggests a significant difference in visual dream stimuli between the blind population and the sighted. The longer participants have been blind, the less visual impressions. Even though one’s eyes or optic nerve may be preventing vision during waking hours, superior visual regions of the brain still hold the key to visual memories stored in the brain and access is only granted during dreaming. In fact, 70% of participants who developed blindness later in life’s dreams contained visual impressions. This means that someone who has been blind for 10 years might find themselves dreaming something along the order of the following image, without working eyes, for example. Working with the memory consolidation theory for a minute, I think this could mean that the stimuli a late blind person encounters during the day may be consolidated with preexisting visual memories.
In addition, blind participants had significantly more auditory and tactile impressions in the dreams in comparison to sighted participants. An equal number of social interactions, emotions, successes and failures, and oddities in the dreams was observed. This suggests that dream content may not differ largely, but the medium in which the content is displayed in dreams differs. For example, while I might have seen the penguins in my dorm room, a blind person with a similar dream might have felt them or heard their little penguin noises more than I did. These results may offer support for the memory consolidation theory. The ability to create a visual image without direct input requires memory retrieval; thus, congenitally blind individuals do not have the visual memories to retrieve to form any visual impressions, whereas late blind and sighted individuals do.
Circling back, these crazy images, sounds, or objects we feel at night might just be a projection of the memory consolidation process. Thus, the dream you had where you kissed your best friend might be more of a result of consolidation and activation of several memories, than a reflection of an unconscious desire to marry him. Sorry, Freud.
Braun, A, R., Balkin, T, J., Wesensten, N, J., Gwadry, F., Carson, R, E., Varga, M., Baldwin, P., Belenky, G., & Herscovitch, P. (1998). Dissociated Pattern of Activity in Visual Cortices and Their Projections During Human Rapid Eye Movement Sleep. Science, 279, 91-95.
Meaidi, A., Jennum, P., Ptito, M., & Kupers, R. (2014). The sensory construction of dreams and nightmare frequency in congenitally blind and late blind individuals. Sleep Medicine, 15, 586–595.