Cannabis and Visual Processing
Introduction
The study of cannabis and endocannabinoids (eCBS) has exploded into almost all parts of the human body. The attention devoted to the effect of cannabis on vision and the eye has been limited compared to other parts of our anatomy. For example, a quick search on Pubmed for “cannabinoid + brain” easily yields over 2000 articles for the last few years. In contrast, a search of “cannabinoid + retina” and “cannabinoid + glaucoma” retrieves a few dozen studies from the last few years.
There are two main pathways to understanding the effects of cannabis on vision. One pathway is to look at the effects of drugs like THC and CBD on the mechanisms of the eye such as increasing or decreasing eye pressure. And the second pathway involves how are brains process that information. “The effect [of THC on vision] is subtle and modest as might be expected since smoking marijuana does not render the smoker blind.”
The Eye-Brain Connection
You see with your brain, not your eyes. The brain is responsible for everything we will ever think, know or experience. Our brains are completely shut off from the world we require it to navigate. All our brain experiences, such as vision, are electro-chemical signals rocketing along neurons at 268 miles per hour. If you put your hand on the back of your head, you are very close the parts of the brain that are extremely important for processing visual information. Our brain relies on the retinas of our eyeballs to break down incoming visual information to specific signals related to color and depth that are brains process to make sense out of the environment.
Visual perception is a complex mechanism that embraces the cognitive processes for the retrieval of information from environmental light in order to construct a meaningful representation of the environment. Briefly, visual processing begins in the retina with the absorption of light by photoreceptors, thus initiating the conversion of light into brain activity.
Cannabis and many other substances can affect visual processing. The brain is full of receptors and proteins that respond to compounds in cannabis. Which may influence the decisions that brain is making about what it is beholding. It seems simple, but modern research shows that this seamless processing of reality is complex. Depending on how you measure it, the brain uses aa third to half of its real estate to help you perceive and make sense of light signals from the retina.
The most obvious application of this research field is to understand the effect on driving ability, which combines potential effects on retina or the eyeball itself, and how we process the flow of signals from our retina to our visual processing centers. Our brains for example can tell us if we are moving next to an object, or if we are moving and the object is standing still, or if both the object and we are standing still while the environment is moving. Sometimes the brain struggles to figure out what is moving, imagine trying to park a car while the car in the adjacent spot pulls out.
Driving is no doubt a direct concern for anything that could affect vision. Since the 1960’s at least 20 studies have been published regarding the effect of THC on driving ability. Three main consistent points have emerged from these studies:
Cannabis produces a consistent, mild to moderate impairment on tracking ability, such as keeping in center of lane and some variability in speed of driving. The effects observed are significantly far less than that seen with a blood alcohol concentration of 0.08%.
Cannabis produces an extra delay in decision time, a “moment of reconsideration” before undertaking an inherently dangerous maneuver.
Cannabis induces more cautious driving habits
A review of this research by GW pharmaceuticals’ researchers conclude “…driving ability does not appear to be substantially impaired by cannabis. Any impairment is well with the range of (or lower than) what is currently produced by pharmaceutical agents which are commonly used for similar conditions.”
The researchers from the US Department of Transportation, National Highway Traffic Safety Administration study published in 1993 concluded, “Drivers under the influence of marijuana retain insight in their performance and will compensate…by slowing down or increasing effort. As a consequence, THC’s adverse effects on driving performance appear relatively small. Of the many psychotropic drugs, licit and illicit, that are available and sued by people who subsequently drive, marijuana may well be among the least harmful.” The significance is that the decrements in driving performance induced by exposure to cannabis with high THC content most significantly result from a non-selective decrease in attentional or processing resources.
In the National Longitudinal Survey of Youths (NLSY79), a nationally representative sample of 12,686 young men and women surveyed in 1979 to 2010 was used. The quality of eyesight of 1304 heavy marijuana users was compared with 1304 respondents with light or no marijuana use. There was no statistically significant difference in the self-reported quality of eyesight among heavy marijuana smokers compared with youths who never used marijuana or are light marijuana users . Among heavy marijuana smokers, males and high school graduates have decreased odds of reporting a poor quality of eyesight.
If science has shown us anything, it’s that studying cannabis is far from straight forward; Cannabis can both improve and impair vision under different situations and the effects may be sex-dependent. However, the precise brain mechanisms underlying impairments due to cannabis use, in particular those in cognitive functions critical for car driving, are still being debated.
Animal Research, Human Driving Studies, and Night Fishing
There is an evolutionary perspective, which adds depth to our discussion and reasons for the prevalence of fish in research studies on cannabis and vision. The receptors that THC stimulate, the cannabinoid receptors (i.e., CB1 receptor) are part of the family of rhodopsin receptors. Rhodopsin receptors were most likely the first G-protein coupled receptors to evolve when our ocean dwelling ancestors wiggled and waddled onto land. This rhodopsin receptor evolved in the eye, or what passed for eyes several hundred million years ago. The purpose of CB1’s great, great, great, great…great grand-daddy was to notify the brain of incoming light. The structure of the receptor was so useful our ancestors began using other versions of this G-protein to carry out other functions that help us to eat, sleep, relax, forget, and to process visual information.
The fish, such as goldfish, are the preferred animal model for basic eye research, and much data in the literature is generated by studying eCBs and THC in these eyeballs. The second most popular animal model are mice, which may represent a better model for studying the effect of cannabinoids on visual processing. Goldfish are used for two main reasons, despite mice perhaps being the superior research model to base human research upon. One is that goldfish are less expensive, and the second is that the use of goldfish provide reproducible mechanism about how eyes work in response to cannabis compounds and eCBs, mechanisms of eyeball function preserved across species from evolution.
It has been known for nearly 50 years that cannabis and the psychoactive constituent THC reduce intraocular pressure (IOP). IOP means the pressure in your eyes. Left untreated, high eye pressure can cause glaucoma and permanent vision loss in some individuals. However, some people can have ocular hypertension without developing any damage to their eyes or vision.
According to studies in ocular hypertension treatment, 4.5 to 9.4 percent of Americans age 40 or older have ocular hypertension, which increases their risk of developing sight-threatening conditions. Elevated IOP remains the chief hallmark and therapeutic target for glaucoma, a major cause of blindness.
THC’s effect on decreasing IOP may be sex-dependent, for example a recent study measured the effect and found it more pronounced in male mice lasting upwards of 8 hours Vs. 4 hours in female mice. A sex-dependent effect has not been confirmed in the goldfish.
While THC exerts beneficial effects on the eye (i.e., IOP and the retina) it can affect the visual processing of information, such has been shown in some driving studies. However, it is a convoluted area of research as it is unclear if THC’s effect is significant on visual processing as opposed to its effect on attention as mentioned above.
Isolated reports exist that describe cases involving seeing spots and blurred vision as a result of using or quitting the use of cannabis. Those reports appear rare, far from predictable or reproducible in controlled settings, and limited by potential product contamination (studies involving ‘street marijuana’) or drug-drug interactions or pre-existing conditions.
The best example of the effect of chronic cannabis use on vision, comes from humans fishing at night. If cannabis impaired vision and visual processing to a significant degree, then coral fishing at night after smoking hash or cannabis flowers would often result in the subjects sinking boats and going hungry. The study of fisherman has been evaluated at least three times. The base study involved subjects smoking kif, taking tinctures, and even up to 20mg of pure THC before navigating coral reefs and hunting for fish at night. West reported his observations in 1991, that Jamaican fishermen who smoked or ingested a crude tincture of cannabis were apparently able to see and navigate their boats through dangerous coral reefs in the darkness of night and able to see fish jumping out of the water in order to know where to throw their nets. The improvement of night vision after consuming cannabis has demonstrated a degree of reproducibility has been observed in both Morrocan and Jamaican fisherman.
Two takeaways from this section are that fish continue to be important for research studies on cannabis and vision. And the second is that the benefits of cannabis improving night vision are may be limited to THC, because CBD blocks the effects of THC on the CB1 receptor in the eye and the brain.
Is CBD Good for Your Eyes?
Like THC, there have been no significant reports of detriment to the eye from CBD use, except for some reports that it can block CB1 receptors thus CBD can block the beneficial effects of THC in eyeballs. Research surrounding cannabis on eye health, vision, and visual processing has been contradictory at times. In this section we will briefly discuss studies on CBD and eye health. For example, three of four studies that have tested CBD for effects on IOP have reported no effect, but the fourth has reported an increase in IOP. Unlike previous work, this fourth study used topically applied cannabinoids in the eyes of mice.
At least one physician has been outspoken about the concerns regarding the potential trade-off of vision for the health benefits of CBD , as there appears to be no risk or toxicity studies looking directly at IOP related to long-term CBD use in humans. Additionally, the semi-well-known study published in 2010 on CBD protecting the eye in diabetes , was retracted. The statement from the journal of Molecular vision reads “The authors made substantive errors in figure images of this article such that the hypotheses were not tested and the conclusions were not supported.” This study is a ghost – no one can read it anymore. Yet, this article continues to be cited as used as a basis for theories and research as if it still existed in the scientific literature.
CBD is a major constituent of cannabis that has been found to be without effect on IOP in most studies. CBD, contrary to expectation, has two opposing effects on IOP and can interfere with the effects of THC in the eye. Of particular importance to consider is its ability to block THC’s activity at CB1 receptors in the eye; effectively blocking THC’s ability to reduce eye pressure.
Terpenes Influence How We Perceive Colors.
Due to limitations of this article’s length and the reader’s stamina for science, a few tantalizing things about terpenes and vision are briefly discussed. Terpenes such as Pinene can increase eye blink frequency. Blinking cleans the surface of your eye of any debris and also nourishes your eye with oxygen and nutrients, keeping your eyes healthy and comfortable.
Ambient air levels of terpenes may have some benefits, but terpenes can also cause eye irritation, hence protective goggle are worn by gardeners spraying cinnamon oil or workers using concentrated forms of essential oils.
Terpenes may play a significant role in perception or visual processing. For example, why did van Gogh use so much yellow in his later paintings such as his portraits and self-portraits? According to one historical account, “Van Gogh also had some digestive problems, for which he may have taken santonin, a terpene, used at the time to treat and prevent intestinal parasites and known to cause yellow vision. He was presumed to have pica, which caused him to have abnormal cravings for other terpenes similar to santonin: thujone, turpentine, camphor and absinthe.” These common terpenes have been reported in various cannabis preparations.
Terpenes also affect how we see our environment, with affecting our physiology. “Hydrocarbon particles released by vegetation (such as terpenes) react with ozone creating “blue haze” that limits vision in the distance.”
Conclusions
Applications of cannabis’ therapeutic potential include persevering vision, protecting us from age-related blindness, treating migraines by altering light sensitivity and brain activity, and macular degenerative diseases via antioxidant and anti-apoptotic effects. The neuroprotectant and brain elasticity imparted from cannabinoids are important for potential benefits in eye-brain health.
It is valuable to understand the mechanisms by which THC and CBD regulate intraocular pressure, particularly at a time when their changing legal status and the perception of phytocannabinoids as safe contribute to a continued growth in their availability and popular embrace. CBD in particular has recently been approved by the FDA as an antiepileptic and is available in many grocery stores. The regulation of ocular pressure by THC and CBD is more complex than previously appreciated. THC acts via a combination of CB1 and other receptors in a sex-dependent manner, while CBD can both raise IOP and interfere with the effects of THC. The potential of CBD to elevate ocular pressure should be evaluated further as a potential side effect, particularly with long-term use.
And until more research clarifies CBD’s effect on the eye or produces reproducible science in fish, mice, and humans hunting fish—please avoid putting pure CBD directly into your eye.
