How Cannabis Can Affect Vision

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It’s a pitch-dark night in Jamaica, and Dr. Manley West is going fishing. He is accompanying local fishermen as they navigate a treacherous reef on their way to fishing grounds. A pharmacologist trained at some of the world’s top medical institutes, West has long been curious about the medicinal value of cannabis, whose use is widespread in his native Jamaica. The fishermen have no compass—and no lights. But they are stoutly fortified with cannabis. West expects the boat to run aground at any moment. Yet the trip goes smoothly. In the morning, assessing the narrow passages through the reef, West becomes convinced that cannabis helped the fishermen see in the dark. He notes that the fishermen waited half an hour after ingesting cannabis before setting sail, a delay that corresponds with cannabis’ maximum effect on behavior.

Was West right?

New Data on Vision and Cannabis

Manley West’s idea was described in a 1991 article he wrote in Nature, but he presented no measurements to support it. Thirty years later, a study by researchers at the University of Granada in Spain assessed participants’ vision in a battery of seven standard vision tests after smoking cannabis.

They did not find enhancement of night vision—nor of other visual abilities, such as depth perception. In fact, the Granada researchers found that cannabis led to fairly consistent deficits in vision. However, the deficits seemingly caused by cannabis are mild. Cannabis does not enhance vision, but neither does it impose a gauzy veil over our eyes.

The Granada study involved 31 adults who were tested once without cannabis, and another time 20 minutes after smoking a joint. The order of these two tests was swapped for half the participants to reduce practice effects, which could confound the results. (Most people improve on these kinds of tests and would be expected to do better the second time, whether they ingest cannabis then or not.)

First, they measured participants’ visual acuity with a standard eye chart, where the task is to identify smaller and smaller letters. Both when they smoked cannabis and when they did not, participants showed what would be considered slightly better than normal acuity by this measure. By a small margin, the cannabis condition led to worse acuity than the non-smoking condition, though this level of performance would still be more than sufficient to pass an eye test at the DMV.

Next, they tested contrast sensitivity, or the ability to detect patterns with different levels of contrast between dark and light. Participants had to judge the orientation of faint stripes as the width of the stripes varied. For most stripe widths, there was no difference between the cannabis condition and the no-cannabis condition. Only when the stripes were very wide, and, curiously, at one particular small width, was performance worse in the cannabis condition.

Even these small differences seem suspect to me: One or both of the impairments could be spurious due to the low sample size. Some conditions such as schizophrenia are known to cause deficits in detecting wider stripes, while other conditions such as cataracts lead to deficits across stripe width. But there are no known mechanisms that would lead to deficits of both wide stripes and one oddball width of narrow stripes.

Since the basic measures of acuity and contrast sensitivity are little if any different after smoking cannabis, we can say that basic spatial vision is essentially intact. However, differences were observed for some specialized visual functions.

The researchers tested stereoacuity using the kind of test that would be administered to screen for amblyopia, which is the condition caused by strabismus (having “crossed eyes”). In amblyopia, the brain ignores visual inputs from one eye since they can’t be aligned or brought into register with inputs from the other eye, so you effectively see with just one eye. This makes it difficult to feel the fullness of 3D. The task in a stereoacuity test is to identify dots that appear to have a three-dimensional shape while wearing glasses of the kind found at 3D movie theaters. After smoking cannabis, participants were significantly worse at this task, though the difference was small. The degradation of stereo vision would be much less than what would result from closing one eye.

Another visual ability tested was accommodation, or the ability to change the shape of your lens to bring either near or far objects into focus. This is like turning the focus wheel on a pair of binoculars, which moves lens elements closer or further away from your retina. (This is in fact how shark eyes accommodate, though ours work by changing lens curvature.) Cannabis again turned out to degrade accommodation, but once again the difference was small. It is conceivable that cannabis causes nearby objects to look a little blurrier, and makes judgments about the distance to nearby objects less accurate.

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The researchers also tested susceptibility to glare of the kind you might experience while driving at night. When it is dark, bright lights can obscure darker objects, even if you aren’t looking directly at the light source. This largely has to do with the fluids in and around your eyeball which scatter incoming light, sending it in many directions. The Granada researchers found that this kind of glare was slightly worse after cannabis, but a second task involving glare showed almost no difference between the two conditions.

In very dark conditions, when the pupil would normally be its widest to let in whatever small amount of light is available, the cannabis condition caused people’s pupils to stay slightly more constricted. The difference was about a hundredth of an inch in pupil diameter. Contrary to Manley West’s idea, all of this could conceivably contribute to worse night vision due to cannabis.

What It All Means

When we use vision, we aren’t usually using just one of the tested visual abilities; we are combining them. Since there were decreases in performance in most metrics, is reasonable to conclude that there are real—albeit small—vision deficits in the real world due to cannabis. This would be especially the case for situations like driving at night, which is already a challenging perceptual task. However, because participants in this study smoked an amount of their choice, meaning that dosage varied, it is hard to translate these findings into practical recommendations.

It should be emphasized that this study did not test motion or color perception, let alone attention, memory, or reaction time, which are known to be affected by cannabis. Impairments to these perceptual functions have a substantial effect, which would dwarf any small decrement in night vision.

But by the same token, I think it is sensible to conclude that cannabis-related changes in things like attention are not due to altered vision at the level of the eye and retina. That is, changes to attention and other functions are not knock-on effects of changes in the eye. Perception is a construction that depends as much on attention, memory, and other internal processes of the cerebral cortex as it does on information from the eye. It is the complex and not-well-understood effects of cannabis on the neuronal networks of our cortex that alter our perception and consciousness.

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