Summary: Smoking marijuana affects several key visual functions, including 3D vision and contrast sensitivity. However, 90% of cannabis users believe the drug has either no, or minimal, effect on their visual abilities.
Source: University of Granada
A study carried out by the University of Granada indicates that smoking cannabis significantly alters key visual functions, such as visual acuity, contrast sensitivity, three-dimensional vision (stereopsis), the ability to focus, and glare sensitivity.
Yet, more than 90% of users believe that using cannabis has no effect on their vision, or only a slight effect.
A group of researchers from the Department of Optics of the University of Granada (UGR) has studied the effects of smoking cannabis on various visual parameters compared to the effect that the users themselves perceive the drug to have on their vision.
This study, led by Carolina Ortiz Herrera and Rosario González Anera, has been published in the journal Scientific Reports. Its main author, Sonia Ortiz Peregrina, explains that cannabis use is on the rise despite being an illegal drug. According to the national Survey on Alcohol, Drugs and Other Addictions in Spain 2019-2020, cannabis use nationally has increased since 2011, with 37% of Spanish adults having used this drug at some time. Approximately 10% consumed it in the last year.
In this study, which had the approval of the Human Research Ethics Committee of the UGR (ref. 921/CCEIH/2019), an exhaustive visual trial was conducted on 31 cannabis users, both when they had not consumed any substance in advance and also when they were under the effect of the drug. The researchers also studied the participants’ perception of the visual effects of having consumed this drug.
The results showed that, following consumption, visual aspects such as visual acuity, contrast sensitivity, three-dimensional vision (stereopsis), the ability to focus, and glare sensitivity significantly worsened.
Despite this, not all subjects reported a worsening of their vision after smoking cannabis. Indeed, 30% reported that their vision had not suffered at all, while 65% responded that it had worsened only slightly. The authors note that the visual parameter that could be most strongly linked to users’ perception of the visual effect is contrast sensitivity.
The study found a negative effect on all of the visual parameters evaluated, with the effect of cannabis on some of the parameters being analysed for the first time in this research. These results, together with the lack of awareness that the participants presented about the visual impairment caused by smoking cannabis, indicate the need to carry out awareness-raising campaigns, as this visual deterioration can pose a danger when performing everyday tasks.
About this visual neuroscience research news
Source: University of Granada
Contact: Sonia Ortiz Peregrina – University of Granada
Image: The image is in the public domain
Original Research: Open access.
“Effects of cannabis on visual function and self-perceived visual quality” by Sonia Ortiz-Peregrina, Carolina Ortiz, Miriam Casares-López, José R. Jiménez & Rosario G. Anera. Scientific Reports
Effects of cannabis on visual function and self-perceived visual quality
Cannabis is one of the most used drugs of abuse in the world. The objective of this study was to analyze the effects of smoking cannabis on vision and to relate these to those perceived by the user. Thirty-one cannabis users participated in this study. Visual function assessment was carried out in a baseline session as well as after smoking cannabis.
We evaluated static visual acuity, contrast sensitivity, stereoacuity, accommodative response, straylight, night-vision disturbances (halos) and pupil size. The participants were also divided into two groups depending on whether they perceived their vision to have worsened after smoking cannabis. A logistic regression analysis was employed to identify which visual test could best predict self-perceived visual effects.
The study found that smoking cannabis has significant adverse effects on all the visual parameters analyzed (p < 0.05). Self-perceived visual quality results revealed that about two thirds of the sample think that smoking cannabis impairs their vision. Contrast sensitivity, specifically for the spatial frequency 18 cpd, was identified as the only visual parameter significantly associated with self-perceived visual quality (Odds Ratio: 1.135; p = 0.040).
Smoking cannabis is associated with negative effects on visual function. Self-perceived visual quality after smoking cannabis could be related to impaired contrast sensitivity.