Regular cannabis use delays the processing of visual information at its very beginning, in the retina, the results of a study published Thursday in JAMA Ophthalmology suggest. This disruption may impact the eyesight of regular pot smokers and edibles eaters, even if the influence is very weak, the authors noted.
“Such an anomaly could be imperceptible for cannabis users,” said Dr. Vincent Laprevote, lead author of the study and a physician at Pole Hospitalo-Universitaire de Psychiatrie du Grand Nancy in Laxou, France. “However, it is important, since it could reflect the changes in the communication between neural cells implied by regular cannabis use.”
Cannabis is known to act on synaptic transmissions within the brain. Since the retina is an easy-to-access extension of the central nervous system, a comparison study of retinal processes in both regular marijuana users and controls might provide more information about the drug’s effects in the brain, hypothesized Laprevote and his co-authors.
The researchers examined whether cannabis disturbs the function of cells known as retinal ganglion cells.
“We are particularly interested in these ganglion cells because they behave as brain cells,” said Laprevote.
These cells are partially responsible for transforming light (visual stimulation) into a series of electrical pulses, called action potentials, in the brain.
“The timing of signals traveling from the retina to the visual brain is critical for normal visual processing,” said Dr. Laura Frishman, a professor and associate dean at the University of Houston College of Optometry, who was not involved in the study.
To understand the impact of marijuana use on retinal ganglion cell function, the researchers measured the electrical and physiological response of these cells in 52 people: 28 regular cannabis users and 24 who did not use the drug.
Laprevote and his co-authors discovered a delayed response time for cannabis users compared with controls. On average, the marijuana users registered 98.6 milliseconds, compared with 88.4 for the people in the
control group.
This result suggests dysfunction in communication between ganglion cells, said Laprevote.
“At present, the authors only tested the retinal ganglion cell signaling,” Frishman noted. She added that this is a flaw, since the recorded measurements reflect the function of other retinal cells and photoreceptors, not just ganglion cells. The researchers, then, did not determine precisely how cannabis affects the retina.
The research was supported, in part, by a grant from the French Interministerial Mission for Combating Drugs and Addictive Behavior.
Dr. Christopher Lyons of the department of ophthalmology and visual sciences at University of British Columbia and Anthony G. Robson of the department of electrophysiology at Moorfields Eye Hospital in London believe the article addresses an important and neglected issue: the possible toxic effects of cannabis.
Any negative effects on the visual system “would also have implications for driving, work and other activities,” Lyons and Robson wrote in an editorial accompanying the study. Though they believe the topic worthy of study, Lyons and Robson point out flaws in the research, including the small number of participants and the inability to determine how pure and how much cannabis each participant consumed.
“Shortcomings in the study design, methods, and data analysis, acknowledged in part by the authors, weaken their conclusions,” wrote Lyons and Robson.
“Of course, this is a preliminary study, and we have to be prudent in the interpretation of the results,” Laprevote said. “We have now to verify if this delay is also present in later stages of visual processing in the brain.”
Frishman also believes further study is warranted. “An effect of cannabis on visual processing would be important,” she said, “because of its impact on many human activities that require precise timing in the visual pathways.”
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