Some lucky human guinea pigs had one of those “good news, bad news” situations recently. Good news was they got to contribute to global medical knowledge by getting high regularly. Bad news was that this regular high-getting has caused their eyesight to worsen, at least according to the findings of a French research team.

The problem, as outlined in a paper published by JAMA Ophthalmology, is not so much that stoners see worse than everyone else than that they see slower. As The Independent put it, a “small but significant delay was found in the time taken for the signals to be processed by the retina” in regular cannabis users, compared to non-users in the control group.

As per the study itself: “This finding provides evidence for a delay of approximately 10 milliseconds in the transmission of action potentials evoked by the retinal ganglion cells.” In other words, regular weed usage slows down the central nervous system, at least as it pertains to retinal signals.

This new study comes after years of several studies which show evidence that cannabis use can actually improve certain eye impairments. That includes an analysis from Nature which says that the cannabis use of pregnant women “improved significantly” the global motion perception of their babies. Global motion perception is “a behavioural measure of processing within the dorsal extrastriate visual cortex that is thought to be particularly vulnerable to abnormal neurodevelopment.”

An official statement from the  American Glaucoma Society also says that cannabis use can decrease the intraocular pressure (IOP) associated with glaucoma. However, the group states, relief from the pressure would require using cannabis around the clock every three hours, a task that is unpragmatic for many people.

These potentially contradictory studies on cannabis and eyesight point to the need for additional research, a point shared by the leader of the French study, Dr. Vincent Laprevote of the Pole Hospitalo-Universitaire de Psychiatrie du Grand Nancy. Laprevote said that, “Independent of debates about its legalisation, it is necessary to gain more knowledge about the different effects of cannabis so that the public can be informed.

“Future studies may shed light on the potential consequences of these retinal dysfunctions for visual cortical processing and whether these dysfunctions are permanent or disappear after cannabis withdrawal.”

Phot via Flickr user Sean MacEntee