Scientists have discovered that applying an electrical current to the brain can improve people's vision temporarily, and the worse your eyesight is, the more powerful the boosting effect.

Researchers found that zapping the visual cortex of the brain for 20 minutes via transcranial direct-current stimulation offered improvements in vision lasting up to 2 hours. But the way the technique works – primarily affecting the brain, not the eyes – means we can't throw out our glasses or contacts just yet.

"It's actually a very simple idea," said psychologist Geoff Woodman from Vanderbilt University. "This kind of stimulation can improve cognitive processing in other brain areas, so if we stimulate the visual system, could we improve processing? Could we make someone's vision better – not at the level of the eye, like Lasik or glasses, but directly at the level of the brain?"

Woodman and his team tested this hypothesis on 20 young, healthy people who had normal or near-normal vision. The group was asked to evaluate the position of two identical vertical lines, and assess whether they were perfectly aligned or slightly offset from one another.

The researchers say this test is more sensitive than reading a standard eye chart (with big letters up the top, getting smaller as they progress to the bottom) and gives them a precise measurement of participants' visual acuity.

After taking the initial test, the group received a very mild electrical current to the back of their brain where the visual cortex is located. Twenty minutes later, they took the visual line evaluation test again, and the researchers found that 75 percent of the participants showed measurable improvement after having had the transcranial stimulation.

When the researchers repeated the experiment and changed the position of the electrodes – ie. applying the current elsewhere in the brain, and not directly to the visual processing centre – the participants' eyesight wasn't improved, which suggests that the vision benefits of stimulation are limited to the visual cortex.

To see how much of an improvement the current stimulation would give people in a real-world scenario, the researchers also had the participants read a standard eye chart before and after having a current applied to their visual cortex.

After stimulation, the participants' improved vision gave them on average the ability to identify one or two small letters that they hadn't been able to identify beforehand.

"We saw that those who came in with poorer vision, who might be on their way to needing glasses, had these big leaps, while others who came in with excellent vision showed no change," said one of the researchers, Robert Reinhart, now with Boston University.

While the research only has a very small sample size, meaning the results would need to be replicated in a larger study before we can jump to any conclusions, it makes sense that applying current to the visual cortex could offer benefits in visual processing.

Researchers have previously found that electrically stimulating other parts of the brain can enhance people's cognitive processes in a number of ways, including boosting their creativity and powers of memory.

It's not yet clear exactly how the direct current improves vision, but the researchers speculate that the electricity might boost visual signals in the brain so certain neurons can process them more quickly. It's also possible that the stimulation introduces some kind of white noise into the visual cortex, which makes it easier for the brain to focus on specific visual information and ignore distractions.

In either case, there are some promising leads here for future research. "Now we have a new tool that could be valuable for researchers investigating fundamental questions about how the visual system works," said Reinhart.

The findings are published in Current Biology.