Transcranial magnetic stimulation (TMS), a focal, non-invasive brain stimulation technique, appears to help improve visual perception of very faint visual targets in healthy study subjects, and eventually could be used to increase visual capabilities in patients with eye defects, according to a study appearing in the online journal PloS ONE.

The study also found that TMS, when combined with visual cues, enhanced visual performance, but only when those cues hinted or predicted at where the faint visual targets would appear. The study provides evidence that the right Frontal Eye Field (FEF) region of the human brain is involved in human conscious vision, the authors said.

“The fact that a combination of TMS and attentional cues can indeed improve visual sensitivity should be considered a proof of concept that visual capabilities can be manipulated and improved through those approaches,” reported the study, which was led by researchers at the Centre de Recherche de I’Institut du Cerveau et la Moelle (CRICM) in Paris.

“On that basis, strategies operating on cerebral sites involved in attentional orienting and conscious access could become a reality to punctually increase visual capabilities in healthy participants,” the authors wrote. “Similar principles could be also applied to clinical rehabilitation, aiming at containing visual acuity losses in patients with retinal defects, and allowing the emergence of episodic or lasting periods of conscious vision in cortically damaged patients.”

The study used single TMS pulses in an effort to modulate Frontal Eye Field activity prior to showing the subjects a very brief visual target, and then analyzed the impact of those pulses on the perception of those low-contrast targets.

The visual targets, which were designed to be at the edge of visual perception, were displayed on an eye tracker screen and were displayed against a grey background. The 13 study subjects were asked to identify when they saw the visual target, and also to determine which way it was oriented.

The experiment itself consisted of 1,400 trials, including 280 trials in which the target didn’t appear. In half of the trials, chosen randomly, a single TMS pulse that lasted 80, 100 or 140 milliseconds was delivered to the subject’s right FEF immediately prior to the target’s appearance. In the other half, a “sham” TMS pulse that didn’t actually reach the brain was delivered. Some series of experiments used visual cues, while others did not.

In trials involving real TMS pulse delivery, detection improved by an average of around 12% when compared to trials involving sham TMS pulse delivery, although in some cases the results did not reach statistical significance.

These results indicate it might be possible to use TMS to improve vision in people with retinal or cornea disease. However, the authors noted limitations to the potential use of the technology – mainly, that the effects didn’t last long and might not work for the entire field of vision.

“The ameliorations demonstrated in our study operate trial-by-trial and remain extremely short lasting,” the authors wrote. “Furthermore, they have been demonstrated for lateralized right or left peripheral detections and thus might not equally occur for targets presented in other locations of the visual hemifield.”

Both of these problems weaken the case for this technique to be used to increase visual perception in either healthy subjects or people with eye damage, the authors said. However, it may be possible to use longer TMS pulses or multi-day TMS regimens combined with special cuing to generate longer-lasting increases in visual perception abilities, they said.

Meanwhile, researchers are studying other potential uses for TMS. The technology is used to treat major depression, and several companies, including Neuronetics Inc., market TMS devices for depression treatment. However, clinical trials have shown only a modest effect for TMS in major depression, and little effect in other neurological and psychiatric conditions, such as Parkinson’s disease and schizophrenia.

However, TMS has proved useful for diagnostic purposes, particularly in determining the severity of spinal cord injuries and strokes.

Source: Chanes L. et al. Manipulation of Pre-Target Activity on the Right Frontal Eye Field Enhances Conscious Visual Perception in Humans. PLoS ONE 7(5):336232. doi:10.1371/journal.pone.0036232.