Retinal stimulation reveals colour never before seen by the human eye

22 Apr 2025 Tami Freeman

Retinal stimulation reveals colour never before seen by the human eye, 22 Apr 2025, Tami Freeman Oz's principle for stimulating the human retina. Novel technique elicits colour beyond the natural range of human vision. Courtesy: Fong et al. Sci. Adv. 10.1126/sciadv.adu1052

Fullscreen Player Information About Brightcove A new retinal stimulation technique called Oz enabled volunteers to see colours beyond human vision's natural range. Developed by researchers at UC Berkeley, Oz works by stimulating individual cone cells in the retina with targeted microdoses of laser light, while compensating for the eye’s motion.

Colour vision is enabled by cone cells in the retina. Most humans have three types of cone cells, L, M, and S (long, medium, and short), which respond to different wavelengths of visible light. During natural human vision, the spectral distribution of light reaching these cone cells determines the colours we see.

Spectral sensitivity curves. The response function of M cone cells overlaps completely with those of L and S cones. (Courtesy: Ben Rudiak-Gould) Some colours, however, simply cannot be seen. The spectral sensitivity curves of the three cone types overlap. In particular, no wavelength of light stimulates only the M cone cells without stimulating nearby L (and sometimes also S) cones.

The Oz approach, however, is fundamentally different. Rather than being based on spectral distribution, colour perception is controlled by shaping the spatial distribution of light on the retina.

In Science Advances, Ren Ng and colleagues described the technique and showed that targeting individual cone cells with a 543 nm laser enabled subjects to see a range of colours in images and videos. Intriguingly, stimulating only the M cone cells sent a colour signal to the brain that never occurs in natural vision.

The Oz laser system uses adaptive optics scanning light ophthalmoscopy (AOSLO) to simultaneously image and stimulate the retina with a raster laser light scan. The device images the retina with infrared light to track eye motion in real time and targets pulses of visible laser light at individual cone cells, at a rate of 105 per second.

The researchers tested a prototype Oz system on five volunteers in a proof-of-principle experiment. In a preparatory step, they used adaptive optics-based optical coherence tomography (AO-OCT) to classify the LMS spectral type of 1000 to 2000 cone cells in each subject’s retina region.

When exclusively targeting M cone cells in these retinal regions, subjects reported seeing a new blue–green colour of unprecedented saturation, which the researchers named “olo”. They could also clearly perceive Oz hues in image and video form, reliably detecting the orientation of a red line and the motion direction of a rotating red dot on olo backgrounds. In colour matching experiments, subjects could only match olo with the closest monochromatic light by desaturating it with white light, demonstrating that olo lies beyond the range of natural vision.

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The team also performed control experiments in which a few microns intentionally “jittered” the Oz microdoses. With the target locations no longer delivered accurately, the subjects perceived the natural colour of the stimulating laser instead. In the image and video recognition experiments, jittering the microdose target locations reduced the task accuracy to a guessing rate.

Ng and colleagues conclude that “Oz represents a new class of experimental platform for vision science and neuroscience [that] will enable diverse new experiments”. They also suggest that the technique could one day help to elicit full colour vision in people with colour blindness.

Oz principle for stimulating the human retina

Novel technique elicits colour beyond the natural range of human vision. Courtesy: Fong et al. Sci. Adv. 10.1126/sciadv.adu1052

A new retinal stimulation technique called Oz enabled volunteers to see colours beyond human vision's natural range. Developed by researchers at UC Berkeley, Oz works by stimulating individual cone cells in the retina with targeted microdoses of laser light, while compensating for the eye’s motion.

Colour vision is enabled by cone cells in the retina. Most humans have three types of cone cells, L, M, and S (long, medium, and short), which respond to different wavelengths of visible light. During natural human vision, the spectral distribution of light reaching these cone cells determines the colours we see.Spectral sensitivity curves. The response function of M cone cells overlaps completely with those of L and S cones. (Courtesy: Ben Rudiak-Gould)

Some colours, however, simply cannot be seen. The spectral sensitivity curves of the three cone types overlap. In particular, no wavelength of light stimulates only the M cone cells without stimulating nearby L (and sometimes also S) cones.

The Oz approach, however, is fundamentally different. Rather than being based on spectral distribution, colour perception is controlled by shaping the spatial distribution of light on the retina.

In Science Advances, Ren Ng and colleagues described the technique and showed that targeting individual cone cells with a 543 nm laser enabled subjects to see a range of colours in images and videos. Intriguingly, stimulating only the M cone cells sent a colour signal to the brain that never occurs in natural vision.

The Oz laser system uses adaptive optics scanning light ophthalmoscopy (AOSLO) to simultaneously image and stimulate the retina with a raster laser light scan. The device pictures the retina with infrared light to track eye motion in real time and targets pulses of visible laser light at individual cone cells, at a rate of 105 per second.

The researchers tested a prototype Oz system on five volunteers in a proof-of-principle experiment. In a preparatory step, they used adaptive optics-based optical coherence tomography (AO-OCT) to classify the LMS spectral type of 1000 to 2000 cone cells in each subject’s retina region.

When exclusively targeting M cone cells in these retinal regions, subjects reported seeing a new blue–green colour of unprecedented saturation, which the researchers named “olo”. They could also clearly perceive Oz hues in image and video form, reliably detecting the orientation of a red line and the motion direction of a rotating red dot on olo backgrounds. In colour matching experiments, subjects could only match olo with the closest monochromatic light by desaturating it with white light, demonstrating that olo lies beyond the range of natural vision.Read more

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The team also performed control experiments in which a few microns intentionally “jittered” the Oz microdoses. With the target locations no longer delivered accurately, the subjects perceived the natural colour of the stimulating laser instead. In the image and video recognition experiments, jittering the microdose target locations reduced the task accuracy to a guessing rate.

Ng and colleagues conclude that “Oz represents a new class of experimental platform for vision science and neuroscience [that] will enable diverse new experiments”. They also suggest that the technique could one day help to elicit complete colour vision in people with colour blindness.

from physicsworld.com  2/5/2025