A recent study published in Science Advances outlined a new approach to displaying color imagery by directly controlling photoreceptor activity via cell-by-cell light delivery with a platform called Oz.
Give me some background.
Quick refresher on cone cells and color vision: In normal color vision, light stimulates one of three types of photoreceptor cones cells embedded in the retina—each sensitive to different wavelengths of light:
- S cones: Shorter, blue wavelengths
- M cones: Medium, greenish wavelengths
- L cones: Longer, reddish wavelengths
Note: An M cone cell must also stimulate its neighboring L and/or S cones because the M cone spectral response function lies between that of the L and S cones and completely overlaps with them.
- Meaning: 85% of light that activates M cones also activates L cones.
Is this the first attempt to isolate M cone stimulation?
Previous studies have selectively excited M cones by targeting light to only one or two cones at a time.
In addition: Aside from cone-targeted methods, researchers have selectively excited M cones using visual pre-adaptation, such as bleaching L photopigment with red light before displaying green light.
However: These attempts rely on fleeting adaptation states and after-images, so they are difficult to measure precisely.
Now bring in Oz.
In theory, novel colors are possible through bypassing the constraints set by the cone spectral sensitivities and activating M cone cells exclusively.
Moreover: Stimulation from the Oz system could target light to only M cones and not L or S, which—in principle—would send a color signal to the brain that never occurs in natural vision.
So what does the Oz system do?
As one report noted: The system utilizes tiny doses of laser light to individually control up to 1,000 photoreceptors within the eye—all at once.
Break this process down for me.
Building on those abovementioned cone-targeted methods, investigators:
- First: Selected a color image and then utilized adaptive optics optical coherence tomography (AO-OCT) to identify the cone cells that needed to be activated in each subject’s retina to see the image.
- Next: Used adaptive optics scanning light ophthalmoscopy (AOSLO) to simultaneously image and stimulate the retina with a raster scan of near-diffraction-limited laser light over a 0.9° square field of view centered at 4° adjacent to a gaze-fixation target.
- Then: Tracked the eye’s motion in real time using nearly-invisible infrared light to image the retina and deliver microdoses of visible-wavelength laser light dynamically targeted at each cone cell within the field of view.
Note: The laser beam was green—the same hue as a green laser pointer.
I have to ask: Where did the name Oz come from?
“We chose Oz to be the name because it was like we were going on a journey to the land of Oz to see this brilliant color that we’d never seen before,” explained James Carl Fong, first author of the study and a doctoral student in electrical engineering and computer science at the University of California (UC) Berkeley.
Now, talk about this study.
The study outlined multiple steps—from principle to prototype to proof of principle—including:
- Theory of cell-by-cell color to make it possible to see color outside of the natural human gamut
- Design of the Oz prototype
- Color matching experiments to formally measure the color seen by participants
- Image and video recognition experiments to determine the scope of Oz
And the findings?
The study included five participants (two of which were from the research team) and confirmed the proof of principle—establishing a partial expansion of colorspace by activating M cones exclusively to elicit a color beyond the natural human gamut, which they called “olo.”
And what happened in these color matching experiments?
Subjects reported that olo appeared blue-green or peacock green of unprecedented saturation when viewed relative to a neutral gray background.
Further: Participants found that they had to desaturate olo by adding white light before they could achieve a color match with the closest monochromatic light—providing proof that olo lies beyond the natural color gamut.
Finally: Additional experiments showed that subjects perceived Oz colors in both image and video form.
Expert opinion?
Investigators noted that when Oz microdoses were intentionally “jittered” by just a few microns, participants perceived the stimulating laser’s natural color (i.e., laser pointer green).
“But when the same Oz microdoses were accurately delivered, subjects could perceive different colors of the rainbow, unprecedented colors beyond the natural human gamut, and imagery like brilliant red lines or rotating dots on an olo background,” the study authors highlighted.
Interesting … so how could Oz be used in the future?
The study authors explained that Oz could support systematic probing of phenomena related to color vision, such as:
- The threshold at which a small number of cones begin to contribute to a stable color
- The nonlinear function of a retinal ganglion cell’s (RGC’s) response to cone activations in its receptive field
- Cone activations underlying visual phenomena that operate near the limits of visual perception (ex., two colored-line illusion, visual loss with high levels of cone dropout)
- Neural plasticity to boost color dimensionality in humans, like eliciting full trichromatic color vision in a red-green colorblind person or eliciting tetrachromacy in a trichromat
Take home.
These findings suggest that the Oz platform allows for humans to perceive unprecedented colors beyond the gamut of human color vision, such as olo, by stimulating M cone cells exclusively.