Prototype Oz system stimulates thousands of M cone cells: On the threshold of new ocular experimentation

(Image credit: ©Lenka_X/AdobeStock)

A new study published in Science Advances describes what the researchers refer to as a new principle, ie, Oz, for displaying color imagery.

“Oz,” researchers said, “optically stimulates individual photoreceptor cells on the retina at population scale to directly control their activation levels."¹

By isolating and focusing on stimulating only the M cells on the retina, they created a new color signal that the human eye is not capable of perceiving, explained first author James Fong, PhD, from the Department of Electrical Engineering & Computer Sciences, University of California, Berkeley.

In addition to the Department of Electrical Engineering & Computer Sciences, Fong was joined in this study by researchers from the Herbert Wertheim School of Optometry & Vision Science, University of California, Berkeley, and the Department of Ophthalmology, University of Washington School of Medicine, Seattle.

“Theoretically, novel colors are possible through bypassing the constraints set by the cone spectral sensitivities and activating M cone cells exclusively," investigators explained. "In practice, we confirm a partial expansion of colorspace toward that theoretical ideal. Attempting to activate M cones exclusively is shown to elicit a color beyond the natural human gamut, formally measured with color matching by human subjects.”

Study methodology

In this experiment, the researchers isolated the M cells, which are one of the retinal photoceptors responsible for color perception and are sensitive to medium wavelengths of light. However, M cells do not respond to natural light. The other cone types are L and S that respond, respectively, to long and short light wavelengths.

To carry out the experiment, they first had to scan the retina with a laser to determine the position of the M cells on the retina. When that was accomplished, they stimulated the M cells with laser micropulsesin human subjects. These laser microdoses were delivered at a rate of 105 per second to a population of 103 cones. 

Five subjects (4 men, 1 woman; age range, 40-57 years) were recruited for this experiment. All subjects self-reported as having normal color vision and no ocular diseases.

Fong and colleagues explained, “Theoretically, Oz enables display of colors that lie beyond the well-known, bounded color gamut of natural human vision.² In normal color vision, any light that stimulates 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 overlaps completely with them.^3,4 However, Oz stimulation can, by definition, 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. Theoretically, Oz expands the natural human color gamut to any (L, M, and S) color coordinate. In practice, we achieve a partial expansion of colorspace toward this theoretical maximum.”

By stimulating individual cells in the retina, the laser pushed their perception beyond its natural limits.⁵

Once the M cells were stimulated, the researchers conducted color matching experiments and collected qualitative judgments of hue and saturation.

“These experiments confirmed that the prototype successfully displays a range of hues in Oz: e.g., from orange to yellow to green to blue-green with a 543-nm stimulating laser that ordinarily looks green. Further, color matching confirms that our attempt at stimulating only M cones displays a color that lies beyond the natural human gamut. We named this new color 'olo,' with the ideal version of olo defined as pure M activation. Subjects report that olo in our prototype system appears blue-green of unprecedented saturation, when viewed relative to a neutral gray background. Subjects find that they must desaturate olo by adding white light before they can achieve a color match with the closest monochromatic light, which lies on the boundary of the gamut, unequivocal proof that olo lies beyond the gamut,” the investigators reported.

The name olo comes from the binary 010, indicating that of the L, M and S cones, only the M cones are switched on.⁵

A bright and colorful future

The experiment under discussion is a first step in what researchers hope will facilitate diverse new experiments. 

“Oz represents a new class of experimental platform for vision science and neuroscience, which strives for complete control of the first neural layer to the brain, programmability of every photoreceptor’s activation at every point in time. Our prototype is an advance toward this class of neural control, and we demonstrate its ability to accurately deliver microdoses to target cones despite the challenges presented by constant fixational eye motion and the optical aberrations of the eye,” they stated.

When the microdoses are delivered accurately, subjects can be made to 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.

They pointed out, for example, the complex functions that the platform has the potential to achieve.

“Oz can support systematic probing of phenomena such as the threshold at which a small number of cones begin to contribute to a stable color percept.^6-10 or the nonlinear function of a retinal ganglion cell’s response to cone activations in its receptive field.^11,12 Oz can reproduce and then enable programmable “micro-adjustments” to probe the cone activations underlying visual phenomena that operate near the limits of visual perception, such as the two colored-line illusion¹³ or visual loss with high levels of cone dropout,^14,15” they said. 

Further, Oz may be able to probe the plasticity of human color vision. The investigators cited, for example, a study in which gene therapy was used to add a third cone type in adult squirrel monkeys, producing trichromatic color vision behavior.¹⁶ Similarly, they reported that Oz can program signals to the human brain as if a subset of cones were filled with a new photopigment type, allowing researchers to probe the qualitative color experience that was not seen in the squirrel monkey study. 

“Such an approach,” Fong and colleagues commented, “can flexibly probe neural plasticity to boost color dimensionality¹⁷ in humans, such as attempting to elicit full trichromatic color vision in a red-green colorblind person, or eliciting tetrachromacy in a human trichromat.”

References

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