New research out of the University of Rochester—and published in the Journal of Neuroscience—has identified rare retinal ganglion cells (RGCs) that may resolve a long-standing, ongoing question of the backstory on color perception.
Let’s briefly revisit the anatomy.
First things first: the retina has three different cone types that can help to detect color and are sensitive to long (L), medium (M), or short (S) wavelengths of light.
RGC connection: Input from these cones are then transmitted by RGCs to the central nervous system.
- The result: Our perception of color
- Example: red-green and blue-yellow → M vs L+S and L vs M+S, respectively.
Go on…
Next thing to know is the “cardinal directions of color space,” which essentially explain what color detection is and provide the following thresholds: L vs M and S vs L+ M
Note: Each of these directions are intended to match the “most common cone-opponent (RGCs) in the primate retina.”
And for color perception: the cone opponency is believed to be established within the visual cortex.
I’m sensing a but …
You guessed right. Investigators noted, however, that the way the eye detects color and the way color may be perceived by humans differs—which doesn’t necessarily correlate with those cardinal directions. Thus, “scientists suspected that while most RGCs follow the cardinal directions, they may work in tandem with small numbers of non-cardinal RGCs to create more complex perceptions,” according to a University of Rochester article.
Which brings us to this research.
Indeed. A team of researchers from the University of Rochester’s Center for Visual Science—plus the Institute of Optics and the Flaum Eye Institute—used adaptive optics (AO) to identify these non-cardinal direction RGCs to determine how human see the following colors:
- Red
- Green
- Blue
- Yellow
Explain adaptive optics.
This is a technique that involves using a deformable mirror in the optical path that blurs the light by changing the light’s reflection just enough to cancel out the effects of the turbulent cells of air.
Translation: AO removes the atmospheric disturbances and allows a telescope to achieve “diffraction-limited imaging” from the ground.
What is it best used for?
Ideally, to correct wavefront distortions imaging and beam shaping for non-imaging applications.
In optical applications: The technique was first reported to be used for studying the human eye in the 1990s, where a camera was developed that made up for the distortions caused by the eye’s natural aberrations.
The resulting image: a clear picture of an individual’s photoreceptor cells as well as access to RGCs, thanks to AO’s ability to correct these natural aberrations.
And in this research?
Investigators combined AO scanning laser ophthalmoscopy with calcium imaging to collect data from three macaque monkeys (Macaca fascicularis) and view visual stimuli and image RGC responses at the center of the subjects’ foveas.Note: Prior to the imaging scans, these monkeys were treated with anesthesia before receiving injections of viral vectors; pupil dilation was conducted using 1% tropicamide and 2.5% phenylephrine.
- Also: each monkey was fitted with a custom contact lens to enhance wavefront correction
What did they find?
They confirmed that “neurons with both L vs M cone opponency and S-cone input are present in the primate fovea.”
Despite being unable to determine how downstream neurons use retinal neuron responses that are in tune with non-cardinal color direction, the researchers reaffirmed that “cone opponent signals in the retinal output are more diverse than suggested by textbook models of color vision.”
Any hypotheses?
They predicted that the L vs M+S and M vs L+S neurons are actually subtypes of midget RGCs (known to be the only L vs M cone-opponent RGC type present in primates).
However: The study authors noted that “anatomical experiments” would need to be conducted in order to confirm cell type.
Were there any limitations?
Among other recommendations, models examining the interaction between cardinal and non-cardinal mechanisms were called out as being unexplored and a potential “productive avenue for future research.”
Ultimately, though, the authors concluded that “comprehensive account of the neural mechanisms of color perception will require consideration of all cone-opponent neurons, including those tuned to non-cardinal color directions.”
Future implications.
Improving our understanding of the retina’s complex neurological processes could potentially lead to novel breakthroughs in how to approach visual restoration caused by retinal diseases.