Summary: A new study reveals greater complexity in how mice perceive color. These findings may inform future approaches to human visual disorders.
Source: University of Colorado Anschutz Medical Campus.
Researchers at the University of Colorado Anschutz Medical Campus have found that color vision in mice is more elaborate than previously believed, revealing neural circuits that could guide future research into human vision disorders.
The study was published in the journal Neuron.
The research team, led by Maureen Stabio, PhD, assistant professor of anatomy and neurobiology at the University of Colorado School of Medicine, identified a previously unrecognized functional property of a little-understood retinal cell type called the M5.
It was already known that mice possess light-sensitive proteins called opsins that allow them to detect a limited range of colors. While studying how M5 cells process visual signals, Stabio and colleagues discovered that these neurons compare input from different opsins and transmit color-opponent signals to the brain for further interpretation.
“We are the first to discover this particular color vision circuit in mice,” Stabio said. “We knew they had opsins, but we didn’t know they possessed the other two requirements for color vision.”
Stabio’s lab focuses on the retina’s cells and circuits, including a subset called intrinsically photosensitive retinal ganglion cells (ipRGCs), which includes the M5 type. ipRGCs are known for roles in non-image-forming vision—such as signaling ambient light levels to the brain’s circadian system—rather than encoding detailed visual scenes.
The team found that the M5 cell, previously the least characterized ipRGC type, appears to participate in both non-image-forming and image-forming pathways. This blurs the line between circuits dedicated to reflexive light sensing and those that contribute to conscious visual perception.
Most surprising was the discovery that M5 cells process color information. Although mice are nocturnal and rely heavily on olfactory and tactile cues, they retain retinal mechanisms capable of distinguishing chromatic contrasts. The M5 cells integrate inputs from cone pathways and show chromatic opponency—specifically ultraviolet (UV)-driven excitation paired with green-driven inhibition.
“What exactly mice use this color information for remains to be determined,” Stabio said, “but we now know a circuit exists that routes chromatic signals into the brain.”
Source: David Kelly, University of Colorado Anschutz Medical Campus
Publisher: Organized by NeuroscienceNews.com
Image source: Public domain image from NeuroscienceNews.com
Original research: Abstract for “The M5 Cell: A Color-Opponent Intrinsically Photosensitive Retinal Ganglion Cell” by Maureen E. Stabio et al., Neuron. Published online December 14, 2017. DOI: 10.1016/j.neuron.2017.11.030
University of Colorado Anschutz Medical Campus. “Little Understood Cells Help Mice See Color.” NeuroscienceNews, December 15, 2017.
Abstract
The M5 Cell: A Color-Opponent Intrinsically Photosensitive Retinal Ganglion Cell
Highlights
- M5 cells form a morphologically and functionally distinct ipRGC subtype.
- They combine intrinsic melanopsin-driven responses with chromatically opponent, cone-based signals.
- Chromatic opponency in M5 cells manifests as ultraviolet-excitatory (UV-ON) and green-inhibitory (green-OFF) responses, mediated by specific bipolar cell inputs.
- M5 cells project to the dorsal lateral geniculate nucleus (dLGN), positioning them to deliver chromatic signals to visual cortex.
Summary
Intrinsically photosensitive retinal ganglion cells (ipRGCs) integrate direct light sensitivity via melanopsin with synaptic input from classical photoreceptors through bipolar cells. This study provides a detailed description of the M5 ipRGC, revealing a distinctive functional characteristic: chromatic opponency, where UV light excites the cell while green light inhibits it. Serial electron microscopy showed that M5 cells receive selective UV-opsin input from Type 9 cone bipolar cells and mixed cone signals from Types 6–8 bipolar cells. Electrophysiological recordings indicate that both excitation and inhibition arrive via the ON channel and that chromatic opponency arises from surround inhibition driven by M-cone signals and mediated by wide-field spiking GABAergic amacrine cells. Importantly, M5 axons innervate the dorsal lateral geniculate nucleus (dLGN), suggesting these cells contribute chromatic information to cortical visual processing. These findings broaden the known influence of melanopsin beyond unconscious reflexes and imply a role for ipRGCs in cortical vision, potentially including aspects of color perception.
Reference: “The M5 Cell: A Color-Opponent Intrinsically Photosensitive Retinal Ganglion Cell” by Maureen E. Stabio, Shai Sabbah, Lauren E. Quattrochi, Marissa C. Ilardi, P. Michelle Fogerson, Megan L. Leyrer, Jordan M. Renna, Min Tae Kim, Inkyu Kim, Matthew Schiel, Kevin L. Briggman, and David M. Berson. Neuron. Published online December 14, 2017. DOI: 10.1016/j.neuron.2017.11.030