Stories about: Michael Do

Sensing light without sight: The visual system’s ‘third eye’

ipRGCs provide non-image vision, responding to light independently of rods and cones
Intrinsically photosensitive retinal ganglion cells, rich in melanopsin, respond to light independently of rods and cones. (Courtesy Elliott Milner, PhD)

Michael Tri H. Do, PhD, is an investigator in the F.M. Kirby Neurobiology Center at Boston Children’s Hospital and an assistant professor of neurology at Harvard Medical School.

Light affects us even without impinging on our awareness. In 1995, Charles Czeisler and colleagues at Harvard Medical School described people who lacked visual perception due to retinal degeneration, but nevertheless responded to light subconsciously — despite being blind, their melatonin level was suppressed, and they appeared to synchronize their circadian clock with the solar day. Across the pond at Oxford, Russell Foster and colleagues were finding the same in mice, and learned that these responses began in the eye.

These discoveries spurred an intense research effort that continues to this day. What system confers subconscious sight, and how does it differ from the system that generates visual experience?

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How do we sense moonlight? Daylight? There’s a cell for that

environmental light sensing must span a wide spectrum of light intensities

To run our circadian clocks, regulate sleep and control hormone levels, we rely on light-sensing neurons known as M1 ganglion cell photoreceptors. Separate from the retina’s rods and cones, M1 cells specialize in “non-image” vision and function even in people who are blind.

Reporting in today’s Cell, neuroscientists at Boston Children’s Hospital describe an unexpected system that M1 cells use to sense changing amounts of environmental illumination. They found that the cells divvy up the job, with particular neurons tuned to different ranges of light intensity.

“As the earth turns, the level of illumination ranges across many orders of magnitude, from starlight to full daylight,” says Michael Do, PhD, of the F.M. Kirby Neurobiology Center at Boston Children’s Hospital, senior author on the paper. “How do you build a sensory system that covers such a broad range? It seems like a straightforward problem, but the solution we found was a lot more complex than expected.”

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Melanopsin, lighting and you

color spectrum melanopsin
A deep-dive view of non-image vision may refine our understanding of light and health.

Back in the day, the 1980s to be specific, there was a brief fad around amber-on-black computer screens (as opposed to green-on-black or white-on-black) for supposed ergonomic reasons. My computer had one, along with its 5 ¼” floppy drives (remember those?).

More recently, with kids texting at night and people logging late hours on computers and devices, there’s been a recognition that artificial light at night is bad for sleep and disruptive to physiology overall, with blue light increasingly recognized as the culprit.

That’s given birth to some new fads. You can now download programs to eliminate blue light from your computer screen at night or buy amber-tinted glasses for computing and gaming to “filter the harsh spectra” of light. Airlines are using “mood” lighting to mimic sunrises and sunsets, which supposedly reduces jetlag.

In a paper in Neuron last week, Alan Emanuel and Michael Do, PhD, of the F.M. Kirby Neurobiology Center at Boston Children’s Hospital and Harvard Medical School provide some science to support and inform these fads, as well as the use of light therapy for conditions like seasonal affective disorder.

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