Author: Michael Tri Do, PhD

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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