An international team of researchers has, for the first time, directly observed a tiny, rapid mechanical 'twitch' in rod photoreceptors at the very instant they detect light. The findings, published in Light: Science & Applications (a Nature journal), could transform how clinicians assess rod cell function and diagnose diseases like age-related macular degeneration (AMD) and retinitis pigmentosa (RP).¹

Rod photoreceptors are responsible for dim-light vision and make up roughly 95 % of the photoreceptors in the human retina. They are also among the first cells to deteriorate in many blinding retinal diseases—yet existing tests of rod function are limited in sensitivity and often uncomfortable for patients.

The new study reveals that when rhodopsin—the light-sensitive pigment inside rods—absorbs a photon, it triggers not only an electrical signal but also a minute, millisecond-scale contraction of the rod’s outer segment. This physical deformation had eluded scientists, largely because traditional electrophysiological tools cannot access the electrical activity within the isolated disk membranes where rhodopsin resides.

Using a technique called optoretinography (ORG)—a noninvasive, high-resolution imaging method based on optical coherence tomography—researchers captured this rapid structural response in both rodent and human eyes in vivo. In rodents, the contraction reached amplitudes of over 200 nanometres within 10 milliseconds of light exposure—far faster than previously measurable movements.

In humans, adaptive optics-enhanced ORG also detected this rapid contraction preceding the more familiar, slower elongation of photoreceptor outer segments at higher light levels. The response scales with stimulus strength and peaks within mere milliseconds after the light flash. Scientists believe the mechanical 'twitch' reflects the early receptor potential—the first step in phototransduction when visual pigment molecules change shape. This electrical activity produces electromechanical forces that appear to temporarily shrink the rod’s outer segment.

“This is the first time we’ve been able to see this phenomenon in rod cells in a living eye,” said senior researchers involved in the project. Measuring these dynamics offers a new, highly sensitive way to visualize retinal function at the cellular level without dyes, electrodes, or invasive probes.

The researchers say this could lead to earlier detection and tracking of degenerative retinal diseases, offering clinicians an objective metric of rod health that precedes the loss of visual sensitivity measured by conventional tests. Because rods are among the first cells affected in conditions such as macular degeneration, this method may help gauge disease progression and therapeutic response much earlier than current clinical tools allow.

Reference

1. Ling, T., Schmetterer, L., Barathi, V. A., et al. (2025). Optoretinography reveals rapid rod photoreceptor movement upon rhodopsin activation. Light: Science & Applications, 14, Article 149. https://doi.org/10.1038/s41377-025-02149-6