An international research team led by Nanyang Technological University (NTU) in Singapore has achieved a groundbreaking milestone by capturing the first-ever mechanical 'twitch' in the eye's night-vision cells, known as rod photoreceptors, at the precise moment they detect light. This tiny contraction, measuring up to 200 nanometres and occurring within about 10 milliseconds, represents an electromechanical response triggered by the activation of rhodopsin, the light-sensitive protein in these cells. The discovery, made possible through a non-invasive imaging technique called optoretinography (ORG), opens new avenues for early detection of retinal degenerative diseases that first impair night vision.
Rod photoreceptors, which constitute approximately 95% of the photoreceptors in the human retina, are specialized for scotopic vision—seeing in dim light conditions. Unlike cone cells responsible for color and daylight vision, rods excel at detecting faint light but are highly vulnerable to degeneration in conditions like retinitis pigmentosa (RP) and age-related macular degeneration (AMD). In Singapore, where an ageing population is rapidly growing, such innovations from local universities like NTU are timely, potentially transforming clinical practices at institutions such as the Singapore Eye Research Institute (SERI).
🔬 Understanding Rod Photoreceptors and Night Vision Mechanics
Rod photoreceptors are elongated cells located at the periphery of the retina, optimized for low-light sensitivity. Each rod contains thousands of stacked disc membranes in its outer segment (OS), packed with rhodopsin molecules. When a photon strikes rhodopsin, it undergoes photoisomerization—changing from 11-cis-retinal to all-trans-retinal—which initiates phototransduction: a cascade converting light into electrical signals sent to the brain via the optic nerve.
Step-by-step, this process unfolds as follows:
- Photon absorption: Rhodopsin activates, triggering a conformational change.
- Signal amplification: Activated rhodopsin (metarhodopsin II) activates transducin, closing cGMP-gated ion channels and hyperpolarizing the cell.
- Mechanical response: The new finding reveals an immediate nanoscale contraction of the OS, likely due to voltage-dependent membrane tension changes in the disc membranes.
- Signal transmission: Hyperpolarization reduces glutamate release, signaling bipolar cells.
This mechanical twitch, previously only inferred from ex vivo studies, links biophysical changes directly to vision onset, enhancing our understanding of why rods fail early in diseases.
The Limitations of Traditional Rod Function Assessment
Conventionally, rod health is evaluated via electroretinography (ERG), which measures electrical responses but requires dark adaptation, contact electrodes, and cannot isolate individual rods. Fundus autofluorescence or OCT angiography provide structural views but miss functional dynamics. Night vision loss, an early symptom in RP and AMD, often goes undetected until advanced stages, delaying interventions.
In Singapore, RP prevalence stands at about 6 cases per 10,000 adults over 40, varying by ethnicity—higher among Indians and Malays. Globally, AMD affects nearly 200 million, projected to rise to 288 million by 2040, with rods degenerating first in dry AMD. NTU's ORG addresses these gaps by offering label-free, high-speed functional imaging.
Decoding Optoretinography: The Breakthrough Imaging Technology
Optoretinography (ORG) leverages phase-sensitive optical coherence tomography (OCT) to detect picometre-to-nanometre displacements in retinal layers caused by light-evoked activity. NTU's implementation combines:
- Ultrahigh-resolution point-scan OCT for rodents, with unsupervised Gaussian mixture model AI to separate rod OS tips from retinal pigment epithelium (RPE).
- Adaptive optics line-scan OCT (200 Hz) for humans, resolving individual rods.
Visual stimuli—5-ms flashes bleaching 0.6% to 62.6% rhodopsin—elicited logarithmic contraction amplitudes, confirming electromechanical coupling. No dyes needed; measurements are non-contact and rapid.
This builds on prior ORG for cones, extending to rods for comprehensive photoreceptor assessment.
Photo by GEE MENG WAH on Unsplash
Key Findings: Quantifying the Rod 'Twitch' in Vivo
In rodents, single flashes caused ~100-200 nm OS contractions within 10 ms, cumulative with multiple flashes. Human rods showed similar bleach-dependent responses, with OS tips contracting faster than inner segments. Voltage models predicted amplitudes matching observations, validating the mechanism.
These dynamics outpace hummingbird wing flaps, highlighting rods' exquisite sensitivity. Cones exhibited parallel responses, suggesting conserved phototransduction mechanics.
| Bleach Level | Rod OS Contraction (nm) | Time to Peak (ms) |
|---|---|---|
| 0.6% | ~50 | <10 |
| 39.4% | ~200 | ~10 |
| 62.6% | Max observed | ~10 |
Spotlight on Singapore's Research Ecosystem: NTU and Collaborators
NTU's School of Chemistry, Chemical Engineering and Biotechnology (CCEB) leads, with Asst Prof Tong Ling pioneering ORG. 'The twitch is the ignition spark of vision,' says Ling. Collaborators include SERI's Prof Leopold Schmetterer, Duke-NUS's Assoc Prof Veluchamy Amutha Barathi, and US partners like Prof Ramkumar Sabesan (University of Washington): 'Rod dysfunction is an early sign... ORG gives unprecedented sensitivity.'
Singapore's biomedical hub—bolstered by A*STAR, SERI, and Duke-NUS—fosters such US-Asia synergies. Independent expert Prof Jost Jonas (Heidelberg) praises ORG's nanoscale visualization potential.NTU CCEB announcement
Transforming Diagnosis of Retinal Diseases in Singapore
RP and AMD burden Singapore's elderly: RP at 1:1667 adults >40; AMD rising with 20% population over 65 by 2030. ORG enables objective rod function tracking, detecting subclinical loss before symptoms. Potential for monitoring gene therapies or visual cycle modulators.
Local trials planned, integrating with SERI's epidemiology studies. Broader: complements NTU's optics research, aiding Singapore's precision medicine push.
Broader Implications and Stakeholder Perspectives
Ophthalmologists gain a tool surpassing ERG for sensitivity. Patients benefit from earlier interventions, reducing blindness risk. Researchers: validates phototransduction models. Industry: spurs ORG devices, with NTU's tech transfer potential.
In Singapore's multi-ethnic context, equitable access via public clinics like SNEC is key. Stakeholders hail it as a leap for ageing Asia.
Future Outlook: Clinical Translation and Singapore's Vision Research
Next: human trials for RP/AMD screening, AI-enhanced ORG commercialization. NTU eyes integration with adaptive optics for population studies. Aligns with Singapore's Healthier SG, emphasizing preventive eye care.
Timeline: preclinical validation 2026, pilots 2027-28. Complements NTU's quantum imaging, positioning Singapore as retinal tech leader.Full study in Light: Science & Applications
NTU's Role in Singapore's Biomedical Innovation Landscape
NTU ranks top globally in materials science/citations, with CCEB driving biotech. Recent feats: AI diagnostics, nanomaterials. This ORG advances NTU's ophthalmology portfolio, training PhDs/postdocs amid Singapore's 5000 PhD target by 2030.
Boosts jobs in research, attracts talent via research positions.
