Insights into Visual System Synchronization

The human brain creates mental representations of the world based on sensory signals. While we perceive simultaneous sensory inputs as synchronized, individual signals vary greatly in transmission speed.

Researchers at the Institute of Molecular and Clinical Ophthalmology Basel (IOB), University of Basel, and Eidgenossische Technische Hochschule (ETH) Zurich have recently explored how the human visual system achieves synchronization despite varying signal speeds. Their findings, published in Nature Neuroscience, reveal an unknown mechanism through which the retina synchronizes arrival times of different visual signals.

"Vision is possible due to photoreceptors detecting light and converting it into electrical signals," explained Felix Franke and Annalisa Bucci, senior author and first author respectively. "The retina sends these signals through retinal ganglion cell axons connecting the eye to the brain. However, vision isn't uniform—only a small central area, the fovea, allows sharp vision."

"Our study was inspired by an anatomical observation: as axons avoid crossing the fovea, visual signals travel through different pathways varying in length. We aimed to understand how these signals remain synchronized for coherent vision," Franke said.

To investigate this synchronization process, researchers utilized organ donors' preserved eyes, recording electrical signals with microelectrode arrays and measuring axon diameters using electron microscopy. They mapped axonal pathways in the retina and developed models to analyze signal timing.

The analysis revealed longer axons are thicker and transmit faster to compensate for their greater length, thus maintaining synchronization. Using adaptive optics scanning laser ophthalmoscopy (AOSLO), they confirmed that human reaction times were consistent across the fovea, supporting the idea of synchronized signal arrivals.

Significantly, unmyelinated axons in the retina are crucial for this synchronization. Axons lack myelination to prevent obstructing vision but contribute greatly to timing synchronization.

"Unmyelinated axons may play a substantial role in temporal synchrony throughout the nervous system," said the authors. "The retina actively fine-tunes signal timings more than previously recognized."

Future research could explore how diseases like glaucoma disrupt this balance, affecting retinal ganglion cells with longer, energy-demanding axons.