Pawan Sinha¹, Chetan Ralekar², Charles Shvartsman¹, Suma Ganesh³; ¹Massachusetts Institute of Technology, ²Indian Institute of Technology, Roorkee, India, ³Dr. Shroff's Charity Eye Hospital, New Delhi, India

Understanding whether the human visual system retains the capacity for myelin-related plasticity beyond early developmental critical periods is essential for both basic neuroscience and clinical practice. Here, we report longitudinal evidence from a cohort of children who experienced profound congenital visual deprivation until late childhood and then received sight-restoring cataract surgery. Using flash visual evoked potentials (fVEPs), we examined how the speed of neural transmission in early visual pathways changes over the year following treatment. Across six participants (mean age 12 years), we measured the latency of the P1 component - a well-established marker of conduction speed in the retino-cortical pathway - at five time points from three days to one year post-surgery. Immediately after treatment, P1 latencies were significantly delayed relative to neurotypical adults, consistent with slowed transmission in long-deprived pathways. However, we observed systematic and statistically significant latency reductions over time, converging to typical adult values by one year. A mixed-effects model revealed a strong effect of log-transformed time on latency (p < 0.001). The rate of convergence was markedly slower, by nearly an order of magnitude, than that documented in infancy, but the eventual approach to normal ranges demonstrates substantial residual plasticity. Comparisons to published neonatal data suggest that some myelination may have occurred prior to treatment due to residual light stimulation through cataracts. Nevertheless, the pronounced post-surgical latency decreases likely reflect activity-dependent increases in myelination within early visual white-matter tracts. This interpretation is supported by our prior structural imaging work showing myelin-associated changes in similar late-sighted populations. These findings challenge long-standing assumptions about the rigidity of myelination timelines, revealing meaningful plasticity in the visual system well past early childhood. They further point to the possibility that experience-dependent myelin remodeling may serve as a modulator of visual learning across the lifespan.

Acknowledgements: This work was supported by grant R01EY020517 from NEI(NIH)