New research findings led by Thomas Krahe and Ary S. Ramoa of Virginia Commonwealth University School of Medicine offer two pieces of good news for treating children with amblyopia. First, the researchers have found evidence that the neural wiring in the brain's visual system is not dismantled by visual deprivation--for example, due to a cataract--during what is known as the "critical period" of vision development. Rather, the wiring is merely deactivated, capable of being rapidly reactivated when vision is restored. And secondly, the researchers wrote in an article published in the October 20, 2005, issue of Neuron, their findings suggest that allowing children with amblyopia to use both eyes--rather than patching the stronger eye to encourage use of the weaker one--enables better recovery. Such findings are clinically important because about three percent of people suffer loss of visual acuity in one eye during early development.
In their experiments, Krahe, Ramoa, and their colleagues first deprived ferrets of vision in one eye for six days. They then uncovered the deprived eye for different periods and followed the course of recovery of the eye.
Measurement of neural signals in the ferret's visual system revealed that restoration of binocular vision began in as little as 30 minutes to two hours and was similar to normal in about four hours.
The researchers also found that blocking protein synthesis in the brain with drugs did not affect recovery. Such protein synthesis is necessary if neuronal regrowth is required for recovery, indicating that the basic visual wiring had been preserved during deprivation.
In a preview of the paper in the same issue of Neuron, Takao Hensch wrote that "The results reported here are wonderfully harmonious with the rapid improvement in the acuity of human infants upon restoring visual input during the critical period. Adapting these principles may also motivate therapeutic strategies to aid the precise recovery of function in older, well-established circuits once thought to be beyond repair."
Krahe, Ramoa, and their colleagues concluded that "The results presented here may have important implications for understanding recovery in infants that have relatively short periods of visual deprivation, as occurs in the case of congenital cataracts. These patients, who typically undergo eye operations within the first one or two months of postnatal life, were found to start recovering visual acuity as early as 1 hr after restoration of normal visual input. Our findings suggest a neurobiological basis for this rapid improvement in visual acuity. Latent deprived eye connections may rapidly reactivate upon recovery of normal vision.
"Another implication of our study is that binocular vision is likely to play an important role in recovery from amblyopia," they wrote. "Although clinical evidence suggests that patching the better eye of a child is required to improve spatial resolution in the amblyopic eye, strong support for a role of binocular recovery in the initial stages of amblyopia has been reported, and large ongoing clinical studies are re-examining the roles of patching and binocular vision in recovery."
The researchers include Thomas E. Krahe, Alexandre E. Medina, Ruben E. de Bittencourt-Navarrete, Raymond J. Colello, and Ary S. Ramoa of the Department of Anatomy and Neurobiology at Virginia Commonwealth University School of Medicine. This work was supported by NIH Grant EY-11508 to A.S.R.
Krahe et al.: "Protein Synthesis Independent Plasticity Mediates Rapid and Precise Recovery of Deprived Eye Responses." Publishing in Neuron, Vol. 48, 329–343, October 20, 2005, DOI 10.1016/j.neuron.2005.09.016, www.neuron.org