Susceptibility to monocular deprivation following immersion in darkness just prior to the critical period peak

Abstract

Immersion of animals in complete darkness highlights that the maturation and plastic capacity of visual circuitry is regulated by visual experience. Recent investigations have shown that imposing 10-days of darkness exposure during juvenile life instates a neurobiological context wherein an animal’s susceptibility to subsequent modification of visual experience is augmented. For instance, immersion of 12-week old cats in complete darkness for 10-days promotes neurobiological changes that render the effects of monocular deprivation (MD) more severe than those observed in age-matched controls. This result indicates that darkness increases an animal’s capacity for plasticity and recapitulates a developmental state akin to a much younger animal. The current study sought to explore whether short-term darkness immersion could similarly promote visual plasticity when applied just prior to the peak of the critical period), an age when sensitivity to visual perturbation is at its biological maximum. Although 7-days of MD at this age produced potent alterations of neuronal soma size in the dorsal lateral geniculate nucleus (dLGN), 10-days of preceding darkness immersion did not increase the subsequent MD effect. Similarly, MD imposed following darkness induced changes in dLGN neurofilament immunoreactivity equivalent, but not greater than those observed in animals subjected to MD alone. These results reveal that the propensity of darkness to promote susceptibility to MD depends on the animal’s initial capacity for visual plasticity. Further, it appears that the ceiling for critical period plasticity may be constrained by a neural environment that is resistant to modification by darkness.