The maturation and experience-dependent plasticity of the developing visual cortex

Abstract

<p>During early postnatal development the visual cortex undergoes substantial anatomical and physiological maturation that has direct implications for the emergence of the various visual acuities. A large body of research has studied this early development, in particular the role of visual experience during development. Surprisingly, a relatively simple aspect of development--growth--has received very little attention. The results from Chapter 2 of this thesis show that there is substantial postnatal growth of the mammalian visual cortex and that this growth is best described as a simple uniform expansion. The development of visual acuities occurs during early postnatal life as a result of the maturing neural circuitry of the visual system and normal binocular visual experience is important for this maturation. Disruption of binocular vision during this period leads to changes in neural circuitry and can result in permanent visual impairments. Numerous studies have demonstrated that a key post-synaptic component of experience-dependent plasticity is activation of the NMDA receptor. In the present set of studies, a monoclonal antibody was used to label neurons expressing the obligatory NMDAR1 receptor subunit in the visual cortex to study the normal development and affect of visual deprivation on the distribution of this population of neurons. Results from the experiments in this thesis show that in normal kittens, there are regularly spaced patches of NMDAR1 expression across the supragranular layers of the visual cortex. These NMDAR1 patches are both spatially and functionally linked to binocular vision. Reducing binocular correlation by monocular deprivation led to a visuotopic loss of NMDAR1 expression whereas reducing visually driven activity, but not binocular correlation by binocular deprivation led to a generalized increase in NMDAR1 expression that may represent a homeostatic response. These results show that NMDA expression is experience-dependent and can be modulated by both the amount and pattern of visual experience. Recovery of NMDAR1 expression after deprivation was promoted by simply introducing binocular vision, however, reverse occlusion did not promote recovery. The results of this thesis may be related to the results of previous anatomical, physiological, and behavioural studies of deprivation that have found visuotopic changes. Furthermore, these results have important implications for the experience-dependent development of the visual cortex and visual functions because of the key role that the NMDA receptor plays in the establishment of functionally relevant neural circuitry.</p>