Our next chapter of interest in Principles of Neural Science is Chapter 49: Experience and the Refinement of Synaptic Connections in Part VII, which is titled Development and the Emergence of Behavior. The chapter is authored by Joshua R. Sanes, a neuroscientist from Harvard who specializes in neuro-regeneration at the level of the retina.
Dr. Sanes begins by posing this question: What accounts for the delayed maturation of our motor, perceptual, and cognitive abilities? One main factor is that the embryonic connectivity of the nervous system is only a rough draft of the neural circuits that exist in our adult selves. Embryonic circuits are refined by our experiences as a repertoire of sensory stimulation. This two-part sequence – genetically determined connectivity followed by experience-dependent reorganization – is a common feature of mammalian neural development. In humans, this latter phase of experience-dependent reorganization is especially prolonged.
At first glance, this delay in human neural development might seem dysfunctional. While it does take a toll, it also confers an advantage. Because our mental abilities are shaped largely by experience, we gain the ability to custom fit our nervous systems to our individual bodies and unique environments. The plasticity of the nervous system in response to experience endures throughout life, and new discoveries are blurring the demarcation between critical and sensitive periods.
Although plasticity of binocular interactions was initially believed to be confined to early periods in life, it is now apparent that critical periods can be “reopened” to some extent in adulthood. In some cases this can be done by altering the environment or the way in which altered experience is delivered. Critical periods can also be reopened by manipulating some of the factors that normally close them in adolescence. Having said that, Sanes notes that plasticity in adulthood is more modest in magnitude and more difficult to trigger as compared to plasticity in childhood.
Critical or sensitive periods occur during development of numerous systems, such as formulation of orderly maps fo auditory, somatosensory and visual input onto relevant sensory cortices. Many of the principles and mechanisms that characterize the plasticity of binocular interactions also regulate these critical periods including the role of spontaneous and experience-dependent activity, competition, alterations in excitatory and inhibitory synapses, and selective growth and pruning of inputs to achieve appropriate patterns of adult connectivity.
In the final analysis, Hanes writes, the existence of critical periods does demonstrate that the brain’s ability to remodel itself declines in adulthood. At first blush this seems disadvantageous, but may actually represent a useful adaptation allowing the brain to adapt to its environment as it develops. In what sense? By buffering the brain against excessive change later, perhaps even enabling skills and memories to persist that would otherwise have been lost. Sane’s last sentence reads: “If this is the case, therapies based on reopening critical periods may come at a cost.” I found myself thinking back to Al Sutton who talked about the need to be prepared for certain emotions that come to the surface when actively modifying the visual system of adult patients, and how beautifully Sue Barry dealt with this in Fixing My Gaze.
True, I thought, therapies based on reopening critical periods may come at a cost, as Sanes cautions. But to paraphrase Mick Jagger, some patients want the opportunity to change, and refuse to be chained to a life where nothing’s gained and nothing’s lost – at such a cost.