Critical Minds and Critical Time

Jessica George

“The habits we form from childhood make no small difference, but rather they make all the difference” - Aristotle

This opening quote was the perspective that Harvard University’s neuromolecular biologist Dr. Takao Hensch implemented in his talk about manipulating brain plasticity. Children are faster than adults when learning motor outputs, languages, and a plethora of other skills necessary for development. You may know names such as Mozart or Serena Williams; their success in composing and tennis, respectively, was the result of an accumulation of years of experience within these fields. It may not be surprising that people who have achieved great performances often start early in life and acquire the skills that made them extraordinary.  

Brain plasticity, or the brain’s “willingness to change,” is one way to explain a child’s malleability to early experience. Interestingly, this plasticity is different throughout an individual’s lifespan.  For example, let’s look at something that mostly all humans eventually acquire in their skills repertoire: the ability to speak and comprehend language. It’s truly extraordinary to see an infant with near-zero comprehension of language develop a full working vocabulary by the age of four. But what’s truly perplexing is that the same child, when older, struggles significantly more when trying to learn a new language. How can humans be so proficient at acquiring a language with zero past experience during one point of their life, yet so much worse at acquiring new languages later in life -- when they have already developed the neural mechanisms necessary for language?

“What makes us all distinct individuals is this period when environment can act dramatically on the circuits and shape them to become tailor made hardware for the environments into which we are thrown.” - Hensch

This period Hensch refers to is the critical period. Critical periods can be understood as windows of opportunity, in which early experiences can shape brain function for a variety of processes, such as vision and language. During the critical period for language, for example, a child’s exposure to language is extremely influential on the wiring of her brain; thus, she learns at a very efficient rate. However once her critical period closes later in life, her plasticity to experience decreases, and thus she learns languages slower and with more effort. So how does experience translate into changing neural connections? Well, these experiences serve as sensory inputs to two types of neurons in the brain: excitatory neurons and inhibitory neurons. It’s through their communication which makes all the magic happen.

Excitatory neurons drive target cells towards being active and inhibitory neurons prevent target cells from firing action potentials (the method of communication between neurons). Historically, research has been more interested in the implications of excitatory cells because they far outnumber inhibitory cells; the accepted thought was that you need to excite the brain to induce change. However, recent research has indicated that inhibitory circuitry is behind these highly plastic periods of learning.

Created by Jessica George

Created by Jessica George

There are a few theories that explain why inhibitory neurons are necessary for critical periods. One explanation is that neural inhibition sharpens the signal-to-noise ratio. Think about an outdated TV set. While watching the screen, sometimes you may see a bunch of little dots that don’t contribute to the the clarity of the picture. These dots are the noise, whereas the picture is the signal. We see in newer TV models that these white dots are almost unrecognizable -- indicating that companies have effectively worked to sharpen the signal-to-noise ratio. This analogy can be understood when thinking of inhibitory cells as the TV companies, incoming sensory information as the picture, and noise as all the extra stuff we pick up that our bodies don’t really want to process. Research has shown that immature brains before critical periods have a lot of this “noise”, or spontaneous activity not driven by sensory input. And just like TV companies worked on strengthening this signal-to-noise ratio, our neural system ultimately works to get stronger. Furthermore, the rules of “neurons that fire together wire together” and “neurons out of sync lose their link” are the underlying mechanisms to how some of this noise is lost and signal strength is increased. Adding inhibitory networks allows responses to get sharper and the noise signal to be preferentially dampened.

So what’s the point of understanding how and why critical periods happen?

Interestingly, understanding environmental effects on neural networks can have great implications for society, particularly in education. Preliminary research on neuroplasticity in cats looked at their visual systems and how the environment influences maturation (Frégnac & Imbert, 1978). That is, early environmental disadvantage (such as perpetual darkness) can prevent visual development in cats. This knowledge of environmental influence on visual systems was also found to apply to humans, as well.  Researchers found that when it came to certain visual deficits (ie Amblyopia or lazy eye), the effectiveness of the treatment interventions was highly dependent on the age of the subject (Holmes et al., 2011).  Dr. Allyson Mackey and Dr. Emily Cooper evaluated implications of such findings in their review Sensory and cognitive plasticity: implications for academic interventions. This idea of critical time periods for development, when extended to other neural systems, reveal that other forms of environmental disadvantage (e.g. low SES status, lack of available resources, early childhood stress, etc.) significantly affect cognitive development. However, Dr. Mackey and Dr. Cooper stressed that it’s important to understand that early interventions are not necessarily optimal if they take place before the critical period opens. Moving toward neuroscientific applications within society, understanding trajectories of developmental neural plasticity could be extremely helpful when determining the timing of interventions aiming to ameliorate environmental disadvantages of children in high-risk communities.

About the Author:
Jessica is a junior studying Cognitive Science at the University of Pennsylvania, with a concentration in Cognitive Neuroscience. She is also double-minoring in Psychology and Urban Education: Policy, Research, and Practice. Jessica has volunteered for The Changing Brain Lab since the Fall of 2017. Any questions can be emailed to