Adolescence as a Sensitive Period of Brain Development

Adolescence as a Sensitive
Period of Brain Development
Delia Fuhrmann,1,* Lisa J. Knoll,1 and Sarah-Jayne Blakemore1
Most research on sensitive periods has focussed on early sensory, motor, and
language development, but it has recently been suggested that adolescence
might represent a second ‘window of opportunity’ in brain development. Here,
we explore three candidate areas of development that are proposed to undergo
sensitive periods in adolescence: memory, the effects of social stress, and drug
use. We describe rodent studies, neuroimaging, and large-scale behavioural
studies in humans that have yielded data that are consistent with heightened
neuroplasticity in adolescence. Critically however, concrete evidence for sensitive periods in adolescence is mostly lacking. To provide conclusive evidence,
experimental studies are needed that directly manipulate environmental input
and compare effects in child, adolescent, and adult groups.
Defining Plasticity and Sensitive Periods
In the 1960s, Wiesel and Hubel investigated the effect of monocular deprivation for 1–4 months
after eye opening. Neurons in the corresponding visual cortex subsequently lost responsiveness
to stimuli directed towards the previously deprived eye and started responding preferentially to
the nondeprived eye [1,2]. Monocular deprivation in the first 3 months of life was also associated
with atrophy in cells in the thalamus receiving input from the deprived eye. Recovery from this
atrophy was very limited, even after 5 years of light exposure. In contrast, monocular deprivation
after 3 months of age produced virtually no physiological, morphological, or behavioural effects
[3,4]. The findings from these studies were taken as evidence that the first few months of life form
a sensitive period for perceptual development, during which neuronal plasticity is heightened [5].
Plasticity describes the ability of the nervous system to adapt its structure and function in
response to environmental demands, experiences, and physiological changes [6]. The human
brain retains a baseline level of plasticity throughout life – this is known as experience-dependent
plasticity, and underlies all learning [7]. Plasticity during sensitive periods, by contrast, is
experience-expectant – an organism ‘expects’ to be exposed to a particular stimulus during
this time [7].
Sensitive periods were originally referred to as ‘critical periods’. This term is used less frequently
now, because it has since become clear that some recovery of function may be possible even
outside the time window of highest sensitivity. In the case of visual development, later research
on monocular deprivation in kittens showed that animals can be trained to use the initially
deprived eye after it is uncovered, and this can bring about a certain level of recovery [8].
Studies on sensitive periods of the visual system in humans have relied on naturally occurring
instances of visual deprivation in individuals born with cataracts, which occlude the lens of the
eye. Sight may be regained after cataract reversal procedures. Cataract reversal studies indicate
differences among sensitive periods for normal visual development, periods of sensitivity to
deprivation, and periods of recovery from deprivation [9