26 CHAPTER ONE
nal connections are only too subject to modification by the environment”
(Hubel and Wiesel 1998, 407; see also Hubel and Wiesel 1970). This means
the visual cortex is embodied and relational, dependent on both its own
activity and its engagement with the world for its self- organization. Yet this
research also points to the limits of developmental plasticity. The kittens
whose eyes had been occluded for too long were never able to see normally;
their visual cortices remained wired as if they had only one eye. Because of
the apparently tight window of plasticity, Hubel and Wiesel retained a view
of the mature brain as a fixed structure whose functions are irreversibly
localized, or tied to specific neural pathways, for life (Clifford 1999).
Uneven Developments
The idea of a “critical window” of plasticity now uneasily coexists with a
view of the brain’s extended development and morphological plasticity.^7
The neuroscientific understanding of human brain development has been
changing dramatically since researchers began to conduct imaging studies
in the 1990s.^8 The human brain was once considered mostly mature by age
five or six; it was thought to achieve this through a wave of prenatal and
postnatal synaptic growth, followed by several years of pruning of unused
synapses, which seems to buttress the efficacy of remaining ones. Neuroim-
aging researchers have claimed, however, that human brains experience a
second wave of synaptic sprouting just before puberty and undergo pruning
for years afterward. Imaging studies of the adolescent brain have reported
changes in, for example, the frontal, parietal, and temporal lobes from the
teen years into the early or midtwenties (Giedd et al. 2004; Petanjek et
al. 2011; Shonkoff and Phillips^ 2014; Sowell et al. 1999). The expansion of
developmental plasticity into early adulthood suggests that experience has
considerably more opportunity to shape the brain than previously thought.
Within the neuroimaged brain, however, development has a “heteroge-
neous temporality” (Gumy 2014, 257); that is, different regions appear to
have different developmental trajectories. The implications of this depend
in part on whether, and to what degree, tasks and aptitudes are localized
with respect to various brain areas.^9 If one subscribes to the localization
thesis that dominates neuroimaging research, this means that the capac-
ity for some tasks may be more plastic than others. The stratification of
plasticity between regions of the brain is leading to new ways of identi-