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loss. Blindness or deafness makes unused brain areas available for other uses, such as sound
and smell ( Amedi et al., 2005 ; Bauer et al., 2017 ). If a blind person uses one finger to read
Braille, the brain area dedicated to that finger expands as the sense of touch invades the
visual cortex that normally helps people see ( Barinaga, 1992 ; Sadato et al., 1996 ). In sighted
people, Braille-reading training produces similar brain changes ( Debowska et al., 2016 ).
Neuroplasticity also helps explain why some studies have found that deaf people
who learned sign language before another language may have enhanced peripheral and
motion-detection vision ( Brooks et al., 2020 ). In deaf people whose native language is
sign, the temporal lobe area dedicated to hearing waits in vain for stimulation. Finally, it
looks for other signals to process, such as those from the visual system used to see and
interpret signs.
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Similar reassignment may occur when disease or damage frees up other brain areas
normally dedicated to specific functions. If a slow-growing left hemisphere tumor disrupts
language (which resides mostly in the left hemisphere), the right hemisphere may compen-
Copyright © Bedford, Freeman & Worth Publishers.
sate ( Thiel et al., 2006 ). If a finger is amputated, the somatosensory cortex that received its
input will begin to receive input from the adjacent fingers, which then become more sensi-
tive ( Oelschläger et al., 2014 ).
Although the brain often attempts self-repair by reorganizing existing tissue, research-

ers are debating whether it can also mend itself through neurogenesis — producing new
neurons ( Kempermann et al., 2018 ). Researchers have found baby neurons deep in the
brains of adult mice, birds, monkeys, and humans ( He & Jin, 2016 ; Jessberger et al., 2008 ).
These neurons may then form connections with neighboring neurons ( Gould, 2007 ; Luna
et al., 2019 ).
Stem cells, which can develop into any type of brain cell, have also been discovered in
the human embryo. If mass-produced in a lab and injected into a damaged brain, might
neural stem cells turn themselves into replacements for lost brain cells? Might surgeons
someday be able to rebuild damaged brains, much as we reseed the grass on damaged
sports fields? Stay tuned. In the meantime, we can all benefit from natural promoters of
neurogenesis, such as exercise, sleep, and nonstressful but stimulating environments ( Liu &
Nusslock, 2018 ; Monteiro et al., 2014 ; Nollet et al., 2019 ).

The Divided Brain

1.4-9 What do split brains reveal about the functions of our two brain hemi-
1.4-9 What do split brains r eveal about the functions of our two brain hemi-

spheres?
spheres?
Our brain’s look-alike left and right hemispheres serve differing functions. This lateralization
becomes apparent after brain damage. Research spanning more than a century has shown
that left-hemisphere accidents, strokes, and tumors can impair reading, writing, speaking,
arithmetic reasoning, and understanding. Similar right-hemisphere damage has less visi-
bly dramatic effects. Does this mean that the right hemisphere is just along for the ride?
Many believed this was the case until the 1960s, when a fascinating chapter in psychology’s
history began to unfold: Researchers found that the “minor” right hemisphere was not so
limited after all.
neurogenesis the formation of
new neurons.
Splitting the Brain corpus callosum [KOR-pus

In the early 1960s, two neurosurgeons speculated that major epileptic seizures were caused kah-LOW-sum] the large band
by an amplification of abnormal brain activity bouncing back and forth between the two of neural fibers connecting the
two brain hemispheres and
cerebral hemispheres, which work together as an integrated system ( Bogen & Vogel, 1962 ). carrying messages between
They wondered if they could end this biological tennis match by severing the corpus them.

,
callosum the wide band of axon fibers connecting the two hemispheres and carrying
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