Lateralization: the brain is not bilaterally the same, functions differ in each
- Historical perspective (WW2, brain damage in adults)
• Adult patients with left hemisphere (LH) damage:
During hospital recovery it was found that 100% of these patients (when
bullet wound was in LEFT side showed some aphasic (aphasia is
problems in language/speech) symptoms.
Approximately 30% will show some recovery, at varying degrees. Those
that showed recovery were all left-handed/ambidextrous patients (LHP).
Least amount of recovery was in right-handed patients.
Since 100% of patients showed some language problems with Left
Hemisphere damage, clearly Left Hemisphere is associated with
• Adult patients with right hemisphere (RH) damage:
Found in these patients that they showed very few language problems
and rarely did it lead to complete aphasia.
The % of patients that did show aphasic symptoms with RH damage
were all LHP. Furthermore, most patients showed recovery from
language problems as a result of RH damage.
Right handed people (RHP) with RH damage did not show aphasia. For
RH wounds, RHP don’t get aphasic symptoms. Right handed people only
had problems when damage is in the Left hemisphere. Suggests that in
RHP language is located on left side. In LHP, language is bilaterally
represented (occurs in both hemispheres) because in LHP, damage to
either hemisphere shows problems in opposite hemisphere.
• RHP Language is on LEFT Side, LHP language is on BOTH sides
RHP: all language is LH. Damage to the LH equals lots of problems and
virtually no recovery. There’s no language on the right so it can’t take
LHP, damage to either side results in language problems however you
get recovery because the side that isn’t damage can process language
and can compensate and learn, so there is some degree of recovery.
Development of Lateralization
- Children (under age of 5) with damage to either hemisphere has equal
probability of inducing aphasic symptoms, which tells us that there are equal
amounts of language found on both sides of a child’s brain. However 100% of
kids will show some recovery.
- In children language is not yet lateralized, because there’s language on both
sides similar to left handed and ambidextrous people.
- Over the course of development, language slowly becomes lateralized to one
side. (E.g. In RHP, children are born with both sides capable of processing
language, however over time RH loses ability to process language and
lateralizes to LH.)
- Neuroplasticity: ability of brain to change, store info, recovery from injury. - Brain is very plastic during development, loses plasticity in
- If a child does have severe brain damage what they will do is remove that
damaged hemisphere because leaving it in there may result in epilepsy. The
other hemisphere will then take over everything.
Acallosal patients (those without a corpus callosum)
- In adults who were born without a corpus callosum, language is localized on
both sides of brain.
- Children with very extensive brain damage, language is always found on the
other intact side.
- It would appear then that when we’re born both hemispheres are capable of
language acquisition. As we mature, and begin to develop language that the
LH seems to be more dominant for language in individuals.
- It would appears because of the fact that a person has damage to LH, or
because hemispheres aren’t connected that during development LH actively
- In individuals who are acallosal, language remains on the RH, but if
connected, all language moves to LH.
- Why is the LH more superior (mainly in right handed people)?
• Left temporal lobe contains planum temporale (PT) which is the primary
language area in temporal lobe. PT in LH is larger, more convoluted, and
heavier by about a third, suggesting that the LH speech area is
anatomically more equipped to handle language.
• Even in other apes left temporal lobe was also larger and heavier. It’s also
true in other animals that communicate, such as birds. LH is anatomically
larger than RH.
• Throughout evolution there has been a predisposition for vocal
communication to be localized in the LH.
• LHP have anatomical advantage due to larger LH for language even though
there’s language in both sides.
• Can also be seen in infants and even before birth (10-31 week of
• Larger anatomical structure in left temporal lobe, language acquisition is
not due to experience, it’s an evolutionary genetically carried
predisposition for vocal communication to be stored on LH.
- Sodium amytal