Lecture 27- Brain development and plasticity

Question 1

What is prenatal development?

Answer 1

Formation of the central nervous system occurs during prenatal development.

Question 2

What the three steps for the how brain structure develops?

Answer 2

The structure of the brain develops in steps:
1. Cell Division (i.e., cell proliferation)
2. Cell Migration
3. Cell Differentiation

The first step in wiring cerebral cortex is generating neurons (cell division).
* Stem cells in the central nervous system divide into two cells.
* After cell division, the newly divided cell migrates away to take up its position in the cortex and
the stem cell remains to undergo more divisions.
* Stem cells continue to divide until all the neurons of the cortex have been generated.
* Cell differentiation refers to the process by which, after migrating, new cells take on the
appearance and characteristics of a neuron or glial cell.

Question 3

What is the ‘wiring’ of the brain?

Answer 3

Neurons extend their axons to the appropriate targets in order to form inter-neuronal connections.

Question 4

What is post natal development how does it differ to the plasticity of the brain in terms of growth trend?

Answer 4

Brain development continues after birth, but the plasticity of the brain declines as we age.

Question 5

What is plasticity?

Answer 5

Plasticity refers to the ability of the nervous system to change.
* Although on a much smaller scale, plasticity of the brain continues after birth.
* The adult brain is comparatively rigid, but still undergoes plasticity (e.g., learning).

Question 6

How are connections in the brain modified by sensory experience?

Answer 6

• At birth, the basic circuitry of the brain is largely in place.
• Fine-tuning of the wiring of the brain is driven by neuronal activity.
• For example, consider the level of overlap
  in the neural projections from the two eyes
  at birth versus after months of sensory
  experience.
• Recall that the LGN and V1 receive
  information about the opposite hemifield
  from both eyes (e.g., in the right
  hemisphere, the LGN and V1 receive
  information about the left visual hemifield
  from both the left and the right eye).
• Recall also that the two eyes do not receive
  the same visual information (see figure).
  Note the discrepancy in the information
  about the left hemifield received by the left
  eye versus the right eye.

Question 7

How do Ocular Dominance Columns in Primary Visual Cortex form?

Answer 7

Top: At birth, the inputs from the LGN
representing the two eyes are intermingled within
striate cortex (V1).
* Bottom: Over the course of early postnatal development, the inputs from the two eyes segregate into ocular dominance columns.
* This fine-tuning of the wiring of the brain is driven, at least in part, by sensory experience.

Question 8

What is the ‘Rewiring the Brain in Newborns: Evidence of Functional Plasticity’ experiment?

Answer 8

What happens if you surgically rewire the brain such that visual input is directed to the auditory
system?
* Background Anatomy of the Subcortical Visual System: Recall that some of the retinal ganglion
cells project their axons out of the eye and to the superior colliculus (SC). That is, normally
retina → SC
* Background Anatomy of the Auditory System: Recall that neurons in the inferior colliculus (IC)
send their axons to the medial geniculate nucleus of the thalamus (MGN), which in turn send
their axons to primary auditory cortex (A1). That is, normally IC → MGN → A1
* Procedure: In newborn ferrets, within one hemisphere the axons of retinal ganglion cells
destined for the superior colliculus (SC) were redirected to the medial geniculate nucleus
(MGN), and the SC and inferior colliculus (IC) were removed. The other hemisphere was left
intact.
* Method: Single-cell recordings in primary auditory cortex (A1)
* Result: After the animals were raised to adulthood, the neurons in primary auditory cortex in the
rewired hemisphere behaved like visual neurons in response to visual stimuli (e.g., retinotopic
organization).

Question 9

What is the “Effects of stimulating visual cortex in adults with impaired vision” experiment?

Answer 9

• Participants (all had visual impairment due to damage before the LGN)
  – Group 1: 10 adults with some residual vision
  – Group 2: 15 adults with very poor residual vision
  – Group 3: 10 adults without any residual vision
• Apparatus: Transcranial magnetic stimulation (TMS)
• Measure: Self-reported experience of phosphenes elicited by TMS over visual cortex
• Task: Report experience of phosphenes.
• Results (% of participants that reported experiencing phosphenes):
  – Group 1 (some residual vision): 100%
  – Group 2 (poor residual vision): 60%
  – Group 3 (no residual vision): 20%
• The results indicate that the effect of activating visual cortex via TMS is altered in people with
  severe visual impairment, as evidenced by a reduction in the ability to elicit phosphenes in
  people with a high degree of visual impairment, especially in those without previous visual
  experience.
• Do you think that the two completely blind participants that experienced phosphenes have
  congenital blindness (i.e., since birth) or that they have had previous visual experience?
• One congenitally blind participant reproducibly reported the experience of a localized sensation
  of warmth (‘like a heating-lamp’) in the contralateral half of his near-grasping space’ when his
  occipital cortex was stimulated.

Question 10

What is the ‘Mental imagery in sighted and congenitally blind adults’ experiment?

Answer 10

• Which brain structure is activated during mental imagery in sighted versus congenitally-blind
  adults?
• Participants:
  – 6 blind adults that were blind from birth
  – 6 sighted adults that were blindfolded
• Task: Produce mental images from animal names versus passive listening to abstract words.
• Apparatus: Functional magnetic resonance imaging (fMRI)
• fMRI Data: Brain activity during passive listening to abstract words was subtracted from brain
  activity during the mental imagery task.
• This subtraction procedure yields a more specific measure of brain activity associated with
  mental imagery.
• Results: In both congenitally blind and sighted participants, the production of mental images was
  associated with activation of visual cortex.
• Conclusion: Primary visual cortex is activated in congenitally blind people and that activation
  occurs in a mental imagery task that involves only verbal instructions.

Question 11

What is training and plasticity like in A1?

Answer 11

Training monkeys to discriminate specific tone frequencies leads to an enlargement of the
cortical regions in which the trained frequencies are represented.
* This change provides a demonstration of functional plasticity within primary auditory cortex.

Question 12

What is training and plasticity like in M1? (experiment)

Answer 12

• How does the human adult motor system respond to training?
• Task: Using your non-dominant hand, perform finger-to-thumb tapping in the specified sequence
  (participants learned either A or B).
• Training involved tapping the designated sequence as fast and accurately as possible for 10-20
  minutes per day.
• Method: Using fMRI, the scientists measured activity-related changes in blood flow in primary
  motor cortex for the practiced sequences and untrained sequences.
• Data comparison: Trained vs. Untrained motor sequence
• The fMRI activity elicited by performing untrained sequential movements (right) was subtracted
  from the activity elicited by performing trained sequential movements (left).
• Results: Greater changes in blood flow occurred in the contralateral primary motor cortex for
  trained than for untrained sequences after only three weeks of training.
• 8 weeks after the final session: Greater changes in blood flow in the contralateral motor cortex
  for trained (left) compared to untrained (right) sequences persisted, even without training in the
  interim.
• Conclusion: In the human adult brain, training can induce relatively rapid changes in brain
  activity that reflect the plastic ability of the nervous system to acquire and retain new information
  and skills.

Question 13

What happens to the brain as it ages?

Answer 13

As we grow older, the brain becomes less plastic and begins to atrophy.
* One of the consequences of this is a decline in memory function, which has been connected to
age-related reductions in the size of the hippocampus.
* Recent research in older adults indicates that engagement in aerobic exercise can increase the
size of the hippocampus (see above graphs).
* Engagement in aerobic exercise also improved the accuracy of spatial memory.