Lecture 2 : Why and how do brains differ?
What does the difference in brain size mean?
- Intelligence?
o Dogs can be very smart -> can learn to associate a large number of words with items
Fast mapping
Learning by exclusion ; naming a word they don’t know and when there is a
novel object in the environment they automatically associate these two
words with each other
(as human children do)
Word processing also has a similar form as in humans
o Don’t see the above in for example casts
Best predictor for brain size is body size: E=CSr
E -> weight of brain
C -> cephalization factor
S -> body weight
r = exponential constant
Average in mammals : 0.66
Encephalization Quotient (EQ) -> Some animals have a larger brain than
they expect based on their body weight. The ratio of ‘C’ over the
expected value of ‘C’ of animal of a given weight. You make one animal
‘1’ and then you can see how much larger the brain of other individuals
is compared to that animal.
- Probably has something to do with intelligence
Within a carnivore family has been investigated what the benefit of a higher EQ is. They did this with
problem solving ability. In a puzzle box with food and they have to get it out.
- Result -> was a weak and two-part relation between success in opening the box and
EQ/brain size. Larger brains were successful, regardless of EQ. Smaller brains were successful
when larger than average EQ. Within a species there is a relation with a relative big brain
There are multiple reasons why animals can have big brains:
- Both social and ecological factors predict ungulate brain size
o Solitary animal vs. social animal
Social hoofed animals have bigger brains than solitary
Other differences
1. Anatomy
a. Sulci and Gyri
i. Lissencephaly (no sulci and gyri)
Folding is not simply a matter of (lack of) space
- Sheep embryos without skull still develop strong folding
- There are genetic determinants, axonal tension, tissue buckling because of cortical thickness
also play a role (less folds with thicker cortex, more with thinner)
Lissencephaly in humans -> leads to severe intellectual disability and untreatable epilepsy. Complete
lissencephaly doesn’t happen very often, but it can also be partially.
, 2. Cortical thickness
a. Neuronal density
b. Glial cells vs. neuron ratio
c. 80% of neurons in cerebellum!!
A better measure may be to look at how many neurons an animal has instead of its actual brain size.
Size, shape and encephalization may depend on:
- Body
o Weight issues ; birds having to fly, protection from cold, fat of whales
o Muscle proportion
- Brain
o Cortex vs. subcortical vs. cerebellum
o Gyration
o Cortical thickness
o Neurons vs glial cells (supportive tissue)
o Neuron density
Nowadays more popular -> looking at differences between individuals within species
- What you can do is looking at cognitive functions and specific brain areas to see if they are
bigger.
o Voxel based morphometry (VBM) ; look at the amount of grey-matter measured with
structural MRI. One study looked at English taxi drivers had a larger hippocampus
correlated with their years of experience.
Difficult to replicate
o Never replicated -> People with conservative political conservation correlated with
larger amygdala (fear?!) and smaller anterior cingulate (control of fear?!)
You can find these effects in 30 to 40 participants, but if you take for example a 1000 these effects
disappear. It seems like there is a relation between characteristics and brain areas, but not very clear.
Synesthetes ; associating words with colors. In these people it was shown with diffusion tensor
imaging that there is an increased fiber connection in the regions that connect V4 color regions with
visual word form area.
More modern approach to individual differences in brain morphology
,Take all these different measures for individual brain morphology in individuals and compare them
between individuals. An independent component analysis shows what patterns of combinations with
characteristics make up ‘different types of brains’. For example one component 6 (consisting of many
parts) is correlated with behavioral and demographic traits that are ‘positive’ and negatively with
traits that are considered ‘negative’. So the more of component 6 the more you have of those
characteristics.
- Is it that your brain looks like this that you have more success or is it because you have a lot
of success your brain will look like this? -> usually a bit of both
Can also do postmortem studies with coloring:
- Silver stain
Cytoarchitecture
The layout of cortical layers, their thickness, cell density, cell
types, fiber orientation and density etc. based on different
ways of coloring slices of brain.
- Golgi ; full coloring of (some) individuals cells
- Nissl ; All cell bodies
- Weigert ; fibers (axons and dendrites)
You can also make maps according to cytoarchitecture, so a
cytoarchitectonic map. This is not a function based on
function, but just structure. Later it was found with the
Brodmann map that these areas do often overlap with the
functional areas, but this is definitely not always the case.
Most determinant for how a neuron functions is what its input is and less what it looks like. Obviously
both play an important role.
Differences between halves
- Lot of functions are lateralized.
Language
o Left visual field = right hemisphere
For example coordinating of your hands. In a study they show that the proximal muscles (shoulder,
trunk) you coordinate with both hemispheres so it is bilateral. More the fine movements with the
fingers for example is way more lateralized, this is also more distal movements). This counts mainly
for the dominant hand, for the other hand it is more bilateral again.
A function that is a lot lateralized is the understanding and production of language. This is mostly to
the left hemisphere and the left a lot less. The classic method of determining where someone’s
language center is, is the WADA test. You give an anesthetic where you anesthetize one hemisphere
of the brain. You can see then when one of the two sides doesn’t work you know which side produces
language. This is also not as black and white as this, but often one hemisphere is dominant.
, Gyri
Left hemisphere is a lot more
smooth than the right
hemisphere. Left is also often bigger than on the right. The sylvian fissure was much flatter on the left
than on the right. This corresponds to differences in size of Heschl’s gyrus and the Planum Temporale.
This corresponds to Wernicke’s area.
BUT
Size of left vs. right Heschl’s gyrus does not match to language lateralization as measuring using
WADA test. People with right hemisphere speech dominance or bilateral speech also have a larger
left side Heschl’s gyrus, as do left hemisphere dominant people.
Also, no difference in gray matter of planum temporale. But clear difference in gray matter density of
Broca’s speech. Could also be that language acquisition is more localized than speech production.
Pyramidal cells
Pyramidal cell bodies are larger on the left than on the right in Heschl’s gyrus, but not in other regions
like Wernicke. If this is linked to language is unknown.
Patches
Connections between cells are organized in patches. Neurons are kind of connected in patches and
between the patches there is not much connection. This has in the visual cortex very functional, but
not in all parts the function is known. You can measure the distance between these patches and that
in area 22 (Wernicke’s ; understanding language) the patches are further from each other on the left
compared to the right. So, this is specific to this language are.
Summary
Macroscopic and microscopic anatomical, next to functional differences clearly exist between the two
hemispheres, but are never totally absolute.
Two hemispheres normally work together because the corpus callosum connects the cortex of the
two hemispheres
- Connections between homotopic (same function), and
also heterotopic areas
- Together with the anterior commissure
- Posterior commissure connects subcortical nuclei
Interhemispheric connectivity ; synchrony between neurons that
respond to the same stimulus that activates cells with receptive
fields in both hemifields.
What does the difference in brain size mean?
- Intelligence?
o Dogs can be very smart -> can learn to associate a large number of words with items
Fast mapping
Learning by exclusion ; naming a word they don’t know and when there is a
novel object in the environment they automatically associate these two
words with each other
(as human children do)
Word processing also has a similar form as in humans
o Don’t see the above in for example casts
Best predictor for brain size is body size: E=CSr
E -> weight of brain
C -> cephalization factor
S -> body weight
r = exponential constant
Average in mammals : 0.66
Encephalization Quotient (EQ) -> Some animals have a larger brain than
they expect based on their body weight. The ratio of ‘C’ over the
expected value of ‘C’ of animal of a given weight. You make one animal
‘1’ and then you can see how much larger the brain of other individuals
is compared to that animal.
- Probably has something to do with intelligence
Within a carnivore family has been investigated what the benefit of a higher EQ is. They did this with
problem solving ability. In a puzzle box with food and they have to get it out.
- Result -> was a weak and two-part relation between success in opening the box and
EQ/brain size. Larger brains were successful, regardless of EQ. Smaller brains were successful
when larger than average EQ. Within a species there is a relation with a relative big brain
There are multiple reasons why animals can have big brains:
- Both social and ecological factors predict ungulate brain size
o Solitary animal vs. social animal
Social hoofed animals have bigger brains than solitary
Other differences
1. Anatomy
a. Sulci and Gyri
i. Lissencephaly (no sulci and gyri)
Folding is not simply a matter of (lack of) space
- Sheep embryos without skull still develop strong folding
- There are genetic determinants, axonal tension, tissue buckling because of cortical thickness
also play a role (less folds with thicker cortex, more with thinner)
Lissencephaly in humans -> leads to severe intellectual disability and untreatable epilepsy. Complete
lissencephaly doesn’t happen very often, but it can also be partially.
, 2. Cortical thickness
a. Neuronal density
b. Glial cells vs. neuron ratio
c. 80% of neurons in cerebellum!!
A better measure may be to look at how many neurons an animal has instead of its actual brain size.
Size, shape and encephalization may depend on:
- Body
o Weight issues ; birds having to fly, protection from cold, fat of whales
o Muscle proportion
- Brain
o Cortex vs. subcortical vs. cerebellum
o Gyration
o Cortical thickness
o Neurons vs glial cells (supportive tissue)
o Neuron density
Nowadays more popular -> looking at differences between individuals within species
- What you can do is looking at cognitive functions and specific brain areas to see if they are
bigger.
o Voxel based morphometry (VBM) ; look at the amount of grey-matter measured with
structural MRI. One study looked at English taxi drivers had a larger hippocampus
correlated with their years of experience.
Difficult to replicate
o Never replicated -> People with conservative political conservation correlated with
larger amygdala (fear?!) and smaller anterior cingulate (control of fear?!)
You can find these effects in 30 to 40 participants, but if you take for example a 1000 these effects
disappear. It seems like there is a relation between characteristics and brain areas, but not very clear.
Synesthetes ; associating words with colors. In these people it was shown with diffusion tensor
imaging that there is an increased fiber connection in the regions that connect V4 color regions with
visual word form area.
More modern approach to individual differences in brain morphology
,Take all these different measures for individual brain morphology in individuals and compare them
between individuals. An independent component analysis shows what patterns of combinations with
characteristics make up ‘different types of brains’. For example one component 6 (consisting of many
parts) is correlated with behavioral and demographic traits that are ‘positive’ and negatively with
traits that are considered ‘negative’. So the more of component 6 the more you have of those
characteristics.
- Is it that your brain looks like this that you have more success or is it because you have a lot
of success your brain will look like this? -> usually a bit of both
Can also do postmortem studies with coloring:
- Silver stain
Cytoarchitecture
The layout of cortical layers, their thickness, cell density, cell
types, fiber orientation and density etc. based on different
ways of coloring slices of brain.
- Golgi ; full coloring of (some) individuals cells
- Nissl ; All cell bodies
- Weigert ; fibers (axons and dendrites)
You can also make maps according to cytoarchitecture, so a
cytoarchitectonic map. This is not a function based on
function, but just structure. Later it was found with the
Brodmann map that these areas do often overlap with the
functional areas, but this is definitely not always the case.
Most determinant for how a neuron functions is what its input is and less what it looks like. Obviously
both play an important role.
Differences between halves
- Lot of functions are lateralized.
Language
o Left visual field = right hemisphere
For example coordinating of your hands. In a study they show that the proximal muscles (shoulder,
trunk) you coordinate with both hemispheres so it is bilateral. More the fine movements with the
fingers for example is way more lateralized, this is also more distal movements). This counts mainly
for the dominant hand, for the other hand it is more bilateral again.
A function that is a lot lateralized is the understanding and production of language. This is mostly to
the left hemisphere and the left a lot less. The classic method of determining where someone’s
language center is, is the WADA test. You give an anesthetic where you anesthetize one hemisphere
of the brain. You can see then when one of the two sides doesn’t work you know which side produces
language. This is also not as black and white as this, but often one hemisphere is dominant.
, Gyri
Left hemisphere is a lot more
smooth than the right
hemisphere. Left is also often bigger than on the right. The sylvian fissure was much flatter on the left
than on the right. This corresponds to differences in size of Heschl’s gyrus and the Planum Temporale.
This corresponds to Wernicke’s area.
BUT
Size of left vs. right Heschl’s gyrus does not match to language lateralization as measuring using
WADA test. People with right hemisphere speech dominance or bilateral speech also have a larger
left side Heschl’s gyrus, as do left hemisphere dominant people.
Also, no difference in gray matter of planum temporale. But clear difference in gray matter density of
Broca’s speech. Could also be that language acquisition is more localized than speech production.
Pyramidal cells
Pyramidal cell bodies are larger on the left than on the right in Heschl’s gyrus, but not in other regions
like Wernicke. If this is linked to language is unknown.
Patches
Connections between cells are organized in patches. Neurons are kind of connected in patches and
between the patches there is not much connection. This has in the visual cortex very functional, but
not in all parts the function is known. You can measure the distance between these patches and that
in area 22 (Wernicke’s ; understanding language) the patches are further from each other on the left
compared to the right. So, this is specific to this language are.
Summary
Macroscopic and microscopic anatomical, next to functional differences clearly exist between the two
hemispheres, but are never totally absolute.
Two hemispheres normally work together because the corpus callosum connects the cortex of the
two hemispheres
- Connections between homotopic (same function), and
also heterotopic areas
- Together with the anterior commissure
- Posterior commissure connects subcortical nuclei
Interhemispheric connectivity ; synchrony between neurons that
respond to the same stimulus that activates cells with receptive
fields in both hemifields.
