A cellular basis of human mental ability - Huib Mansvelder
How is it possible for human cognitive ability to be higher than any other species? To answer this
question we need to study the human neocortex and compare it to other species, which professor
Huib Mansvelder explained in his guest lecture1. The neocortex is responsible for cognition, in
humans the neocortex is folded much more than any other species. This causes a big contact area
in relation to the size of a human, which would be a first implication as to why human cognition is
exceptional.
When looking at the cortical layers of the neocortex, the second and third layers (L2/3) turn out to
be thicker in humans compared to other species. Further research has shown that the thickness of
these layers are in a positive correlation with IQ. The thicker these layers, specifically in the left
side of the brain, the higher the IQ. Additionally, the neurons in these thicker layers are also larger,
the cell density is higher and the dendrites are longer as well.
When comparing humans to mice, there are 2 times more synapses per neuron, which could
indicate that humans indeed process more information than other species. Another reason could
be the high failure rate of synaptic communication between pyramidal neurons. In mice 25% of the
APs do not result in the release of glutamate, in human cells, failure is rare. Lastly, there is a big
difference in synaptic depression between humans and other species, such as mice. In mice, the
depression- which causes a smaller effect in the postsynaptic neuron- happens easily after a few
fired APs. In humans not so much. Additionally, a human neuron needs less than half a second to
recover from this, while a mouse neuron needs a longer period of time.
All together, the thicker L2/3, the longer dendrites, more synapses and faster recovery from
depression can explain why human cognition is incomparable to the other species.
A great proportion of Mansvelder’s pitch is based on the relation or correlation between temporal
cortex gray matter and IQ. A given example of the systems in this brain region is the language
system. IQ can be subdivided in many categories. One of the categories studied in this research is
verbal IQ, this is why the temporal cortex is relevantly mentioned in the pitch. A conclusion drawn
from this system is that temporal cortical L2/3 thickness positively correlates with a higher verbal
IQ. Another brain region which was often discussed was the neocortex. This is because the
neocortex is associated with thinking and processing, hence intelligence. As it is already
mentioned, the human neocortex is bigger and more folded, creating a greater surface area,
resulting in more neuronal complexity.
We found the strong positive correlation between
the thickness of cortex in the second and third
layers and IQ in the human brain to be the most
inspiring part of this lecture. We think it is
fascinating how something seemingly small as
the size of these layers can have such big
influences on our IQ and cognitive abilities
compared to other species. Also the human brain
consisting of 86 billion neurons was a fascinating
fact.
Figure 1. Human versus mouse L2/L3 pyramidal cells
from the temporal cortex. ResearchGate. 2
1 Huib Mansvelder, guest lecture 20 September 2023, Vrije Universiteit
2 https://www.researchgate.net/figure/Human-versus-mouse-L2-L3-pyramidal-cells-from-the-temporal-cortex-A-PCA-
of_fig8_319852620
How is it possible for human cognitive ability to be higher than any other species? To answer this
question we need to study the human neocortex and compare it to other species, which professor
Huib Mansvelder explained in his guest lecture1. The neocortex is responsible for cognition, in
humans the neocortex is folded much more than any other species. This causes a big contact area
in relation to the size of a human, which would be a first implication as to why human cognition is
exceptional.
When looking at the cortical layers of the neocortex, the second and third layers (L2/3) turn out to
be thicker in humans compared to other species. Further research has shown that the thickness of
these layers are in a positive correlation with IQ. The thicker these layers, specifically in the left
side of the brain, the higher the IQ. Additionally, the neurons in these thicker layers are also larger,
the cell density is higher and the dendrites are longer as well.
When comparing humans to mice, there are 2 times more synapses per neuron, which could
indicate that humans indeed process more information than other species. Another reason could
be the high failure rate of synaptic communication between pyramidal neurons. In mice 25% of the
APs do not result in the release of glutamate, in human cells, failure is rare. Lastly, there is a big
difference in synaptic depression between humans and other species, such as mice. In mice, the
depression- which causes a smaller effect in the postsynaptic neuron- happens easily after a few
fired APs. In humans not so much. Additionally, a human neuron needs less than half a second to
recover from this, while a mouse neuron needs a longer period of time.
All together, the thicker L2/3, the longer dendrites, more synapses and faster recovery from
depression can explain why human cognition is incomparable to the other species.
A great proportion of Mansvelder’s pitch is based on the relation or correlation between temporal
cortex gray matter and IQ. A given example of the systems in this brain region is the language
system. IQ can be subdivided in many categories. One of the categories studied in this research is
verbal IQ, this is why the temporal cortex is relevantly mentioned in the pitch. A conclusion drawn
from this system is that temporal cortical L2/3 thickness positively correlates with a higher verbal
IQ. Another brain region which was often discussed was the neocortex. This is because the
neocortex is associated with thinking and processing, hence intelligence. As it is already
mentioned, the human neocortex is bigger and more folded, creating a greater surface area,
resulting in more neuronal complexity.
We found the strong positive correlation between
the thickness of cortex in the second and third
layers and IQ in the human brain to be the most
inspiring part of this lecture. We think it is
fascinating how something seemingly small as
the size of these layers can have such big
influences on our IQ and cognitive abilities
compared to other species. Also the human brain
consisting of 86 billion neurons was a fascinating
fact.
Figure 1. Human versus mouse L2/L3 pyramidal cells
from the temporal cortex. ResearchGate. 2
1 Huib Mansvelder, guest lecture 20 September 2023, Vrije Universiteit
2 https://www.researchgate.net/figure/Human-versus-mouse-L2-L3-pyramidal-cells-from-the-temporal-cortex-A-PCA-
of_fig8_319852620
