Transcript Lecture 8B

Mental Synthesis theory:
• the mechanism of imagining novel objects
involves temporal SYNCHRONIZATION
of independent neuronal ensembles.
Synchronization
no enhanced
connections
Asynchronously
firing neuronal
ensembles
Perceived as
two different
objects
Synchronously
firing neuronal
ensembles
Perceived as
one morphed
image
The lateral prefrontal cortex as a puppeteer
PFC
Puppeteer
Puppets in the
posterior cortex
Synchronization
Mental
Synthesis
Lateral PFC
• Since the lateral prefrontal cortex plays such a
central role in mental synthesis in humans, it
would be interesting to see if a homologous brain
area was present in other animals.
• The lateral prefrontal cortex is present in all
primates but is absent in other animals including
other mammals (Striedter GF, 2005; Petrides,
2002).
• Striedter GF, 2005: “Nonprimate mammals do have a PFC, but it apparently
consists of only two major regions, rather than three as in primates. The
two conserved prefrontal regions are the orbital prefrontal region, whose
neurons respond preferentially to external stimuli that are likely to be
rewarding or otherwise significant (Tremblay, 1999; Schoenbaum, 2001),
and the anterior cingulate cortex, which mainly process information about
the body’s internal state (Nauta, 1971; Luu, 2003). Collectively, these two
regions contribute to what we might call the “emotional” aspect of decision
making (Damasio, 1994; Dias, 1996; Allman, 2001). The third prefrontal
region, which is generally known as the lateral, or granular prefrontal
cortex, is apparently unique to primates (Preuss, 1995) and is concerned
mainly with the “rational” aspects of decision making. Its neurons respond
less rapidly than orbitofrontal neurons to rewarding stimuli and are more
selective for the physical attributes of the stimuli, such as their spatial
location (Wallis, 2003). Without those lateral prefrontal neurons, primates
become less able to retrieve and manipulate information about objects in
the outside world (Owen, 1999). In the context of decision making, this
probably means that the lateral prefrontal cortex helps primates to consider
alternative interpretations of external objects and to construct alternative
scenarios of how to interact with them. ... The notion of the lateral
prefrontal cortex being a primate innovation originated with Brodmann and
has recently been championed by Todd Preuss (1995a). Central to their
argument is that small celled granular layer that characterizes the lateral
prefrontal cortex in primates is lacking in most other mammals. This finding
does indeed suggest that the lateral PFC is unique to primates.”
What is so special about
primates?
55-million-year-old primate fossil in
Chinadubbed Archicebus Achilles
• Separated from the rest of mammals some 70 million years
ago.
• The mammalian ancestors of primates were small nocturnal
animals who spent most of their time underground. Following
the demise of dinosaurs some 66 million years ago, the
competition for aboveground resources diminished and
primates evolved into diurnal arboreal animals.
• Primates came to occupy the evolutionary niche that heavily
depends on vision rather than olfaction for food search and
predator avoidance. Reliance on an enhanced sense of vision
became the defining feature of all primates.
• most primate-typical features reflect
the development of the visual system:
– primate eyes evolved to a more frontally
placed position (presumably to improve
depth perception)
– Primates also acquired an additional color
receptor to better differentiate red from
green
actual
Perceived
• Most mammals, including
dogs and cats, have only two
different kinds of
UV
color receptors.
• Why?
• A remote vertebrate ancestor
of all mammals possessed 4
color receptors- most fish,
UV
reptiles and birds still have 4
color receptors
• However nocturnal mammalian
ancestors lost two of four
cones in the retina at the time
of dinosaurs  all mammals,
with the exception of some
primates still have only 2 color
receptors.
• Some primates (including apes)
have acquired the third color
receptors.
• Why?
• Some people (2.4% of males)
still have two color receptors.
• Primates primarily use vision to direct
skilled movements, as well as to search
for food and avoid predators.
• Rodents can also grasp and manipulate
objects using the tips of their digits,
however rodents are much less visual.
Their skilled movements are primarily
directed by the sense of touch and
olfaction: rats without vision are able to
locate and reach for food as quickly as
they did before being blinded, but rats
without olfaction, who therefore rely
more on vision, are significantly slower
(Whishaw, 2003).
Primates occupied the “finebranch niche”
• Collecting ripe fruits and leaves, and
hunting for insects.
• Primates also acquired more direct corticospinal
projections (connections between the cortical
neurons and the motor neurons in the spinal
cord, which increase fine control over muscles)
• an additional cortical area (called the ventral premotor cortical
area) specialized for arm and mouth movements (Preuss, 1996).
• Furthermore, the control of hand and foot movement benefitted
from an increased somatosensory cortex, which, in primates,
added several cortical areas that have no homologues in nonprimates.
• Striedter concludes: “diverse evolutionary changes in the visual,
motor and somatosensory systems all interacted to give early
primates exceptionally good hand-eye coordination, which must
have come in handy in the fine-branch niche”.
• A primate’s survival in the “fine-branch niche” depended on its
ability to visually detect predators, prey, and edible objects.
• Consequently, primates were under constant evolutionary
pressure to recognize those objects faster and from a greater
distance.
• Visually identifying ripe fruits, leaves, and insects relies on a
primate’s ability to separate those objects from the background
and from other objects.
• Fruits are normally hidden under leaves, leaves in the rainforest
merge into thick foliage, and insects tend to camouflage their
appearance.
• Predators are also often camouflaged and therefore merge into
the background.
• Furthermore fruits, leaves, insects, and predators often remain
motionless, and visual detection of motionless objects is
significantly more difficult than detection of moving objects.
• Can you find a camouflaged frog
amongst the dead leaves in this illustration
from the 2015 Johnston Club calendar?
•
A Leopard conceals herself in vegetation at the base of a tree in Kruger National Park, Transvaal, South Africa
• A wolf peering out from behind a tree trunk in an
autumn Montana forest
What part of the brain is the primary source of topdown attentional control in object perception?
• What part of the brain is the primary source of topdown attentional control in object perception?
• Meet our old friend the prefrontal cortex. The
prefrontal cortex is involved in the maintenance of
goal-related information as well as in attentional
selection and focus shifting (Fuster JM, 2008).
• To succeed in an environment where prey and
predators were often motionless, camouflaged, and
partially hidden, primates had to rely heavily on their
prefrontal cortex to interpret ambiguous sensory input
throughout the 70 million years of their evolution.
• The prefrontal cortex of diurnal arboreal animals, who
relied on their sense of vision to detect prey and
predator from afar, was under constant evolutionary
pressure to improve its ability to actively steer
perception to coordinate subjective goals with
objective reality.
Primates primarily detect
predators and prey visually
Prosimians in
Madagascar
Visual
System
Modern Humans
• Primates
exposed
to greater predation
by venomous snakes
acquired a better
visual system
compared to
primates not
exposed to
venomous snakes
(Isbell LA, 2009).
New World
Old World
Homo habilis
monkeys of South monkeys and
America
apes (catarrhines)
(platyrrhines)*
Immobile
Predators
Species
THE SNAKE DETECTION THEORY
(LYNNE A. ISBELL, 2009)
interrupted exposure
to venomous snakes
continuous coexistence
with venomous snakes for
over 60 million years
*The last exposure of the New World monkeys of South America (platyrrhines) to rattlesnakes lasted for only three million years
• Recall: some animals cannot
visually detect a still object
even when it is unambiguous
• To locate an immobile prey,
most predators use smell or
others senses (e.g. infrared
radiation), not vision.
• For these reptiles it was
evolutionarily “easier” to
acquire this additional system
rather than to develop
computational support for the
existing visual sensory input.
• Visual primates, on the other
hand, “chose” to expand their
computational system in order
to improve detection of
Only few snakes can detect infrared radiation: A python (top) and
illustrating the positions of the pit organs. Red arrows
motionless ambiguous objects. rattlesnake
point to the pit organs; Black arrow point to the nostril.
Prosimians in
Madagascar
New World
Old World
monkeys of South monkeys and
America
apes (catarrhines)
(platyrrhines)
Homo habilis
Modern Humans
Visual
System
Immobile Predators
Species
The Mental Synthesis theory (Vyshedskiy, 2008) extends
the Snake Detection theory to the evolution of hominins
• Hominins foraging in the savanna were exposed to an even greater range of IMMOBILE STALKING predators…
• the brain volume increased relatively slowly in
australopiths from 350 cm3 to 450 cm3 over 3.5
million years,
• but then increased significantly in two major
growth spurts:
– around 1.8 million years ago and
– about 100,000 years ago
• The lateral prefrontal cortex underwent the
greatest transformation under this
evolutionary pressure.
Different brain areas are
scaled up differently:
• the volume of the
olfactory bulb in
humans is only 30% as
large as would be
expected in a primate
brain of our size,
• the volume of V1 is
only 60% as large as
expected.
• Remarkably, the
volume of Brodmann
area 10 (frontopolar
cortex) is nearly 200%
as large as expected for
a primate brain of our
size.
• Brodmann measured the PFC size as percent of the total
cortex using the cytoarchitectonic method:
–
–
–
–
–
–
29% of in humans,
17% in the chimpanzee,
11.5% in the gibbon and the macaque,
8.5% in the lemur,
7% in the dog, and
3.5% in the cat
• Semendeferi K, 2001 (using a precise cytoarchitectonic
method): neuronal density in human Brodmann area 10 was
around half of that of other primates  Brodmann area 10 in
humans has a significantly greater portion of its volume
dedicated to connections between neurons
• Conclusion: Humans’ Brodmann area 10 is nearly 200% as
large as expected for a primate brain of our size and that
increase in size is due to greater number of connections and
myelination (Schoenemann PT, 2005)
Greater differential myelination
Slower
conduction
velocity
Faster
conduction
velocity
Slower
conduction
velocity
Faster
conduction
velocity
Faster
conduction
velocity
Faster
conduction
velocity
• Without a mechanism that could equalize transit times, the
signal from the prefrontal cortex would arrive to its targets in the
posterior cortex at different times.
• This synchronization mechanism poses a serious challenge
that every human needs to solve during development:
• These connections must be fine-tuned to become synchronous.
More myelin ->
faster conduction
velocity
Less myelin ->
slower conduction
velocity
• Hypothesis: myelination is the primary factor producing uniform
conduction time throughout the cortex
The lateral prefrontal cortex as a puppeteer
PFC
Puppeteer
Puppets in the
posterior cortex
Synchronization
Mental
Synthesis
• - DANIEL J. POVINELLI (20min, 2013) https://www.youtube.com/watch?v=kX49dlbf
G9E
• - Herbert S. Terrace - Nim Chimpsky DVD