nature requires nurture initial wiring is genetically controlled sperry experiment but...
Post on 21-Dec-2015
214 views
TRANSCRIPT
![Page 1: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/1.jpg)
Nature requires Nurture
Initial wiring is genetically controlledSperry Experiment
But environmental input critical in early development Occular dominance columns
Hubel and Wiesel experiment
![Page 2: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/2.jpg)
Innervation of the Optic tectum
Ganglion Cells in the frog retina (maps particular regions in space) map systematically to cells in the optic tectum (movement in specific directions). The retinal cells “see” in a direction and map to tectal cells which cause the frog
to move in that direction The image of the external stimulus is inverted in the retina and the mapping
from the retina to the optic tectum reverts to the original image. The Nasal ganglion cells of the retina map to the posterior region of the
Optic tectum and the temporal ganglion cells map to the anterior region of the tectum
![Page 3: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/3.jpg)
Sperry’s experiment
Sperry took advantage of the fact that in amphibians, the optic nerve will regrow after it has been interrupted
Sperry cut the optic nerve and simultaneously rotated the eye 180 degrees in the eye socket.
In 'learning’ movements to catch prey, the part of the retina now looking forward (backward) should connect to the part of the brain which causes forward (backward) movement.
![Page 4: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/4.jpg)
Sperry’s findings
After regeneration, his animals responded to prey items in
front by turning around and to prey items behind by moving forward.
and kept doing this even though they never
succeeded in reaching the prey.
![Page 5: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/5.jpg)
![Page 6: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/6.jpg)
Conclusion from experiment
The conclusion from this (and some supporting experiments) is
that the pattern of connections between retina and tectum, and the movement information represented is not based on experience.
It is innate based on the initial distribution of chemical markers in the brain.
![Page 7: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/7.jpg)
Lecture Overview
Summary Overview Development from embryo Initial wiring Activity dependent fine tuning
![Page 8: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/8.jpg)
The role of the environment
The development of ocular dominance columns Cat and later monkey (Hubel and Wiesel)
Retinal input connects to the LGN (Thalamus) LGN is composed of layers. Each layer receives input (axons)
from a single eye LGN connects to layer IV of the visual cortex
The visual cortex develops ocular dominance columns Cells that are connected to similar layers in the LGN get stacked
together in columns forming stripes.
http://neuro.med.harvard.edu/site/dh/
![Page 9: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/9.jpg)
VISUAL CORTEX
LGN
![Page 10: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/10.jpg)
Monocular deprivation critical period Hubel and Wiesel deprived one of the eyes of
the cat (later macaque monkey) at various times 1 week – 12 weeks (in the monkey case) 4 weeks – 4 months (for the cat).
The found that the ocular dominance cell formation was most severely degraded if deprivation occurred at 1 – 9 weeks after birth.
Deprivation after the plastic period had no long-term effect.
![Page 11: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/11.jpg)
![Page 12: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/12.jpg)
![Page 13: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/13.jpg)
C I
1
C I
7
C I
5
C I
6
C I
3
C I
4
C I
2
Cat Striate Cortex Layer IV
Monkey Striate Cortex Area 17 (V1) Layer IV
CLOSED EYE
OPEN EYE
![Page 14: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/14.jpg)
Critical Periods in Development
There are critical periods in development (pre and post-natal) where stimulation is essential for fine tuning of brain connections.
Other examples of columnsOrientation columns
![Page 15: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/15.jpg)
Pre-Natal Tuning: Internally generated tuning signals But in the womb, what provides the feedback to establish which
neural circuits are the right ones to strengthen? Not a problem for motor circuits - the infant moves its limbs to refine the
feedback and control networks. But there is no vision in the womb.
--Systematic moving patterns of activity are spontaneously generated pre-natally in the retina.
A predictable pattern, changing over time, provides excellent training data for tuning the connections between visual maps.
The pre-natal development of the auditory system Research indicates that infants, immediately after birth, preferentially
recognize the sounds of their native language over others. The assumption is that similar activity-dependent tuning mechanisms work with speech signals perceived in the womb.
![Page 16: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/16.jpg)
Post-natal environmental tuning
The pre-natal tuning of neural connections using simulated activity can work quite well – a newborn colt or calf is essentially functional at birth.
This is necessary because the herd is always on the move.
For many animals, including people, experience is absolutely necessary for normal development (as in the kitten experiment).
For a similar reason, if a human child has one weak eye, the doctor will sometimes place a patch over the stronger one, forcing the weaker eye to gain experience.
![Page 17: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/17.jpg)
Adult Plasticity and Regeneration
The brain has an amazing ability to reorganize itself through new pathways and connections rapidly.
• Through Practice:• London cab drivers, motor regions for the skilled
• After damage or injury • Undamaged neurons make new connections and take
over functionality or establish new functions• But requires stimulation (phantom limb sensations)
• Stimulation standard technique for stroke victim rehabilitation
![Page 18: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/18.jpg)
When nerve stimulation changes, as with amputation, the brain reorganizes. In one theory, signals from a finger and thumb of an uninjured person travel independantly to separate regions in the brain's thalamus (left). After amputation, however, neurons that formerly responded to signals from the finger respond to signals from the thumb (right).
![Page 19: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/19.jpg)
Possible explanation for the recovery mechanism The initial pruning of connections leaves some
redundant connections that are inhibited by the more active neural tissue.
When there is damage to an area, the lateral inhibition is removed and the redundant connections become active
The then can undergo activity based tuning based on stimulation.
Great area for research.
![Page 20: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/20.jpg)
Summary
Both genetic factors and activity dependent factors play a role in developing the brain architecture and circuitry. There are critical developmental periods where
nurture is essential, but there is also a great ability for the adult brain to regenerate.
Next: What computational models satisfy some of the biological constraints.
Question: What is the relevance of development and learning in language and thought?
![Page 21: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/21.jpg)
Connectionist Models: Basics
Srini NarayananCS182/CogSci110/Ling109Spring 2006
![Page 22: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/22.jpg)
![Page 23: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/23.jpg)
Neural networks abstract from the details of real neurons
Conductivity delays are neglected An output signal is either discrete (e.g., 0 or
1) or it is a real-valued number (e.g., between 0 and 1)
Net input is calculated as the weighted sum of the input signals
Net input is transformed into an output signal via a simple function (e.g., a threshold function)
![Page 24: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/24.jpg)
The McCullough-Pitts Neuron
yj: output from unit j
Wij: weight on connection from j to i
xi: weighted sum of input to unit i
xi f
yj wij
yi
xi = ∑j wij yj
yi = f(xi – i)
ti : target
Threshold
![Page 25: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/25.jpg)
Mapping from neuron
Nervous System Computational Abstraction
Neuron Node
Dendrites Input link and propagation
Cell Body Combination function, threshold, activation function
Axon Output link
Spike rate Output
Synaptic strength Connection strength/weight
![Page 26: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/26.jpg)
Simple Threshold Linear Unit
![Page 27: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/27.jpg)
Simple Neuron Model
1
![Page 28: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/28.jpg)
A Simple Example
a = x1w1+x2w2+x3w3... +xnwn
a= 1*x1 + 0.5*x2 +0.1*x3
x1 =0, x2 = 1, x3 =0
Net(input) = f = 0.5
Threshold bias = 1
Net(input) – threshold bias< 0
Output = 0
.
![Page 29: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/29.jpg)
Simple Neuron Model
1
1
1
1
![Page 30: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/30.jpg)
Simple Neuron Model
1
1
1
1
1
![Page 31: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/31.jpg)
Simple Neuron Model
1
0
1
1
![Page 32: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/32.jpg)
Simple Neuron Model
1
0
1
1
0
![Page 33: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/33.jpg)
Different Activation Functions
Threshold Activation Function (step) Piecewise Linear Activation Function Sigmoid Activation Funtion Gaussian Activation Function
Radial Basis Function
BIAS UNITWith X0 = 1
![Page 34: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/34.jpg)
Types of Activation functions
![Page 35: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/35.jpg)
The Sigmoid Function
x=neti
y=a
![Page 36: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/36.jpg)
The Sigmoid Function
x=neti
y=a Output=0
Output=1
![Page 37: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/37.jpg)
The Sigmoid Function
x=neti
y=a Output=0
Output=1
Sensitivity to input
![Page 38: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/38.jpg)
Changing the exponent k(neti)K >1
K < 1
![Page 39: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/39.jpg)
Radial Basis Function
2axe)( xf
![Page 40: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/40.jpg)
Stochastic units Replace the binary threshold units by
binary stochastic units that make biased random decisions.The “temperature” controls the amount of
noise
Twsij ijj
esp
1
1)( 1
temperature
![Page 41: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/41.jpg)
Types of Neuron parameters
The form of the function - e.g. linear, sigma-pi, cubic.
The activation-output relation - linear, hard-limiter, or sigmoidal.
The nature of the signals used to communicate between nodes - analogue or boolean.
The dynamics of the node - deterministic or stochastic.
![Page 42: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/42.jpg)
Computing other functions
McCollough-Pitts Neurons can compute logical functions.AND, NOT, OR
![Page 43: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/43.jpg)
Computing other functions: the OR function
• Assume a binary threshold activation function.
• What should you set w01, w02 and w0b to be so that you can get the right answers for y0?
i1 i2 y0
0 0 0
0 1 1
1 0 1
1 1 1
x0 f
i1 w01
y0i2
b=1
w02
w0b
![Page 44: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/44.jpg)
Many answers would work
y = f (w01i1 + w02i2 + w0bb)
recall the threshold function
the separation happens when w01i1 + w02i2 + w0bb = 0
move things around and you get
i2 = - (w01/w02)i1 - (w0bb/w02)
i2
i1
![Page 45: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/45.jpg)
Decision Hyperplane
The two classes are therefore separated by the `decision' line which is defined by putting the activation equal to the threshold.
It turns out that it is possible to generalise this result to TLUs with n inputs.
In 3-D the two classes are separated by a decision-plane.
In n-D this becomes a decision-hyperplane.
![Page 46: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/46.jpg)
Linearly separable patterns
Linearly Separable PatternsPERCEPTRON is an architecture which can solve this type of decision boundary problem. An "on" response in the output node represents one class, and an "off" response represents the other.
![Page 47: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/47.jpg)
The XOR Function
X1/X2 X2 = 0 X2 = 1
X1= 0 0 1
X1 = 1 1 0
![Page 48: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/48.jpg)
The Input Pattern Space
![Page 49: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/49.jpg)
The Decision planes
![Page 50: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/50.jpg)
Computing other relations
The 2/3 node is a useful function that activates its outputs (3) if any (2) of its 3 inputs are active
Such a node is also called a triangle node and will be useful for lots of representations.
![Page 51: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/51.jpg)
Triangle nodes and McCullough-Pitts Neurons?
B C
A
A B C
![Page 52: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/52.jpg)
Representing concepts using triangle nodes triangle
nodes:
when two of the neurons fire, the third also fires
![Page 53: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/53.jpg)
“They all rose”
triangle nodes:
when two of the neurons fire, the third also fires
model of spreading activation
![Page 54: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/54.jpg)
Link to Vision: The Necker Cube
![Page 55: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/55.jpg)
Basic Ideas behind the model
Parallel activation streams. Top down and bottom up activation combine to
determine the best matching structure. Triangle nodes bind features of objects to values Mutual inhibition and competition between
structures Mental connections are active neural
connections
![Page 56: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/56.jpg)
5 levels of Neural Theory of Language
Cognition and Language
Computation
Structured Connectionism
Computational Neurobiology
Biology
MidtermQuiz Finals
Neural Development
Triangle Nodes
Neural Net
Spatial Relation
Motor Control Metaphor
SHRUTI
Grammar
abst
ract
ion
![Page 57: Nature requires Nurture Initial wiring is genetically controlled Sperry Experiment But environmental input critical in early development Occular dominance](https://reader030.vdocument.in/reader030/viewer/2022032704/56649d6b5503460f94a49e5c/html5/thumbnails/57.jpg)
Can we formalize/model these intuitions What is a neurally plausible computational
model of spreading activation that captures these features.
What does semantics mean in neurally embodied termsWhat are the neural substrates of concepts
that underlie verbs, nouns, spatial predicates?