Announcements
• a3 is out, due 2/15 11:59pm
• Please please please start early
• quiz will be graded in about a week.
• a1 will be graded shortly—use glookup to see
your grade
Where we stand
• Last Week
– Imaging studies
– Connectionist representation
• This Week
– Backprop
– traditional AI
• Coming up
– Neurophysiology of color
The Big (and complicated) Picture
Psycholinguistics
Experiments
Spatial
Relation
Motor
Control
Metaphor Grammar
Cognition and Language
abstraction
Computation
Chang
Model
Bailey
Model
Structured
Connectionism
Neural Net
Regier
& Learning
Triangle Nodes
Narayanan
Model
Model
SHRUTI
Computational Neurobiology
Visual System
Neural
Development
Quiz
Biology
Midterm
Finals
Quiz!
1. How is fMRI used? How is TMS used?
2. What systems are active when we observe a
person picking up a glass?
3. What is the biological mechanism for shortterm memory? Long-term memory?
4. Why is Hebb’s rule not the complete story for
the learning that goes on in the brain?
Quiz!
1. How is fMRI used? How is TMS used?
2. What systems are active when we observe a
person picking up a glass?
3. What is the biological mechanism for shortterm memory? Long-term memory?
4. Why is Hebb’s rule not the complete story for
the learning that goes on in the brain?
Imaging Techniques
• fMRI Measures the magnetic resonance of
cranial blood flow, which varies with
oxygenation
• fMRI has very good spatial resolution (mmscale) but not-so-great temporal resolution (25 seconds)
• TMS induces a current in the brain, the
movement of which indicates interconnections
• TMS can be used with fMRI…
Quiz!
1. How is fMRI used? How is TMS used?
2. What systems are active when we observe a
person picking up a glass?
3. What is the biological mechanism for shortterm memory? Long-term memory?
4. Why is Hebb’s rule not the complete story for
the learning that goes on in the brain?
The Mirror Circuit in Monkeys
• top:
monkey sees
experimenter grasp
an object
• bottom:
monkey sees
experimenter
reaches his hand
behind a screen to
grasp an object
this is what we see in a monkey…
measuring a neuron in the parietal area
Somatotopy
• top:
humans watching foot ,
hand and mouth
actions without an object
• bottom:
humans watching same
actions with an object
• What can we learn from
these two experiments?
Buccino et al., 2001
integrated, multi-modal
representation of
actions, along with the
objects and locations
Quiz!
1. How is fMRI used? How is TMS used?
2. What systems are active when we observe a
person picking up a glass?
3. What is the biological mechanism for shortterm memory? Long-term memory?
4. Why is Hebb’s rule not the complete story for
the learning that goes on in the brain?
Two ways of looking at memory:
Memory
Declarative
Episodic
facts about a
situation
Non-Declarative
Semantic
general facts
Procedural
skills
Two ways of looking at memory:
Memory
Short Term Memory
Long Term Memory
electrical
changes
structural
changes
LTP
LTP and Hebb’s Rule
• Hebb’s Rule:
neurons that fire together wire together
strengthen
weaken
• Long Term Potentiation (LTP) is the biological
basis of Hebb’s Rule
• Calcium channels is the key mechanism
Quiz!
1. How is fMRI used? How is TMS used?
2. What systems are active when we observe a
person picking up a glass?
3. What is the biological mechanism for shortterm memory? Long-term memory?
4. Why is Hebb’s rule not the complete story for
the learning that goes on in the brain?
Why is Hebb’s rule incomplete?
• here’s a contrived example:
tastebud
tastes rotten
eats food
gets sick
drinks water
• should you “punish” all the connections?
The McCullough-Pitts Neuron
yj
wij
xi
f
yi
ti : target
xi = ∑j wij yj
yi = f(xi)
yj: output from unit j
Wij: weight on connection from j to i
xi: weighted sum of input to unit i
Let’s try an example: the OR function
i1
i2
b=1
w01
w02
w0b
x0
f
y0
i1
i2
y0
0
0
0
0
1
1
1
0
1
1
1
1
• Assume you have a threshold function centered at the
origin
• What should you set w01, w02 and w0b to be so that
you can get the right answers for y0?
Many answers would work
y = f (w01i1 + w02i2 + w0bb)
i2
recall the threshold function
the separation happens when
w01i1 + w02i2 + w0bb = 0
i1
move things around and you get
i2 = - (w01/w02)i1 - (w0bb/w02)
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