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Transcript of Announcements
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
Cognition and Language
Computation
Structured Connectionism
Computational Neurobiology
Biology
MidtermQuiz Finals
Neural Development
Triangle Nodes
Neural Net & Learning
Spatial Relation
Motor Control Metaphor
SHRUTI
Grammar
abst
ract
ion
Regier Model
Bailey Model
Narayanan Model
Chang Model
Visual System
Psycholinguistics Experiments
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 short-term 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 short-term 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 (mm-scale) but not-so-great temporal resolution (2-5 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 short-term 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 objectthis 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?
integrated, multi-modal representation of actions, along with the objects and locationsBuccino et al., 2001
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 short-term memory? Long-term memory?
4. Why is Hebb’s rule not the complete story for the learning that goes on in the brain?
Declarative Non-Declarative
Episodic Semantic Procedural
Memory
Two ways of looking at memory:
facts about a situation
general facts skills
Memory
Short Term Memory Long Term Memory
Two ways of looking at memory:
electrical changes
structural changes
LTP
• Hebb’s Rule: neurons that fire together wire together
• Long Term Potentiation (LTP) is the biological basis of Hebb’s Rule
• Calcium channels is the key mechanism
LTP and Hebb’s Rule
strengthen
weaken
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 short-term 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:
• should you “punish” all the connections?
tastebud tastes rotten eats food gets sick
drinks water
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)
ti : target
Let’s try an example: the OR function
• 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?
i1 i2 y0
0 0 0
0 1 1
1 0 1
1 1 1
x0 f
i1 w01
y0i2
b=1
w02
w0b
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
Anonymous Feedback: Lectures
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Any particularly confusing topic?
Anonymous Feedback: Sections
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