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

feel free to comment on each instructor seperately

How is the pace?

Do you find the material interesting? too dense? too slow?

What will be most helpful to you in getting the most out of lectures?

Any particularly confusing topic?

Anonymous Feedback: Sections

Have sections been useful?

Any feedback on our styles of presentation?

How will sections be most helpful to you?

Any other comments?