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 abstraction 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 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? integrated, multi-modal representation of actions, along with the objects and locations Buccino 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?

DeclarativeNon-Declarative EpisodicSemanticProcedural Memory Two ways of looking at memory: facts about a situation general facts skills

Memory Short Term MemoryLong 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 strengthenweaken

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? tastebudtastes rotteneats foodgets sick drinks water

The McCullough-Pitts Neuron y j : output from unit j W ij : weight on connection from j to i x i : weighted sum of input to unit i xixi f yjyj w ij yiyi x i = ∑ j w ij y j y i = f(x i ) t i : target

Let’s try an example: the OR function Assume you have a threshold function centered at the origin What should you set w 01, w 02 and w 0b to be so that you can get the right answers for y 0 ? i1i1 i2i2 y0y x0x0 f i1i1 w 01 y0y0 i2i2 b=1 w 02 w 0b

Many answers would work y = f (w 01 i 1 + w 02 i 2 + w 0b b) recall the threshold function the separation happens when w 01 i 1 + w 02 i 2 + w 0b b = 0 move things around and you get i 2 = - (w 01/ w 02 )i 1 - (w 0b b/w 02 ) i2i2 i1i1

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?