brief introduction to computational & statistical...
TRANSCRIPT
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Brief introduction to computational & statistical
neuroscience
Jonathan Pillow
Lecture #1Statistical Modeling and Analysis of Neural Data
Spring 20181
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What is computational neuroscience?
2. Study how the brain behaves as a computer• Brain is a machine for processing information &
computing relevant outputs
• Machine for statistical inference
1. Computational/statistical tools to study the brain.• Extract structure from noisy data
• Build models that capture behavior of neurons
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Mind-Brain Problem
What is the relationship of the mind to the brain?
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The brain as a computer:
“The brain computes! This is accepted as a truism by the majority of neuroscientists engaged in discovering the principles employed in the design and operation of nervous systems. What is meant here is that any brain takes the incoming sensory data, encodes them into various biophysical variables, such as the membrane potential or neuronal firing rates, and subsequently performs a very large number of ill-specified operations, frequently termed computations, on these variables to extract relevant features from the input. The outcome of some of these computations can be stored for later access and will, ultimately, control the motor output of the animal in appropriate ways.”
- Christof Koch, Biophysics of Computation
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Short history of brain metaphors:
• hydraulic device (Descartes, 17th C.)• mill (Leibniz, 17th C.)• telegraph (Sherrington, early 20th C.)• telephone switchboard (20th C.)• digital computer (late 20th C.)
• quantum computer? (Penrose, 1989)
• convolutional neural network? (21st C.)
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BrainSensoryInput
MotorOutput
• The physical parts of the brain are important only insofar as they represent steps in a formal calculation.
• Any physical device implementing the same formal system would have the same “mind properties” as a brain.
What does it mean to claim the brain is a computer?
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BrainSensoryInput
MotorOutput
Claim: Most neuroscientists take it for granted that the brain is a computer.
They are devoted to finding out which computer (i.e., what formal structure? what algorithms does the brain implement?).
What does it mean to claim the brain is a computer?
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What is (some of) the evidence that the brain is a computer?
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Mathematical model of sensory neurons
photoreceptors
bipolar cells
retinal ganglion cells
the retina
detect light
output cells (send all visual information to the brain)
to brain!9
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Mathematical model of sensory neurons
photoreceptors
bipolar cells
retinal ganglion cells
Difference of light in “center” and light in the “surround”
+ --
the retina
what mathematical operation?
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Mathematical model of sensory neurons
photoreceptors
bipolar cells
retinal ganglion cells
Difference of light in “center” and light in the “surround”
+ --
stimulus
lots of spikes!
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Mathematical model of sensory neurons
photoreceptors
bipolar cells
retinal ganglion cells
Difference of light in “center” and light in the “surround”
+ --
stimulus
few spikes
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Mathematical model of sensory neurons
photoreceptors
bipolar cells
retinal ganglion cells
Difference of light in “center” and light in the “surround”
+ --
stimulus
more spikes
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Mach Bands
Each stripe has constant luminance
Then why does it look like there’s a gradient?
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Mach Bands
Each stripe has constant luminance
Then why does it look like there’s a gradient?
- + - Cell on right edge
- + -Cell on left edge
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The Neural Coding Problem
• How does the brain take stimuli and “code” them with sequences of spikes?
spikesstimulus“encoding function”
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stimulus spikes
membrane potential
calcium imaging
fMRI
neural activity
• How are stimuli and actions encoded in neural activity?• How are representations transformed between brain areas?
Questions:
The Neural Coding Problem
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stimulus spikes
membrane potential
fMRI
Approach: • develop flexible statistical models of P(y|x) • quantify information coding strategies and mechanisms
encoding models
calcium imaging
neural activityThe Neural Coding Problem
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Lightness Illusion
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Hermann illusion
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This magical slide can track where you’re looking
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Color Computations
Beau Lotto22
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Color Computations
Beau Lotto23
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an image can fool 2/3 of the population (and spark hostility across the globe)
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Turns out: percept depends on statistical inferences brain makes about the light source!
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color after-images
• neurons adjust their response properties after prolonged exposure to an image
• we can compute (and predict) these changes!• red —> green after-image • blue —> yellow after-image • black —> white after-image
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Helmholtz: perception as “optimal inference”
“Perception is our best guess as to what is in the world, given our current sensory evidence
and our prior experience.”
“perception is our best guess as to what is in the world, given our
current sensory evidence and our prior experience.”
perception - alan stocker © 2009
perception as optimal inference
helmholtz 1821-1894
P(world | sense data) ∝ P(sense data | world) P(world)
(given by past experience)
Prior(given by laws of physics;
ambiguous because many world statescould give rise to same sense data)
LikelihoodPosterior(resulting beliefs about
the world)
Bayesian Models for Perception
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what is perception?
percept
• seeing• hearing• touching• smelling• tasting• orienting
“bottom-up”
“top-down”
statistical knowledge about the structure of the world
prior (“top down”)
likelihood (“bottom up”)
posterior
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Many different 3D scenes can give rise to the same 2D retinal image
The Ames Room
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Many different 3D scenes can give rise to the same 2D retinal image
The Ames Room
How does our brain go about deciding which interpretation?
A
B
P(image | A) and P(image | B) are equal! (both A and B could have generated this image)
Let’s use Bayes’ rule:
P(A | image) = P(image | A) P(A) / Z P(B | image) = P(image | B) P(B) / Z
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Neural prostheses: Neurons can be replaced by other entities (silicon chips) that have different physical structure but carry out the same (or similar) mathematical operations, allowing the organism to produce (“compute”) the same behavior.
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Cochlear implants(using a “different computer” to encode auditory signals)
microphone
transmitter receiver
cochlea
electrode array
to brain
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Direct neural control of movement Schwartz Lab (Pitt)
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Direct neural control of movement Schwartz Lab (Pitt)
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If we understand the mathematical operations carried out by different parts of the brain, we could (in theory) replace them with new parts that perform the same computations!
Interchangeability: replacing neurons with silicon
BrainSensory
InputMotorOutput
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Our goal: figure out how the brain works.
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10 microns
There are about 10 billion cubes of this size in your brain!
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Tungsten Electrode
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Kelly, Smith, Samonds, Kohn, Bonds & Movshon, 2007
“Utah” array (96 channels)
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Coming soon:
neuropixel probe (1K electrodes)
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Neurons are noisy
0 0.2 0.4 0.6 0.8 1
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Time (s)
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Retinal responses to white noise stimuli
Shlens, Field, Gauthier, Greschner, Sher , Litke & Chichilnisky (2009).
(ON parasol cells )
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This is a great time to study computational / statistical neuroscience
• We are about to get incredible data.
• Computers are getting extremely fast.
• Advances in statistical/mathematical techniques are allowing us to gain a deep understanding of neural data and neural information processing capabilities
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For Next Time
• Install Python (instructions will be posted online)
• Review Linear Algebra basics
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Quick review of the basics• vectors• vector norm (“L2 norm”)• unit vector• inner product (“dot product”)• linear projection• orthogonality• linear dependence / independence• outer product• matrices• matrix multiplication (matrix-vector,
matrix-matrix)• basis, span, vector space
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