comp neuro
view markdownintroduction
overview
 does biology have a cutoff level (likecutoffs in computers below which fluctuations don’t matter)
 core principles underlying these two questions
 how do brains work?
 how do you build an intelligent machine?
 lacking: insight from neuro that can help build machine
 scales: cortex, column, neuron, synapses
 physics: theory and practice are much closer
 are there principles?
 “god is a hacker”  francis crick
 theorists are lazy  ramon y cajal
 things seemed like mush but became more clear  horace barlow
 principles of neural design book
 felleman & van essen 1991
 ascending layers (e.g. v1> v2): goes from superficial to deep layers
 descending layers (e.g. v2 > v1): deep layers to superficial
 solari & stoner 2011 “cognitive consilience”  layers thicknesses change in different parts of the brain
 motor cortex has much smaller input (layer 4), since it is mostly output
historical ai
 people: turing, von neumman, marvin minsky, mccarthy…
 ai: birth at 1956 conference
 vision: marvin minsky thought it would be a summer project
 lighthill debate 1973  was ai worth funding?
 intelligence tends to be developed by young children…
 cortex grew very rapidly
historical cybernetics/nns
 people: norbert weiner, mcculloch & pitts, rosenblatt
 neuro
 hubel & weisel (1962, 1965) simple, complex, hypercomplex cells
 neocognitron fukushima 1980
 david marr: theory, representation, implementation
types of models
 three types
 descriptive brain model  encode / decode external stimuli
 mechanistic brian cell / network model  simulate the behavior of a single neuron / network
 interpretive (or normative) brain model  why do brain circuits operate how they do
 receptive field  the things that make a neuron fire
 retina has oncenter / offsurround cells  stimulated by points
 then, V1 has differently shaped receptive fields

efficient coding hypothesis  learns different combinations (e.g. lines) that can efficiently represent images
 sparse coding (Olshausen and Field 1996)
 ICA (Bell and Sejnowski 1997)
 Predictive Coding (Rao and Ballard 1999)
 brain is trying to learn faithful and efficient representations of an animal’s natural environment
 same goes for auditory cortex
 brain is trying to learn faithful and efficient representations of an animal’s natural environment
biophysical models
modeling neurons
 nernst battery
 osmosis (for each ion)
 electrostatic forces (for each ion)
 together these yield Nernst potential $E = \frac{k_B T}{zq} ln \frac{[in]}{[out]}$
 T is temp
 q is ionic charge
 z is num charges
 part of voltage is accounted for by nernst battery $V_{rest}$
 yields $\tau \frac{dV}{dt} = V + V_\infty$ where $\tau=R_mC_m=r_mc_m$
 equivalently, $\tau_m \frac{dV}{dt} = ((VE_L)  g_s(t)(VE_s) r_m) + I_e R_m $
 together these yield Nernst potential $E = \frac{k_B T}{zq} ln \frac{[in]}{[out]}$
simplified model neurons
 integrateandfire neuron
 passive membrane (neuron charges)
 when V = V$_{thresh}$, a spike is fired
 then V = V$_{reset}$
 doesn’t have good modeling near threshold
 can include threshold by saying
 when V = V$_{max}$, a spike is fired
 then V = V$_{reset}$
 modeling multiple variables
 also model a K current
 can capture things like resonance
 theta neuron (Ermentrout and Kopell)
 often used for periodically firing neurons (it fires spontaneously)
a forest of dendrites
 cable theory  Kelvin
 voltage V is a function of both x and t
 separate into sections that don’t depend on x
 coupling conductances link the sections (based on area of compartments / branching)
 Rall model for dendrites
 if branches obey a certain branching ratio, can replace each pair of branches with a single cable segment with equivalent surface area and electrotonic length
 $d_1^{3/2} = d_{11}^{3/2} + d_{12}^{3/2}$
 if branches obey a certain branching ratio, can replace each pair of branches with a single cable segment with equivalent surface area and electrotonic length
 dendritic computation (London and Hausser 2005)
 hippocampus  when inputs arrive at soma, similiar shape no matter where they come in = synaptic scaling
 where inputs enter influences how they sum
 dendrites can generate spikes (usually calcium) / backpropagating spikes
 ex. Jeffress model  sound localized based on timing difference between ears
 ex. direction selectivity in retinal ganglion cells  if events arive at dendrite far > close, all get to soma at same time and add
circuitmodeling basics
 membrane has capacitance $C_m$
 force for diffusion, force for drift
 can write down diffeq for this, which yields an equilibrium
 $\tau = RC$
 bigger $\tau$ is slower
 to increase capacitance
 could have larger diameter
 $C_m \propto D$
 axial resistance $R_A \propto 1/D^2$ (not same as membrane lerk), thus bigger axons actually charge faster
action potentials
 channel/receptor types
 ionotropic: $G_{ion}$ = f(molecules outside)
 something binds and opens channel
 metabotropic: $G_{ion}$ = f(molecules inside)
 doesn’t directly open a channel: indirect
 others
 photoreceptor
 hair cell
 voltagegated (active  provide gain; might not require active ATP, other channels are all passive)
 ionotropic: $G_{ion}$ = f(molecules outside)
physics of computation
 based on carver mead: drift and diffusion are at the heart of everything
 different things realted by the Boltzmann distr. (ex. distr of air molecules vs elevation. Subject to gravity and diffusion upwards since they’re colliding)
 nernst potential
 currentvoltage relation of voltagegated channels
 currentvoltage relation of MOS transistor
 these things are all like transistor: energy barrier that must be overcome
 neuromorphic examples
 differential pair sigmoid yields sigmoidlike function
 can compute tanh function really simply to simulate
 silicon retina
 lateral inhibition exists (gap junctions in horizontal cells)
 mead & mahowald 1989  analog VLSI retina (centersurround receptive field is very low energy)
 differential pair sigmoid yields sigmoidlike function
 computation requires energy (otherwise signals would dissipate)
 von neumann architecture: CPU  bus (data / address)  Memory
 moore’s law ending (in terms of cost, clock speed, etc.)
 ex. errors increase as device size decreases (and can’t tolerate any errors)
 moore’s law ending (in terms of cost, clock speed, etc.)
 neuromorphic computing
 brain ~ 20 Watts
 exploit intrinsic transistor physics (need extremely small amounts of current)
 exploit electronics laws kirchoff’s law, ohm’s law
 new materials (ex. memristor  3d crossbar array)
 can’t just do biological mimicry  need to understand the principles
 von neumann architecture: CPU  bus (data / address)  Memory
spiking neurons
 passive membrane model was leaky integrator
 voltagegaed channels were more complicated
 can be though of as leaky integrateandfire neuron (LIF)
 this charges up and then fires a spike, has refractory period, then starts charging up again
 rate coding hypothesis  signal conveyed is the rate of spiking (bruno thinks this is usually too simple)
 spiking irregulariy is largely due to noise and doesn’t convey information
 some neurons (e.g. neurons in LIP) might actually just convey a rate
 linearnonlinearpoisson model (LNP)  sometimes called GLM (generalized linear model)
 based on observation that variance in firing rate $\propto$ mean firing rate
 plotting mean vs variance = 1 $\implies$ Poisson output
 these led people to model firing rates as Poisson $\frac {\lambda^n e^{\lambda}} {n!}$
 bruno doesn’t really believe the firing is random (just an effect of other things we can’t measure)
 ex. fly H1 neuron 1997
 constant stimulus looks very Poisson
 moving stimulus looks very Bernoulli
 based on observation that variance in firing rate $\propto$ mean firing rate
 spike timing hypothesis
 spiece timing can be very precise in response to timevarying signals (mainen & sejnowski 1995; bair & koch 1996)
 often see precise timing
 encoding: stimulus $\to$ spikes
 decoding: spikes $\to$ representation
 encoding + decoding are related through the joint distr. over simulus and repsonse (see Bialek spikes book)
 nonlinear encoding function can yield linear decoding
 able to directly decode spikes using a kernel to reproduce signal (seems to say you need spikes  rates would not be good enough)
 some reactions happen too fast to average spikes (e.g. 30 ms)
 estimating information rate: bits (usually better than snr  can calculate between them)  usually 23 bits/spike
neural coding
neural encoding
defining neural code
 extracellular
 fMRI
 averaged over space
 slow, requires seconds
 EEG
 noisy
 averaged, but faster
 multielectrode array
 record from several individual neurons at once
 calcium imaging
 cells have calcium indicator that fluoresce when calcium enters a cell
 fMRI
 intracellular  can use patch electrodes
 raster plot
 replay a movie many times and record from retinal ganglion cells during movie
 encoding: P(response  stimulus)
 tuning curve  neuron’s response (ex. firing rate) as a function of stimulus
 orientation / color selective cells are distributed in organized fashion
 some neurons fire to a concept, like “Pamela Anderson”
 retina (simple) > V1 (orientations) > V4 (combinations) > ?
 also massive feedback
 decoding: P(stimulus  response)
simple encoding
 want P(response  stimulus)
 response := firing rate r(t)
 stimulus := s

simple linear model
 r(t) = c * s(t)
 weighted linear model  takes into account previous states weighted by f
 temporal filtering
 r(t) = $f_0 \cdot s_0 + … + f_t \cdot s_t = \sum s_{tk} f_k$ where f weights stimulus over time
 could also make this an integral, yielding a convolution:
 r(t) = $\int_{\infty}^t d\tau : s(t\tau) f(\tau)$
 a linear system can be thought of as a system that searches for portions of the signal that resemble its filter f
 leaky integrator  sums its inputs with f decaying exponentially into the past
 flaws
 no negative firing rates
 no extremely high firing rates
 can add a nonlinear function g of the linear sum can fix this
 r(t) = $g(\int_{\infty}^t d\tau : s(t\tau) f(\tau))$
 spatial filtering
 r(x,y) = $\sum_{x’,y’} s_{xx’,yy’} f_{x’,y’}$ where f again is spatial weights that represent the spatial field
 could also write this as a convolution
 for a retinal center surround cell, f is positive for small $\Delta x$ and then negative for large $\Delta x$
 can be calculated as a narrow, large positive Gaussian + spread out negative Gaussian  can combine above to make spatiotemporal filtering  filtering = convolution = projection
 temporal filtering
feature selection
 P(responsestimulus) is very hard to get
 stimulus can be highdimensional (e.g. video)
 stimulus can take on many values
 need to keep track of stimulus over time
 solution: sample P(responses) to many stimuli to characterize what in input triggers responses
 find vector f that captures features that lead to spike
 dimensionality reduction  ex. discretize
 value at each time $t_i$ is new dimension
 commonly use Gaussian white noise
 time step sets cutoff of highest frequency present
 prior distribution  distribution of stimulus
 multivariate Gaussian  Gaussian in any dimension, or any linear combination of dimensions
 look at where spiketriggering points are and calculate spiketriggered average f of features that led to spike
 use this f as filter
 determining the nonlinear input/output function g
 replace stimulus in P(spikestimulus) with P(spike$s_1$), where s1 is our filtered stimulus
 use bayes rule $g=P(spikes_1)=\frac{P(s_1spike)P(spike)}{P(s_1)}$
 if $P(s_1spike) \approx P(s_1)$ then response doesn’t seem to have to do with stimulus
 replace stimulus in P(spikestimulus) with P(spike$s_1$), where s1 is our filtered stimulus
 incorporating many features $f_1,…,f_n$
 here, each $f_i$ is a vector of weights
 $r(t) = g(f_1\cdot s,f_2 \cdot s,…,f_n \cdot s)$
 could use PCA  discovers lowdimensional structure in highdimensional data
 each f represents a feature (maybe a curve over time) that fires the neuron
variability

hidden assumptions about timevarying firing rate and single spikes
 smooth function RFT can miss some stimuli

statistics of stimulus can effect P(spikestimulus)
 Gaussian white noise is nice because no way to filter it to get structure
 identifying good filter
 want $P(s_fspike)$ to differ from $P(s_f)$ where $s_f$ is calculated via the filter
 instead of PCA, could look for f that directly maximizes this difference (Sharpee & Bialek, 2004)
 KullbackLeibler divergence  calculates difference between 2 distributions
 $D_{KL}(P(s),Q(s)) = \int ds P(s) log_2 P(s) / Q(s)$
 maximizing KL divergence is equivalent to maximizing mutual info between spike and stimulus
 this is because we are looking for most informative feature
 this technique doesn’t require that our stimulus is white noise, so can use natural stimuli
 maximization isn’t guaranteed to uniquely converge
 modeling the noise
 need to go from r(t) > spike times
 divide time T into n bins with p = probability of firing per bin
 over some chunk T, number of spikes follows binomial distribution (n, p)
 mean = np
 var = np(1p)
 if n gets very large, binomial approximates Poisson
 $\lambda$ = spikes in some set time
 mean = $\lambda$
 var = $\lambda$
1. can test if distr is Poisson with Fano factor=mean/var=1
 interspike intervals have exponential distribution  if fires a lot, this can be bad assumption (due to refractory period)
 $\lambda$ = spikes in some set time
 generalized linear model adds explicit spikegeneration / postspike filter (Pillow et al. 2008)
 postspike filter models refractory period
 Paninski showed that using exponential nonlinearity allows this to be optimized
 could add in firing of other neurons
 timerescaling theorem  tests how well we have captured influences on spiking (Brown et al 2001)
 scaled ISIs ($t_{i1}t_i$) r(t) should be exponential
neural decoding
neural decoding and signal detection
 decoding: P(stimulus  response)  ex. you hear noise and want to tell what it is
 here r = response = firing rate
 monkey is trained to move eyes in same direction as dot pattern (Britten et al. 92)
 when dots all move in same direction (100% coherence), easy
 neuron recorded in MT  tracks dots
 count firing rate when monkey tracks in right direction
 count firing rate when monkey tracks in wrong direction
 as coherence decreases, these firing rates blur
 need to get P(+ or   r)
 can set a threshold on r by maximizing likelihood
 P(r+) and P(r) are likelihoods
 NeymanPearson lemma  likelihood ratio test is the most efficient statistic, in that is has the most power for a given size
 $\frac{p(r+)}{p(r)} > 1?$
 can set a threshold on r by maximizing likelihood
 when dots all move in same direction (100% coherence), easy
 accumulated evidence  we can accumulate evidence over time by multiplying these probabilities
 instead we take sum the logs, and compare to 0
 $\sum_i ln \frac{p(r_i+)}{p(r_i)} > 0?$
 once we hit some threshold for this sum, we can make a decision + or 
 experimental evidence (Kiani, Hanks, & Shadlen, Nat. Neurosci 2006)
 monkey is making decision about whether dots are moving left/right
 neuron firing rates increase over time, representing integrated evidence
 neuron always seems to stop at same firing rate
 priors  ex. tiger is much less likely then breeze
 scale P(+r) by prior P(+)
 neuroscience ex. photoreceptor cells P(noiser) is much larger than P(signalr)
 therefore threshold on r is high to minimize total mistakes
 cost of acting/not acting
 loss for predicting + when it is : $L_ \cdot P[+r]$
 loss for predicting  when it is +: $L_+ \cdot P[r]$
 cut your losses: answer + when average Loss$+$ < Loss$$
 i.e. $L_+ \cdot P[r]$ < $L_ \cdot P[+r]$
 rewriting with Baye’s rule yields new test:
 $\frac{p(r+)}{p(r)}> L_+ \cdot P[] / L_ \cdot P[+]$
 here the loss term replaces the 1 in the NeymanPearson lemma
population coding and bayesian estimation
 population vector  sums vectors for cells that point in different directions weighted by their firing rates
 ex. cricket cercal cells sense wind in different directions
 since neuron can’t have negative firing rate, need overcomplete basis so that can record wind in both directions along an axis
 can do the same thing for direction of arm movement in a neural prosthesis
 not general  some neurons aren’t tuned, are noisier
 not optimal  making use of all information in the stimulus/response distributions
 bayesian inference
 $p(sr) = \frac{p(rs)p(s)}{p( r)}$
 maximum likelihood: s* which maximizes p(rs)
 MAP = maximum $a:posteriori$: s* which mazimizes p(sr)
 simple continuous stimulus example
 setup
 s  orientation of an edge
 each neuron’s average firing rate=tuning curve $f_a(s)$ is Gaussian (in s)
 let $r_a$ be number of spikes for neuron a
 assume receptive fields of neurons span s: $\sum r_a (s)$ is const
 solving
 maximizing loglikelihood with respect to s  take derivative and set to 0
 soln $s^* = \frac{\sum r_a s_a / \sigma_a^2}{\sum r_a / \sigma_a^2}$
 if all the $\sigma$ are same, $s^* = \frac{\sum r_a s_a}{\sum r_a}$
 this is the population vector
 maximum a posteriori
 $ln : p(sr) = ln : P(rs) + ln : p(s) = ln : P(r )$
 $s^* = \frac{T \sum r_a s_a / \sigma^2a + s{prior} / \sigma^2{prior}}{T \sum r_a / \sigma^2_a + 1/\sigma^2{prior}}$
 this takes into account the prior
 narrow prior makes it matter more
 doesn’t incorporate correlations in the population
 maximizing loglikelihood with respect to s  take derivative and set to 0
 setup
stimulus reconstruction
 decoding s > $s^*$
 want an estimator $s_{Bayes}=s_B$ given some response r
 error function $L(s,s_{B})=(ss_{B})^2$
 minimize $\int ds : L(s,s_{B}) : p(sr)$ by taking derivative with respect to $s_B$
 $s_B = \int ds : p(sr) : s$  the conditional mean (spiketriggered average)
 add in spiketriggered average at each spike
 if spiketriggered average looks exponential, can never have smooth downwards stimulus
 could use 2 neurons (like in H1) and replay the second with negative sign
 LGN neurons can reconstruct a video, but with noise
 recreated 1 sec long movies  (Jack Gallant  Nishimoto et al. 2011, Current Biology)
 voxelbased encoding model samples ton of prior clips and predicts signal
 get p(rs)
 pick best p(rs) by comparing predicted signal to actual signal
 input is filtered to extract certain features
 filtered again to account for slow timescale of BOLD signal
 decoding
 maximize p(sr) by maximizing p(rs) p(s), and assume p(s) uniform
 30 signals that have highest match to predicted signal are averaged
 yields pretty good pictures
 voxelbased encoding model samples ton of prior clips and predicts signal
information theory
information and entropy
 surprise for seeing a spike h(p) = $log_2 (p)$
 entropy = average information
 code might not align spikes with what we are encoding
 how much of the variability in r is encoding s
 define q as en error
 $P(r_+s=+)=1q$
 $P(r_s=+)=q$
 similar for when s=
 total entropy: $H(R ) =  P(r_+) log P(r_+)  P(r_)log P(r_)$
 noise entropy: $H(RS=+) = q log q  (1q) log (1q)$
 mutual info I(S;R) = $H(R )  H(RS) $ = total entropy  average noise entropy
 = $D_{KL} (P(R,S), P(R )P(S))$
 define q as en error
 grandma’s famous mutual info recipe
 for each s
 P(Rs)  take one stimulus and repeat many times (or run for a long time)
 H(Rs)  noise entropy
 $H(RS)=\sum_s P(s) H(Rs)$
 $H(R ) $ calculated using $P(R ) = \sum_s P(s) P(Rs)$
 for each s
information in spike trains
 information in spike patterns
 divide pattern into time bins of 0 (no spike) and 1 (spike)
 binary words w with letter size $\Delta t$, length T (Reinagel & Reid 2000)
 can create histogram of each word
 can calculate entropy of word  look at distribution of words for just one stimulus
 distribution should be narrower  calculate $H_{noise}$  average over time with random stimuli and calculate entropy
 varied parameters of word: length of bin (dt) and length of word (T)
 there’s some limit to dt at which information stops increasing
 this represents temporal resolution at which jitter doesn’t stop response from identifying info about the stimulus
 corrections for finite sample size (Panzeri, Nemenman,…)
 information in single spikes  how much info does single spike tell us about stimulus
 don’t have to know encoding, mutual info doesn’t care
 calculate entropy for random stimulus  $p=\bar{r} \Delta t$ where $\bar{r}$ is the mean firing rate
 calculate entropy for specific stimulus
 let $P(r=1s) = r(t) \Delta t$
 let $P(r=0s) = 1  r(t) \Delta t$
 get r(t) by having simulus on for long time
 ergodicity  a time average is equivalent to averging over the s ensemble
 info per spike $I(r,s) = \frac{1}{T} \int_0^T dt \frac{r(t)}{\bar{r}} log \frac{r(t)}{\bar{r}}$
 timing precision reduces r(t)
 low mean spike rate > high info per spike
 ex. rat runs through place field and only fires when it’s in place field
 spikes can be sharper, more / less frequent
coding principles
 natural stimuli
 huge dynamic range  variations over many orders of magnitude (ex. brightness)
 power law scaling  structure at many scales (ex. far away things)
 efficient coding  in order to have maximum entropy output, a good encoder should match its outputs to the distribution of its inputs
 want to use each of our “symbols” (ex. different firing rates) equally often
 should assign equal areas of input stimulus PDF to each symbol
 adaptataion to stimulus statistics
 feature adaptation (Atick and Redlich)
 spatial filtering properties in retina / LGN change with varying light levels
 at low light levels surround becomes weaker
 feature adaptation (Atick and Redlich)
 coding sechemes
 redundancy reduction
 population code $P(R_1,R_2)$
 entropy $H(R_1,R_2) \leq H(R_1) + H(R_2)$  being independent would maximize entropy
 correlations can be good
 error correction and robust coding
 correlations can help discrimination
 retina neurons are redundant (Berry, Chichilnisky)
 more recently, sparse coding
 penalize weights of basis functions
 instead, we get localized features
 redundancy reduction
 we ignored the behavioral feedback loop
computing with networks
modeling connections between neurons
 model effects of synapse by using synaptic conductance $g_s$ with reversal potential $E_s$
 $g_s = g_{s,max} \cdot P_{rel} \cdot P_s$
 $P_{rel}$  probability of release given an input spike
 $P_s$  probability of postsynaptic channel opening = fraction of channels opened
 $g_s = g_{s,max} \cdot P_{rel} \cdot P_s$
 basic synapse model
 assume $P_{rel}=1$
 model $P_s$ with kinetic model
 open based on $\alpha_s$
 close based on $\beta_s$
 yields $\frac{dP_s}{dt} = \alpha_s (1P_s)  \beta_s P_s$
 3 synapse types
 AMPA  wellfit by exponential
 GAMA  fit by “alpha” function  has some delay
 NMDA  fit by “alpha” function  has some delay
 linear filter model of a synapse
 pick filter (ex. K(t) ~ exponential)
 $g_s = g_{s,max} \sum K(tt_i)$
 network of integrateandfire neurons
 if 2 neurons inhibit each other, get synchrony (fire at the same time
intro to network models
 comparing spiking models to firingrate models
 advantages
 spike timing
 spike correlations / synchrony between neurons
 disadvantages
 computationally expensive
 uses linear filter model of a synapse
 advantages
 developing a firingrate model
 replace spike train $\rho_1(t) \to u_1(t)$
 can’t make this replacement when there are correlations / synchrony?
 input current $I_s$: $\tau_s \frac{dI_s}{dt}=I_s + \mathbf{w} \cdot \mathbf{u}$
 works only if we let K be exponential
 output firing rate: $\tau_r \frac{d\nu}{dt} = \nu + F(I_s(t))$
 if synapses are fast ($\tau_s « \tau_r$)
 $\tau_r \frac{d\nu}{dt} = \nu + F(\mathbf{w} \cdot \mathbf{u}))$
 if synapses are slow ($\tau_r « \tau_s$)
 $\nu = F(I_s(t))$
 if static inputs (input doesn’t change)  this is like artificial neural network, where F is sigmoid
 $\nu_{\infty} = F(\mathbf{w} \cdot \mathbf{u})$
 could make these all vectors to extend to multiple output neurons
 replace spike train $\rho_1(t) \to u_1(t)$
 recurrent networks
 $\tau \frac{d\mathbf{v}}{dt} = \mathbf{v} + F(W\mathbf{u} + M \mathbf{v})$
 $\mathbf{v}$ is decay
 $W\mathbf{u}$ is input
 $M \mathbf{v}$ is feedback
 with constant input, $v_{\infty} = W \mathbf{u}$
 ex. edge detectors
 V1 neurons are basically computing derivatives
 $\tau \frac{d\mathbf{v}}{dt} = \mathbf{v} + F(W\mathbf{u} + M \mathbf{v})$
recurrent networks
 linear recurrent network: $\tau \frac{d\mathbf{v}}{dt} = \mathbf{v} + W\mathbf{u} + M \mathbf{v}$
 let $\mathbf{h} = W\mathbf{u}$
 want to investiage different M
 can solve eq for $\mathbf{v}$ using eigenvectors
 suppose M (NxN) is symmetric (connections are equal in both directions)
 $\to$ M has N orthogonal eigenvectors / eigenvalues
 let $e_i$ be the orthonormal eigenvectors
 output vector $\mathbf{v}(t) = \sum c_i (t) \mathbf{e_i}$
 allows us to get a closedform solution for $c_i(t)$
 eigenvalues determine network stability
 if any $\lambda_i > 1, \mathbf{v}(t)$ explodes $\implies$ network is unstable
 otherwise stable and converges to steadystate value
 $\mathbf{v}_\infty = \sum \frac{h\cdot e_i}{1\lambda_i} e_i$
 amplification of input projection by a factor of $\frac{1}{1\lambda_i}$
 if any $\lambda_i > 1, \mathbf{v}(t)$ explodes $\implies$ network is unstable
 suppose M (NxN) is symmetric (connections are equal in both directions)
 ex. each output neuron codes for an angle between 180 to 180
 define M as cosine function of relative angle
 excitation nearby, inhibition further away
 memory in linear recurrent networks
 suppose $\lambda_1=1$ and all other $\lambda_i < 1$
 then $\tau \frac{dc_1}{dt} = h \cdot e_1$  keeps memory of input
 ex. memory of eye position in medial vestibular nucleus (Seung et al. 2000)
 integrator neuron maintains persistent activity
 nonlinear recurrent networks: $\tau \frac{d\mathbf{v}}{dt} = \mathbf{v} + F(\mathbf{h}+ M \mathbf{v})$
 ex. rectification linearity F(x) = max(0,x)
 ensures that firing rates never go below
 can have eigenvalues > 1 but stable due to rectification
 can perform selective “attention”
 network performs “winnertakesall” input selection
 gain modulation  adding constant amount to input h multiplies the output
 also maintains memory
 ex. rectification linearity F(x) = max(0,x)
 nonsymmetric recurrent networks
 ex. excitatory and inhibitory neurons
 linear stability analysis  find fixed points and take partial derivatives
 use eigenvalues to determine dynamics of the nonlinear network near a fixed point
hopfield nets
 hopfield nets can store / retrieve memories
 fully connected (no input/output)  activations are what matter
 can memorize patterns  starting with noisy patterns can converge to these patterns
 marrpogio stereo algorithm
 hopfield threeway connections
 $E =  \sum_{i, j, k} T_{i, j, k} V_i V_j V_k$ (self connections set to 0)
 update to $V_i$ is now bilinear
 $E =  \sum_{i, j, k} T_{i, j, k} V_i V_j V_k$ (self connections set to 0)
 hopfield nets are all you need
 keys: each input has a key vector which “represents info about this input” (e.g. this is a noun)
 queries: each input has a query vector which “asks for other inputs that would be useful context” (e.g. what adjectives describe this word)
 in selfattention these queries also come from the input whereas in just regular attention they come from somewhere else (e.g. the output of a translation task)
 transformer finds similarity between each key with each query then takes softmax  this provides weights for each of the inputs, as context for the original input
 in transformer, these weights are used to weight the values but in hopfield nets we would take a weighted sum of the keys and feed it back as the input
 as we update becomes more skewed towards the things that match the most
learning
supervised learning
 net talk was major breakthrough (words > audio) Sejnowski & Rosenberg 1987
 people looked for worldcentric receptive fields (so neurons responded to things not relative to retina but relative to body) but didn’t find them
 however, they did find gain fields: (Zipser & Anderson, 1987)
 gain changes based on what retina is pointing at
 trained nn to go from pixels to headcentered coordinate frame
 yielded gain fields
 pouget et al. were able to find that this helped having 2 pop vectors: one for retina, one for eye, then add to account for it
 however, they did find gain fields: (Zipser & Anderson, 1987)
 support vector networks (vapnik et al.)  svms early inspired from nns
 dendritic nonlinearities (hausser & mel 03)
 example to think about neurons due this: $u = w_1 x_1 + w_2x_2 + w_{12}x_1x_2$
 $y=\sigma(u)$
 somestimes called sigmapi unit since it’s a sum of products
 exponential number of params…could be fixed w/ kernel trick?
 could also incorporate geometry constraint…
unsupervised learning
 born w/ extremely strong priors on weights in different areas
 barlow 1961, attneave 1954: efficient coding hypothesis = redundancy reduction hypothesis
 representation: compression / usefulness
 easier to store prior probabilities (because inputs are independent)
 relich 93: redundancy reduction for unsupervised learning (text ex. learns words from text w/out spaces)
hebbian learning and pca
 pca can also be thought of as a tool for decorrelation (in pc dimension, tends to be less correlated)
 hebbian learning = fire together, wire together: $\Delta w_{ab} \propto <a, b>$ note: $<a, b>$ is correlation of a and b (average over time)
 linear hebbian learning (perceptron with linear output)
 $\dot{w}_i \propto <y, x_i> \propto \sum_j w_j <x_j, x_i>$ since weights change relatively slowly
 synapse couldn’t do this, would grow too large
 oja’s rule (hebbian learning w/ weight decay so ws don’t get too big)
 points to correct direction
 sanger’s rule: for multiple neurons, fit residuals of other neurons
 competitive learning rule: winner take all
 population nonlinearity is a max
 gets stuck in local minima (basically kmeans)
 pca only really good when data is gaussian
 interesting problems are nongaussian, nonlinear, nonconvex
 pca: yields checkerboards that get increasingly complex (because images are smooth, can describe with smaller checkerboards)
 this is what jpeg does
 very similar to discrete cosine transform (DCT)
 very hard for neurons to get receptive fields that look like this
 retina: does whitening (yields centersurround receptive fields)
 easier to build
 gets more even outputs
 only has ~1.5 million fibers
synaptic plasticity, hebb’s rule, and statistical learning
 if 2 spikes keep firing at same time, get LTP  longterm potentiation
 if input fires, but not B then could get LTD  longterm depression
 Hebb rule $\tau_w \frac{d\mathbf{w}}{dt} = \mathbf{x}v$
 $\mathbf{x}$  input
 $v$  output
 translates to $\mathbf{w}_{i+1}=\mathbf{w}_i + \epsilon \cdot \mathbf{x}v$
 average effect of the rule is to change based on correlation matrix $\mathbf{x}^T\mathbf{x}$
 covariance rule: $\tau_w \frac{d\mathbf{w}}{dt} = \mathbf{x}(vE[v])$
 includes LTD as well as LTP
 Oja’s rule: $\tau_w \frac{d\mathbf{w}}{dt} = \mathbf{x}v \alpha v^2 \mathbf{w}$ where $\alpha>0$
 stability
 Hebb rule  derivative of w is always positive $\implies$ w grows without bound
 covariance rule  derivative of w is still always positive $\implies$ w grows without bound
 could add constraint that $w=1$ and normalize w after every step
 Oja’s rule  $w = 1/\sqrt{alpha}$, so stable
 solving Hebb rule $\tau_w \frac{d\mathbf{w}}{dt} = Q w$ where Q represents correlation matrix
 write w(t) in terms of eigenvectors of Q
 lets us solve for $\mathbf{w}(t)=\sum_i c_i(0)exp(\lambda_i t / \tau_w) \mathbf{e}_i$
 when t is large, largest eigenvalue dominates
 hebbian learning implements PCA
 hebbian learning learns w aligned with principal eigenvector of input correlation matrix
 this is same as PCA
intro to unsupervised learning

 most active neuron is the one whose w is closest to x
 competitive learning
 updating weights given a new input
 pick a cluster (corresponds to most active neuron)
 set weight vector for that cluster to running average of all inputs in that cluster
 $\Delta w = \epsilon \cdot (\mathbf{x}  \mathbf{w})$
 related to selforganizing maps = kohonen maps
 in selforganizing maps also update other neurons in the neighborhood of the winner
 update winner closer
 update neighbors to also be closer
 ex. V1 has orientation preference maps that do this
 updating weights given a new input
sparse coding and predictive coding
 eigenface  Turk and Pentland 1991
 eigenvectors of the input covariance matrix are good features
 can represent images using sum of eigenvectors (orthonormal basis)
 suppose you use only first M principal eigenvectors
 then there is some noise
 can use this for compression
 not good for local components of an image (e.g. parts of face, local edges)
 if you assume Gausian noise, maximizing likelihood = minimizing squared error
 generative model
 images X
 causes
 likelihood P(X=xC=c)
 Gaussian
 proportional to $exp(xGc)$
 want posterior P(CX)
 prior p(C )
 assume priors causes are independent
 want sparse distribution
 has heavy tail (superGaussian distribution)
 then P(C ) = $k \cdot \prod exp(g(C_i))$
 can implement sparse coding in a recurrent neural network
 Olshausen & Field, 1996  learns receptive fields in V1
 sparse coding is a special case of predicive coding
 there is usually a feedback connection for every feedforward connection (Rao & Ballard, 1999)
sparse, distributed coding

\[\underset {\mathbf{D}} \min \underset t \sum \underset {\mathbf{h^{(t)}}} \min \mathbf{x^{(t)}}  \mathbf{Dh^{(t)}}_2^2 + \lambda \mathbf{h^{(t)}}_1\]
 D is like autoencoder output weight matrix
 h is more complicated  requires solving inner minimization problem
 outer loop is not quite lasso  weights are not what is penalized
 barlow 1972: want to represent stimulus with minimum active neurons
 neurons farther in cortex are more silent
 v1 is highly overcomplete (dimensionality expansion)
 codes: dense > sparse, distributed $n \choose k$ > local (grandmother cells)
 energy argument  bruno doesn’t think it’s a big deal (could just not have a brain)
 PCA: autoencoder when you enforce weights to be orthonormal
 retina must output encoded inputs as spikes, lower dimension > uses whitening
 cortex
 sparse coding different kind of autencoder bottleneck (imposes sparsity)
 using bottlenecks in autoencoders forces you to find structure in data
 v1 simplecell receptive fields are localized, oriented, and bandpass
 higherorder image statistics
 phase alignment
 orientation (requires at least 3 points stats (like orientation)
 motion
 how to learn sparse repr?
 foldiak 1990 forming sparse reprs by local antihebbian learning
 driven by inputs and gets lateral inhibition and sum threshold
 neurons drift towards some firing rate naturally (adjust threshold naturally)
 use higherorder statistics
 projection pursuit (field 1994)  maximize nongaussianity of projections
 CLT says random projections should look gaussian
 gaborfilter response histogram over natural images look nonGaussian (sparse)  peaked at 0
 doesn’t work for graded signals
 projection pursuit (field 1994)  maximize nongaussianity of projections
 sparse coding for graded signals: olshausen & field, 1996
 $\underset{Image}{I(x, y)} = \sum_i a_i \phi_i (x, y) + \epsilon (x,y)$

loss function $\frac{1}{2} I  \phi a ^2 + \lambda \sum_i C(a_i)$  can think about difference between $L_1$ and $L_2$ as having preferred directions (for the same length of vector)  prefer directions which some zeros
 in terms of optimization, smooth near zero
 there is a network implementation
 $a_i$are calculated by solvin optimization for each image, $\phi$ is learned more slowly
 can you get $a_i$ closed form soln?
 wavelets invented in 1980s/1990s for sparsity + compression
 these tuning curves match those of real v1 neurons
 applications
 for time, have spatiotemporal basis where local wavelet moves
 sparse coding of natural sounds
 audition like a movie with two pixels (each ear sounds independent)
 converges to gamma tone functions, which is what auditory fibers look like
 sparse coding to neural recordings  finds spikes in neurons
 learns that different layers activate together, different frequencies come out
 found place cell bases for LFP in hippocampus
 nonnegative matrix factorization  like sparse coding but enforces nonnegative
 can explicitly enforce nonnegativity
 LCA algorithm lets us implement sparse coding in biologically plausible local manner
 explaining away  neural responses at the population should be decodable (shouldn’t be ambiguous)
 good project: understanding properties of sparse coding bases

SNR = $VAR(I) / VAR( I \phi A )$  can run on data after whitening
 graph is of power vs frequency (images go down as $1/f$), need to weighten with f
 don’t whiten highest frequencies (because really just noise)
 need to do this softly  roughly what the retina does
 as a result higher spatial frequency activations have less variance
 whitening effect on sparse coding
 if you don’t whiten, have some directions that have much more variance
 projects
 applying to different types of data (ex. auditory)
 adding more bases as time goes on
 combining convolution w/ sparse coding?
 people didn’t see sparsity for a while because they were using very specific stimuli and specific neurons
 now people with less biased sampling are finding more sparsity
 in cortex anasthesia tends to lower firing rates, but opposite in hippocampus
selforganizing maps
 homunculus  3d map corresponds to map in cortex (sensory + motor)
 visual cortex
 visual cortex mostly devoted to center
 different neurons in same regions sensitive to different orientations (changing smoothly)
 orientation constant along column
 orientation maps not found in mice (but in cats, monkeys)
 direction selective cells as well
 maps are plastic  cortex devoted to particular tasks expands (not passive, needs to be active)
 kids therapy with tonetracking video games at higher and higher frequencies
ml analogies
Brain theories
 Computational Theory of Mind
 Classical associationism
 Connectionism Situated cognition Memoryprediction framework Fractal Theory: https://www.youtube.com/watch?v=axaH4HFzA24 Brain sheets are made of cortical columns (about .3mm diameter, 1000 neurons / column) Have ~6 layers
brain as a computer
 Brain as a Computer – Analog VLSI and Neural Systems by Mead (VLSI – very large scale integration) Brain Computer Analogy Process info Signals represented by potential Signals are amplified = gain Power supply Knowledge is not stored in knowledge of the parts, but in their connections Based on electrically charged entities interacting with energy barriers http://en.wikipedia.org/wiki/Computational_theory_of_mind http://scienceblogs.com/developingintelligence/2007/03/27/whythebrainisnotlikeaco/ Brain’ storage capacity is about 2.5 petabytes (Scientific American, 2005) Electronics Voltage can be thought of as water in a reservoir at a height It can flow down, but the water will never reach above the initial voltage A capacitor is like a tank that collects the water under the reservoir The capacitance is the crosssectional area of the tank Capacitance – electrical charge required to raise the potential by 1 volt Conductance = 1/ resistance = mho, siemens We could also say the word is a computer with individuals being the processors – with all the wasted thoughts we have – the solution is probably to identify global problems and channel people’s focus towards working on them Brain chip: http://www.research.ibm.com/articles/brainchip.shtml Differences: What Can AI Get from Neuroscience? Brains are not digital Brains don’t have a CPU Memories are not separable from processing Asynchronous and continuous Details of brain substrate matter Feedback and Circular Causality Asking questions Brains has lots of sensors Lots of cellular diversity NI uses lots of parallelism Delays are part of the computation
Brain v. Deep Learning
 http://timdettmers.com/
 problems with brain simulations:
 Not possible to test specific scientific hypotheses (compare this to the large hadron collider project with its perfectly defined hypotheses)
 Does not simulate real brain processing (no firing connections, no biological interactions)
 Does not give any insight into the functionality of brain processing (the meaning of the simulated activity is not assessed)
 Neuron information processing parts
 Dendritic spikes are like first layer of conv net
 Neurons will typically have a genome that is different from the original genome that you were assigned to at birth. Neurons may have additional or fewer chromosomes and have sequences of information removed or added from certain chromosomes.
 http://timdettmers.com/2015/03/26/convolutiondeeplearning/
 The adult brain has 86 billion neurons, about 10 trillion synapse, and about 300 billion dendrites (treelike structures with synapses on them
probabilistic models + inference
Wiener filter
has Gaussian prior + likelihood
gaussians are everywhere because of CLT, max entropy (subject to power constraint)
 for gaussian function, $d/dx f(x) = x f(x)$
boltzmann machines
 hinton & sejnowski 1983
 starts with a hopfield net (states $s_i$ weights $\lambda_{ij}$) where states are $\pm 1$
 define energy function $E(\mathbf{s}) =  \sum_{ij} \lambda_{ij} s_i s_j$
 assume Boltzmann distr $P(s) = \frac{1}{z} \exp ( \beta \phi(s))$
 learning rule is basically expectation over data  expectation over model
 could use wakesleep algorithm
 during day, calculate expectation over data via Hebbian learning (in Hopfield net this would store minima)
 during night, would run antihebbian by doing random walk over network (in Hopfield net this would remove spurious local minima)
 learn via gibbs sampling (prob for one node conditioned on others is sigmoid)
 can add hidden units to allow for learning higherorder interactions (not just pairwise)
 restricted boltzmann machine: no connections between “visible” units and no connections between “hidden units”
 computationally easier (sampling is independent) but less rich
 stacked rbm: hinton & salakhutdinov (hinton argues this is first paper to launch deep learning)
 don’t train layers jointly
 learn weights with rbms as encoder
 then decoder is just transpose of weights
 finally, run finetuning on autoencoder
 able to separate units in hidden layer
 cool  didn’t actually need decoder
 in rbm
 when measuring true distr, don’t see hidden vals
 instead observe visible units and conditionally sample over hidden units

$P(h v) = \prod_i P(h_i v)$ ~ easy to sample from

when measuring sampled distr., just sample $P(h v)$ then sample $P(v h)$
 when measuring true distr, don’t see hidden vals
 ising model  only visible units
 basically just replicates pairwise statistics (kind of like pca)
 pairwise statistics basically say “when I’m on, are my neighbors on?”
 need 3point statistics to learn a line
 basically just replicates pairwise statistics (kind of like pca)
 generating textures
 learn the distribution of pixels in 3x3 patches
 then maximize this distribution  can yield textures
 reducing the dimensionality of data with neural networks