nips nips2007 nips2007-140 knowledge-graph by maker-knowledge-mining
Source: pdf
Author: Jonathan W. Pillow, Peter E. Latham
Abstract: Point process encoding models provide powerful statistical methods for understanding the responses of neurons to sensory stimuli. Although these models have been successfully applied to neurons in the early sensory pathway, they have fared less well capturing the response properties of neurons in deeper brain areas, owing in part to the fact that they do not take into account multiple stages of processing. Here we introduce a new twist on the point-process modeling approach: we include unobserved as well as observed spiking neurons in a joint encoding model. The resulting model exhibits richer dynamics and more highly nonlinear response properties, making it more powerful and more flexible for fitting neural data. More importantly, it allows us to estimate connectivity patterns among neurons (both observed and unobserved), and may provide insight into how networks process sensory input. We formulate the estimation procedure using variational EM and the wake-sleep algorithm, and illustrate the model’s performance using a simulated example network consisting of two coupled neurons.
Reference: text
sentIndex sentText sentNum sentScore
1 Neural characterization in partially observed populations of spiking neurons Jonathan W. [sent-1, score-0.548]
2 uk Abstract Point process encoding models provide powerful statistical methods for understanding the responses of neurons to sensory stimuli. [sent-8, score-0.513]
3 Although these models have been successfully applied to neurons in the early sensory pathway, they have fared less well capturing the response properties of neurons in deeper brain areas, owing in part to the fact that they do not take into account multiple stages of processing. [sent-9, score-0.734]
4 Here we introduce a new twist on the point-process modeling approach: we include unobserved as well as observed spiking neurons in a joint encoding model. [sent-10, score-0.663]
5 The resulting model exhibits richer dynamics and more highly nonlinear response properties, making it more powerful and more flexible for fitting neural data. [sent-11, score-0.228]
6 More importantly, it allows us to estimate connectivity patterns among neurons (both observed and unobserved), and may provide insight into how networks process sensory input. [sent-12, score-0.45]
7 We formulate the estimation procedure using variational EM and the wake-sleep algorithm, and illustrate the model’s performance using a simulated example network consisting of two coupled neurons. [sent-13, score-0.238]
8 1 Introduction A central goal of computational neuroscience is to understand how the brain transforms sensory input into spike trains, and considerable effort has focused on the development of statistical models that can describe this transformation. [sent-14, score-0.4]
9 One of the most successful of these is the linear-nonlinearPoisson (LNP) cascade model, which describes a cell’s response in terms of a linear filter (or receptive field), an output nonlinearity, and an instantaneous spiking point process [1–5]. [sent-15, score-0.403]
10 Recent efforts have generalized this model to incorporate spike-history and multi-neuronal dependencies, which greatly enhances the model’s flexibility, allowing it to capture non-Poisson spiking statistics and joint responses of an entire population of neurons [6–10]. [sent-16, score-0.644]
11 Point process models accurately describe the spiking responses of neurons in the early visual pathway to light, and of cortical neurons to injected currents. [sent-17, score-0.816]
12 Such failings are in some ways not surprising: the cascade model’s stimulus sensitivity is described with a single linear filter, whereas responses in the brain reflect multiple stages of nonlinear processing, adaptation on multiple timescales, and recurrent feedback from higher-level areas. [sent-19, score-0.493]
13 Here we extend the point-process modeling framework to incorporate a set of unobserved or “hidden” neurons, whose spike trains are unknown and treated as hidden or latent variables. [sent-21, score-0.569]
14 The unobserved neurons respond to the stimulus and to synaptic inputs from other neurons, and their spiking 1 activity can in turn affect the responses of the observed neurons. [sent-22, score-1.013]
15 Although this expanded model offers considerably greater flexibility in describing an observed set of neural responses, it is more difficult to fit to data. [sent-25, score-0.208]
16 Computing the likelihood of an observed set of spike trains requires integrating out the probability distribution over hidden activity, and we need sophisticated algorithms to find the maximum likelihood estimate of model parameters. [sent-26, score-0.733]
17 Both algorithms make use of a novel proposal density to capture the dependence of hidden spikes on the observed spike trains, which allows for fast sampling of hidden neurons’ activity. [sent-28, score-0.945]
18 We show that a single-cell model used to characterize the observed neuron performs poorly, while a coupled two-cell model estimated using the wake-sleep algorithm performs much more accurately. [sent-30, score-0.447]
19 2 Multi-neuronal point-process encoding model We begin with a description of the encoding model, which generalizes the LNP model to incorporate non-Poisson spiking and coupling between neurons. [sent-31, score-0.526]
20 In this section we do not distinguish between observed and unobserved spikes, but will do so in the next. [sent-34, score-0.19]
21 Let xt denote the stimulus at time t, and y t and zt denote the number of spikes elicited by two neurons at t, where t ∈ [0, T ] is an index over time. [sent-35, score-0.917]
22 Note that x t is a vector containing all elements of the stimulus that are causally related to the (scalar) responses y t and zt at time t. [sent-36, score-0.607]
23 Typically ∆ is sufficiently small that we observe only zero or one spike in every bin: y t , zt ∈ {0, 1}. [sent-43, score-0.477]
24 The conditional intensity (or instantaneous spike rate) of each cell depends on both the stimulus and the recent spiking history via a bank of linear filters. [sent-44, score-0.994]
25 Let y [t−τ,t) and z[t−τ,t) denote the (vector) spike train histories at time t. [sent-45, score-0.297]
26 The nonlinear function, f , maps the input to the instantaneous spike rate of each cell. [sent-53, score-0.306]
27 2 = stimulus filter x > point-process model ky stochastic nonlinearity spiking + ky neuron y kz neuron z stimulus y xt hyy hyy coupling filters exp(·) f post-spike filter x equivalent model diagram neuron y spikes hzy x1 x2 x3 x4 . [sent-55, score-2.562]
28 hzy neuron z spikes + causal structure + y[t-τ,t) yt hyz ? [sent-64, score-0.913]
29 time z[t-τ,t) z time hzz Figure 1: Schematic of generalized linear point-process (glpp) encoding model. [sent-65, score-0.319]
30 a, Diagram of model parameters for a pair of coupled neurons. [sent-66, score-0.157]
31 For each cell, the parameters consist of a stimulus filter (e. [sent-67, score-0.272]
32 , ky ), a spike-train history filter (hyy ), and a filter capturing coupling from the spike train history of the other cell (hzy ). [sent-69, score-0.86]
33 The filter outputs are summed, pass through an exponential nonlinearity, and drive spiking via an instantaneous point process. [sent-70, score-0.238]
34 b, Equivalent diagram showing just the parameters of the neuron y, as used for drawing a sample yt . [sent-71, score-0.393]
35 Gray boxes highlight the stimulus vector xt and spike train history vectors that form the input to the model on this time step. [sent-72, score-0.71]
36 c, Simplified graphical model of the glpp causal structure, which allows us to visualize how the likelihood factorizes. [sent-73, score-0.303]
37 Red arrows highlight the dependency structure for a single time bin of the response y3 . [sent-76, score-0.241]
38 Equation 1 is equivalent to f applied to a linear convolution of the stimulus and spike trains with their respective filters; a schematic is shown in figure 1. [sent-78, score-0.68]
39 The probability of observing y t spikes in a bin of size ∆ is given by a Poisson distribution with rate parameter λyt ∆, (λyt ∆)yt −λyt ∆ e , yt ! [sent-79, score-0.387]
40 This factorization is possible because λyt and λzt depend only on the process history up to time t, making y t and zt conditionally independent given the stimulus and spike histories up to t (see Fig. [sent-82, score-0.829]
41 If the response at time t were to depend on both the past and future response, we would have a causal loop , preventing factorization and making both sampling and likelihood evaluation very difficult. [sent-84, score-0.229]
42 We can write the log-likelihood simply as log P (Y, Z|X, θ) = (yt log λyt + zt log λzt − ∆λyt − ∆λzt ) + c, t where c is a constant that does not depend on θ. [sent-87, score-0.337]
43 3 (4) 3 Generalized Expectation-Maximization and Wake-Sleep Maximizing log P (Y, Z|X, θ) is straightforward if both Y and Z are observed, but here we are interested in the case where Y is observed and Z is “hidden”. [sent-88, score-0.155]
44 The log-likelihood of the observed data is given by L(θ) ≡ log P (Y |θ) = log P (Y, Z|θ), (5) Z where we have dropped X to simplify notation (all probabilities can henceforth be taken to also depend on X). [sent-90, score-0.195]
45 The variational E-step involves minimizing D KL (Q, P ) with respect to φ, which remains positive if Q does not approximate P exactly; the variational M-step is unchanged from the standard algorithm. [sent-109, score-0.188]
46 In the “wake” step (identical to the M-step), we fit the true model parameters θ by maximizing (an approximation to) the log-probability of the observed data Y . [sent-114, score-0.251]
47 We can therefore think of the wake phase as learning a model of the data (parametrized by θ), and the sleep phase as learning a consistent internal description of that model (parametrized by φ). [sent-116, score-0.4]
48 In the next section we show that considerations of the spike generation process can provide us with a good choice for Q. [sent-119, score-0.26]
49 a, Conditional model schematic, which allows zt to depend on the observed response both before and after t. [sent-121, score-0.483]
50 b, Graphical model showing causal structure of the acausal model, with arrows indicating dependency. [sent-122, score-0.314]
51 The observed spike responses (gray circles) are no longer dependent variables, but regarded as fixed, external data, which is necessary for computing Q(zt |Y, φ). [sent-123, score-0.519]
52 Red arrows illustrate the dependency structure for a single bin of the hidden response, z3 . [sent-124, score-0.251]
53 , the hidden neuron spikes at time t), it is highly likely that y t+1 = 1 (i. [sent-127, score-0.469]
54 Consequently, under the true P (Z|Y, θ), which is the probability over Z in all time bins given Y in all time bins, if y t+1 = 1 there is a high probability that zt = 1. [sent-130, score-0.291]
55 In other words, z t exhibits an acausal dependence on y t+1 . [sent-131, score-0.238]
56 But this acausal dependence is not captured in Equation 3, which expresses the probability over z t as depending only on past events at time t, ignoring the future event y t+1 = 1. [sent-132, score-0.237]
57 Thus we have ˜ ˜ ˜ ˜ λzt = exp(kz · xt + hzz · z[t−τ,t) + hzy · y[t−τ,t+τ ) ). [sent-134, score-0.52]
58 (10) ˜ ˜ ˜ ˜ As above, kz , hzz and hzy are linear filters; the important difference is that hzy · y[t−τ,t+τ ) is a sum over past and future time: from t − τ to t + τ − ∆. [sent-135, score-0.91]
59 For this model, the parameters are ˜ ˜ ˜ φ = (kz , hzz , hzy ). [sent-136, score-0.488]
60 We now have a straightforward way to implement the wake-sleep algorithm, using samples from Q to perform the wake phase (estimating θ), and samples from P (Y, Z|θ) to perform the sleep phase (estimating φ). [sent-138, score-0.386]
61 The algorithm works as follows: • Wake: Draw samples {Zi } ∼ Q(Z|Y, φ), where Y are the observed spike trains and φ is the current set of parameters for the acausal point-process model Q. [sent-139, score-0.736]
62 5 • Sleep: Draw samples {Yj , Zj } ∼ P (Y, Z|θ), the true encoding distribution with current parameters θ. [sent-143, score-0.178]
63 To perform a variational E-step, we draw samples (as above) from Q and use them to evaluate both the KL divergence D KL Q(Z|Y, φ)||P (Z|Y, θ) and its gradient with respect to φ. [sent-150, score-0.163]
64 Such samples can be used to evaluate the true log-likelihood, for comparison with the variational lower bound, and for noisy gradient ascent of the likelihood to examine how closely these approximate methods converge to the true ML estimate. [sent-154, score-0.238]
65 For fully observed data, such samples also provide a useful means for measuring how much the entropy of one neuron’s response is reduced by knowing the responses of its neighbors. [sent-155, score-0.403]
66 5 Simulations: a two-neuron example To verify the method, we applied it to a pair of neurons (as depicted in fig. [sent-156, score-0.258]
67 1), simulated using a stimulus consisting of a long presentation of white noise. [sent-157, score-0.309]
68 We denoted one of the neurons ”observed” and the other ”hidden”. [sent-158, score-0.228]
69 The cells have similarly-shaped biphasic stimulus filters with opposite sign, like those commonly observed in ON and OFF retinal ganglion cells. [sent-161, score-0.507]
70 We assume that the ON-like cell is observed, while the OFF-like cell is hidden. [sent-162, score-0.362]
71 The hidden cell has a strong positive coupling filter h zy onto the observed cell, which allows spiking activity in the hidden cell to excite the observed cell (despite the fact that the two cells receive opposite-sign stimulus input). [sent-164, score-1.691]
72 For simplicity, we assume no coupling from the observed to the hidden cell 2 . [sent-165, score-0.551]
73 3b shows rasters of the two cells’ responses to a repeated presentations of a 1s Gaussian whitenoise stimulus with a framerate of 100Hz. [sent-170, score-0.43]
74 Note that the temporal structure of the observed cell’s response is strongly correlated with that of the hidden cell due to the strong coupling from hidden to observed (and the fact that the hidden cell receives slightly stronger stimulus drive). [sent-171, score-1.49]
75 Our first task is to examine whether a standard, single-cell glpp model can capture the mapping from stimuli to spike responses. [sent-172, score-0.55]
76 3c shows the parameters obtained from such a fit to the observed data, using 10s of the response to a non-repeating white noise stimulus (1000 samples, 251 spikes). [sent-174, score-0.53]
77 Note that the estimated stimulus filter (red) has much lower amplitude than the stimulus filter of the true model (gray). [sent-175, score-0.571]
78 3d shows the parameters obtained for an observed and a hidden neuron, estimated using wake-sleep as described in section 4. [sent-177, score-0.287]
79 3e-f shows a comparison of the performance of the two models, indicating that the coupled model estimated with wake-sleep does a much better job of capturing the temporal structure of the observed neuron’s response (accounting for 60% vs. [sent-179, score-0.41]
80 15% of 2 Although the stimulus and spike-history filters bear a rough similarity to those observed in retinal ganglion cells, the coupling used here is unlike coupling filters observed (to our knowledge) between ON and OFF cells in retinal data; it is assumed purely for demonstration purposes. [sent-180, score-0.886]
81 2 0 -10 -20 hzy 0 -10 -20 0 0 hyy hzz @ coupled-model estimate using variational EM 0. [sent-182, score-0.695]
82 05 A single-cell model estimate GWN stimulus B hidden observed raster raster raster comparison coupled singletrue model cell observed observed 2 0 -2 ky > ? [sent-183, score-1.451]
83 true parameters 2 0 -2 hidden = psth comparison true rate (Hz) 100 0 0. [sent-184, score-0.293]
84 The top row shows the filters determining the input to the observed cell, while the bottom row shows those influencing the hidden cell. [sent-188, score-0.261]
85 b, Raster of spike responses of observed and hidden cells to a repeated, 1s Gaussian white noise stimulus (top). [sent-189, score-1.015]
86 c, Parameter estimates for a single-cell glpp model fit to the observed cell’s response, using just the stimulus and observed data (estimates in red; true observedcell filters in gray). [sent-190, score-0.717]
87 d, Parameters obtained using wake-sleep to estimate a coupled glpp model, again using only the stimulus and observed spike times. [sent-191, score-0.868]
88 e, Response raster of true observed cell (obtained by simulating the true two-cell model), estimated single-cell model and estimated coupled model. [sent-192, score-0.574]
89 f, Peri-stimulus time histogram (PSTH) of the above rasters showing that the coupled model gives much higher accuracy predicting the true response. [sent-193, score-0.204]
90 The single-cell model, by contrast, exhibits much worse performance, which is unsurprising given that the standard glpp encoding model can capture only quasi-linear stimulus dependencies. [sent-195, score-0.594]
91 6 Discussion Although most statistical models of spike trains posit a direct pathway from sensory stimuli to neuronal responses, neurons are in fact embedded in highly recurrent networks that exhibit dynamics on a broad range of time-scales. [sent-196, score-0.777]
92 To take into account the fact that neural responses are driven by both stimuli and network activity, and to understand the role of network interactions, we proposed a model incorporating both hidden and observed spikes. [sent-197, score-0.645]
93 We regard the observed spike responses as those recorded during a typical experiment, while the responses of unobserved neurons are modeled as latent variables (unrecorded, but exerting influence on the observed responses). [sent-198, score-1.081]
94 The resulting model is tractable, as the latent variables can be integrated out using approximate sampling methods, and optimization using variational EM or wake-sleep provides an approximate maximum likelihood estimate of the model parameters. [sent-199, score-0.225]
95 As shown by a simple example, certain settings of model parameters necessitate the incorporation unobserved spikes, as the standard single-stage encoding model does not accurately describe the data. [sent-200, score-0.273]
96 The model offers a promising tool for analyzing network structure and networkbased computations carried out in higher sensory areas, particularly in the context where data are only available from a restricted set of neurons recorded within a larger population. [sent-202, score-0.369]
97 A point process framework for relating neural spiking activity to spiking history, neural ensemble and extrinsic covariate effects. [sent-257, score-0.464]
98 Maximum likelihood estimation of cascade point-process neural encoding models. [sent-262, score-0.216]
99 Correlations and coding with multi-neuronal spike trains in primate retina. [sent-276, score-0.348]
100 Analyzing functional connectivity using a network likelihood model of ensemble neural spiking activity. [sent-291, score-0.411]
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