Latent models of stepping and ramping: an update on (the debate over) single-trial dynamics in LIP Jonathan Pillow Princeton Neuroscience Institute, Princeton University Comp. Neurosci. Workshop: Computation, Cognition and the Brain Rutgers University May 30, 2018
“random dots” decision-making task fixed duration or RT fixate targets motion saccade RF
“random dots” decision-making task fixed duration or RT fixate targets motion saccade RF Q : what are the latent dynamics of spike trains in LIP during sensory evidence accumulation?
What do we mean by latent dynamics? “direct” sensory encoding model sensory s stimulus spike spike r response response e.g., “LN” cascade: nonlinearity stimulus filter
What do we mean by latent dynamics? latent variable model sensory s stimulus noisy x latent variable spike rate spike spike r response response
What do we mean by latent dynamics? latent variable model sensory s stimulus noisy x latent variable spike rate spike spike r response response
What do we mean by latent dynamics? latent variable model sensory s stimulus noisy x latent variable spike rate spike spike r response response
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Classic account: drift-diffusion model (DDM) • neuron integrates motion “evidence” Accumulated evidence [logLR] time [Gold & Shadlen 2007]
Classic account: drift-diffusion model (DDM) • neuron integrates motion “evidence” LIP response ... IN [spikes/sec] Roitman & Shadlen, 2002 Gold & Shadlen, 2002 = Huk & Shadlen, 2005 Yang & Shadlen, 2007 Accumulated Churchland & Shadlen, 2008 evidence OUT ... [logLR] time [sec]
normative modeling approach: what LIP neurons ought to do, based on a theory of “optimal” decision-making examples : • log-probability (Shadlen & Newsome 1996) • expected utility (Platt & Glimcher 1999) • posterior probability (Beck et al 2008) • change in the RL value function (Seo, Barraclough, & Lee 2009) descriptive modeling approach: • find most accurate statistical description of spike responses • agnostic about function (“l et the data speak for themselves”) example : • variance of conditional expectation (varCE) - (Churchland et al 2011)
classical analysis of LIP responses motion on IN motion sp/s OUT motion sp/s [Shadlen & Newsome, 2001]
but what are the dynamics on single trials? motion on IN motion spike train • averaging obscures sp/s single-trial dynamics noisy “ramping” our goal: infer latent dynamics from spike trains discrete stepping
Chapter 1: Formalizing the models [Latimer et al 2015]
ramping (“diffusion-to-bound”) model noise variance bound height slope initial spike rate 200 400 600 time after motion onset (ms) Poisson spikes: latent state: spiking:
ramping (“diffusion-to-bound”) model noise variance bound height initial spike rate 200 400 600 time after motion onset (ms) Poisson spikes: latent state: spiking:
ramping (“diffusion-to-bound”) model noise variance bound height initial spike rate 200 400 600 time after motion onset (ms) Poisson spikes: latent state: , , 8 model parameters: spiking:
stepping (“discrete switching”) model • semi-Markov model probability of “in” step step time distribution 200 400 600 time after motion onset (ms) spikes: spiking: step times latent
stepping (“discrete switching”) model • semi-Markov model probability of “in” step step time distribution 200 400 600 time after motion onset (ms) spikes: spiking: step times latent
stepping (“discrete switching”) model • semi-Markov model probability of “in” step step time distribution 200 400 600 time after motion onset (ms) spikes: spiking: step times 14 parameters: latent
Fitting: difficult for both models coherence parameters likelihood: 60 bound spike rate (Hz) requires summing over all 30 possible latent paths specified by model • Bayesian inference: use MCMC to compute this integral [Latimer et al 2015]
Which model is better? Q: how to compare models with different numbers of parameters? naively, more parameters ⟹ more flexibility to fit the data • but with too many parameters, we will overfit (fit noise in data)!
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