136 lines
4.7 KiB
TeX
136 lines
4.7 KiB
TeX
\documentclass[addpoints,11pt]{exam}
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\usepackage{url}
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\usepackage{color}
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\usepackage{hyperref}
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\pagestyle{headandfoot}
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\runningheadrule
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\firstpageheadrule
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\firstpageheader{Scientific Computing}{Project Assignment}{11/05/2014
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-- 11/06/2014}
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%\runningheader{Homework 01}{Page \thepage\ of \numpages}{23. October 2014}
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\firstpagefooter{}{}{{\bf Supervisor:} Jan Benda}
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\runningfooter{}{}{}
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\pointsinmargin
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\bracketedpoints
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%\printanswers
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%\shadedsolutions
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%%%%% listings %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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\usepackage{listings}
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\lstset{
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basicstyle=\ttfamily,
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numbers=left,
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showstringspaces=false,
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language=Matlab,
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breaklines=true,
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breakautoindent=true,
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columns=flexible,
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frame=single,
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% captionpos=t,
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xleftmargin=2em,
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xrightmargin=1em,
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% aboveskip=11pt,
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%title=\lstname,
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% title={\protect\filename@parse{\lstname}\protect\filename@base.\protect\filename@ext}
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}
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\begin{document}
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%%%%%%%%%%%%%%%%%%%%% Submission instructions %%%%%%%%%%%%%%%%%%%%%%%%%
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\sffamily
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% \begin{flushright}
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% \gradetable[h][questions]
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% \end{flushright}
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\begin{center}
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\input{../disclaimer.tex}
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\end{center}
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%%%%%%%%%%%%%% Questions %%%%%%%%%%%%%%%%%%%%%%%%%
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\begin{questions}
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\question You are recording the activity of a neuron in response to
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two different stimuli $I_1$ and $I_2$ (think of them, for example,
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of two sound waves with different intensities $I_1$ and
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$I_2$). Within an observation time of duration $W$ the neuron
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responds stochastically with $n_i$ spikes.
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How well can an upstream neuron discriminate the two stimuli based
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on the spike counts $n_i$? How does this depend on the slope of the
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tuning curve of the neural responses? How is this related to the
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fano factor (the ratio between the variance and the mean of the
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spike counts)?
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The neuron is implemented in the file \texttt{lifboltzmanspikes.m}.
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Call it with the following parameters:
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\begin{lstlisting}
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trials = 10;
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tmax = 50.0;
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Dnoise = 1.0;
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imax = 25.0;
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ithresh = 10.0;
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slope=0.2;
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input = 10.0;
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spikes = lifboltzmanspikes( trials, input, tmax, Dnoise, imax, ithresh, slope );
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\end{lstlisting}
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The returned \texttt{spikes} is a cell array with \texttt{trials} elements, each being a vector
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of spike times (in seconds) computed for a duration of \texttt{tmax} seconds.
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The input is set via the \texttt{input} variable.
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For the two inputs use $I_1=10$ and $I_2=I_1 + 1$.
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\begin{parts}
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\part
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First, show two raster plots for the responses to the two differrent stimuli.
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\part Measure the tuning curve of the neuron with respect to the input. That is,
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compute the mean firing rate as a function of the input
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strength. Find an appropriate range of input values. Do this for
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different values of the \texttt{slope} parameter (values between
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0.1 and 2.0).
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\part Generate histograms of the spike counts within $W=200$\,ms of the
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responses to the two differrent stimuli $I_1$ and $I_2$. How do they depend on the slope
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of the tuning curve of the neuron?
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\part Think about a measure based on the spike count histograms that quantifies how well
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the two stimuli can be distinguished based on the spike
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counts. Plot the dependence of this measure as a function of the observation time $W$.
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For which slopes can the two stimuli be well discriminated?
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\underline{Hint:} A possible readout is to set a threshold $n_{thresh}$ for
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the observed spike count. Any response smaller than the threshold
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assumes that the stimulus was $I_1$, any response larger than the
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threshold assumes that the stimulus was $I_2$. What is the
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probability that the stimulus was indeed $I_1$ or $I_2$,
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respectively? Find the threshold $n_{thresh}$ that
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results in the best discrimination performance.
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\part Also plot the Fano factor as a function of the slope. How is it related to the discriminability?
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\uplevel{If you still have time you can continue with the following questions:}
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\part You may change the difference between the two stimuli $I_1$ and $I_2$
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as well as the intrinsic noise of the neuron via \texttt{Dnoise}
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(change it in factors of ten, higher values will make the
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responses more variable) and repeat your analysis.
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\part For $I_1=10$ the mean firing is about $80$\,Hz. When changing the slope of the tuning curve
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this firing rate may also change. Improve your analysis by finding for each slope the input
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that results exactly in a firing rate of $80$\,Hz. Set $I_2$ on unit above $I_1$.
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\part How does the dependence of the stimulus discrimination performance on the slope change
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when you set both $I_1$ and $I_2$ such that they evoke $80$ and
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$100$\,Hz firing rate, respectively?
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\end{parts}
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\end{questions}
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\end{document}
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