A NEURON Programming Tutorial - Part E


We have run a reasonable number of simulations since the first tutorial (part A). Quite often we will want to analyse or record certain simulation results and perform large numbers of simulations as fast as possible (for example in parameter searching). In this tutorial we will explore methods of getting data out of neuron to be stored or analysed in other packages. We will also consider ways of speeding up the simulations and the consequences and decision we take in doing this.

Picking Vectors from the Plots

PickVector option of the Graph menu Everything that is plotted can be saved to a specified data filename. For example, if we are plotting the voltage of the subthalamic projection neuron and want to record a particular voltage trace (our example simulations are in sthE.hoc), we can do this from the Graph Properties menu. As described in part B, this menu appears after a right click on the graph of interest.

From this menu select the Pick Vector option.

Now any plot on this graph can be selected simply by clicking on the particular plotted line of interest. In our example (left), the graph contains three plots (showing the voltage at the soma of three subthalamic cells). We can only select one plot at a time to save.

Click on the voltage trace of the third subthalamic cell (the blue plotted line) this will be selected and we can save the associated vectors using the NEURON Main Menu toolbar.

The Main Menu toolbar (described in part A) contains a Vector menu allowing vectors to be saved or retrieved from data files.

Select Save File from the Vector menu.

This pops up a file dialogue window allowing you to enter a filename for saving the selected vector data. Once you have entered a filename and saved the data it is a good idea to look at the file in a text editor to see how the vectors are saved. The beginning of the file should look something like this:

label:SThcells[2].soma.v( 0.5 )
0       -65
0.025   -65.0488
0.05    -65.0844
0.075   -65.1125
0.1     -65.136

The first line of the file is a text string identifying the data being plotted, the second line is the number of data points in the file (here 32001), and finally the third line onward contains the data. Each pair of points is the time and, in our example, soma voltage at that time.

Recording Data with Vectors

Often it is more convenient to save data without having to plot it first. This can be done in hoc using Vector and File objects. As introduced in part A we must create an object variable for each object we wish to create. In our example, we want to record both the time and voltage at the soma of the third projection neuron, so we only require two vector objects (which we will call rect and recv):

objref rect, recv

This creates two variables for holding the vector objects. Now the vectors must be created, as we have also seen before, we do this with the new command (compare to the creation of the IClamp in part A):

rect = new Vector()
recv = new Vector()

We now have two vector objects. There are may things we can do with vectors, but all we wish to do here is record the particular time and voltage values of a simulation. If we know the names of the variables in hoc that we wish to save, we can do this with the record function in the vector object. As described in part C our subthalamic cells are defined as an array of object variables (SThcells[0] to SThcells[nSThcells]). The third cell is SThcells[2]. The voltage at the midpoint of the soma is then given by the variable name (see part C):


To record this voltage, we prepend an "&" to this variable name, and give it as an argument to the record function of the particular vector object where were want it saved:


The variable SThcells[0].soma.v(0.5) (i.e. the voltage in the centre of the third cells soma) will be recorded into this vector on each simulation. Note, that only one simulation is held in the vector. If a second simulation is run, it will overwrite any previous simulation data in this vector.

To record the time information, the second vector object (rect) can be setup to record the time variable t.


Now, run a simulation (either from the RunControl window, or using the hoc command run()). The time and voltage data from these variables will be recorded in our two vectors (don't worry if "resize_chunk" messages appear in the terminal, these just indicate the vector objects are growing in size).

To see the data in the vectors, we can use the vector object function printf to display the data in the terminal:


However, it is generally not that useful to print the data onto the screen, as there is usually a lot of data.

Saving the Vectors

All we want to do now is save the vectors to file. The simplest way of doing this from the hoc programming point of view is to save each vector to a different file. To do this we must create file objects for each file. As with any object, first a variable must be defined for the object (we will call ours savv and savt).

objref savv, savt

We now create the file objects using the new command:

savv = new File()
savt = new File()

The function wopen in the file object opens a file for writing to. It takes as an argument the name of the file (we will call our files "cell3somav.dat" and "cell3somat.dat").


We can now save the data in our vectors (rect and recv) using, again, the vector object printf function, but this time with the file object as an argument:


Finally, the files should be closed:


This example creates two text files called "cell3somav.dat" and "cell3somat.dat" containing the voltage and time data respectively. These data files differ from the files generated from the graph PickVector method above. Here there is one data vector per file rather than having two vectors in the same file as above. The first two lines of the file generated using the PickVector method (containing the variable name and number of lines) are also absent from each file using this method.

Saving the Vectors in one file

To make one output file in the same format as the PickVector method, we have to do some more hoc programming. A simple way of doing this is to use a for loop and the printf function.

First, we create a single file object that opens a file called "cell3soma.dat":

objref savdata
savdata = new File()

We can also put useful data, such as the variable name(s) we are saving (e.g. SThcells[2].soma.v(0.5)), and the number of vectors in the file, we can write this to the file first using the printf function in the File object:

savdata.printf("t SThcells[2].soma.v(0.5)\n")

The printf function in a File object is somewhat different from the printf in a Vector (seen above), and will be familiar to C programmers. The first time it is used above, it has only one argument which is the text that will be printed to the file. This text is enclosed in quotes and in programming terms is known as a string. The "\n" at the end of the string inserts a newline. The second time it is used it has two arguments. The first argument is a string, but this time it contains a percentage sign followed by a single character, the letter "d". This "%d" is a placeholder for an integer number that is given to the function as an additional argument. The argument in this example is rect.size(). The function size is in the Vector object and returns the size of the vector, here, the size of the time vector rect.

The next step is to create a for loop (inctroduced in part B) that again uses the printf function in the File object to write the vectors to the file. As in the PickVector method above, we want each line of our file to have two numbers (time and voltage of the SThcells[2].soma at that time):

for i=0,rect.size()-1 {
    savdata.printf("%g %g\n", rect.x(i), recv.x(i))

In this case the printf function has three arguments. As before the first argument is a string of text containing placeholders for numbers. They are "%g" rather than "%d" because the arguments are real rather than integer numbers. It is possible to alter the formatting of the numbers by using letters other than "g" and "d" after the percentage sign; see the online manual for more information about printf. The first string argument contains two "%g" placeholders, indicating two real numbers are requred as additional arguments. These real numbers are the data from the time and voltage vectors. The x function in the Vector object returns the data at a given location in the vector. For example, rect.x(0), is the first ellement in the rect vector, and rect.x(1), is the second etc. In our for loop, we use x(i) to cycle through each vector ellement, so that each data pair gets written to the file in sequence.

Finally, we close the file as shown above.


Saving the Vectors more efficiently

The use of a for loop may be inefficient for large vectors, as the hoc interpreter has to perform as many file printf statements as there are elements in the vectors. A more efficient way of doing this is to use Matrix objects. The sample code is below. For further explanation, consult the Matrix section of the online manual.

objref savdata
savdata = new File()

savdata.printf("t SThcells[2].soma.v(0.5)\n")

objref tempmatrix
tempmatrix = new Matrix()
tempmatrix.setcol(0, rect)
tempmatrix.setcol(1, recv)
tempmatrix.fprint(savdata, " %g")

The method of recording data into vector objects, then saving the objects to files may seem like a lot of work compared to clicking on a graph. However, as we have stressed before, the usefulness comes from including variants of such commands in hoc scripts. These scripts can then be executed performing various and often complex I/O functions. The hoc objects and functions we have introduced also have many options and uses well beyond what we have described. We refer the reader to the NEURON help and reference web pages for exploring the different ways and methods of getting data out of NEURON.

Speed of Simulation

The speed of a simulation is proportional to the number of segments and inversely proportional to the time step. Therefore, to speed up a simulation, we can reduce the number of segments (by using nseg) or increase dt. However, both these measures will decrease the accuracy of the simulation. Hines and Carnevale (2001) discuss the tradeoff between accuracy and speed in an accessible fashion.

They also discuss another strategy for speeding up simulations: variable time step methods. The principle of a variable time step method is that the time step is longer when quantities are not changing much (such as between action potentials) and shorter when quantities are changing quickly (such as during an action potential). NEURON incorporates a standard variable time step method called cvode. We will show basic use of cvode in NEURON.

By default, NEURON uses fixed time step integration. The command


returns 0, indicating that variable time steps are not being used. To turn on the variable time step integration we can type:


(We could also select the Tools-VariableTimeStep dialog and click on Use variable dt.)

When you try running the simulation again, you should see that it runs much faster than before turning cvode. (On a fast machine, you may not notice the lines on the graph being drawn.) To turn off cvode, either type:


or deselect Use variable dt in the Tools-VariableTimeStep dialog.

In the fixed-timestep method of simulation, the accuracy of the simulation can be changed by specifying the timestep. In the variable time step method, there is no one time step to change; instead we specify a tolerance, that is the maximum estimated error allowed at each time step. Smaller tolerances will lead to more accurate but longer simulations. To find what the tolerance is, we type


We see that the default value (in NEURON 5.4 and newer) is 0.001. This gives about the same accuracy as a fixed time step of 25μs on the Hodgkin-Huxley action potential simulation at 6.3°C. You can investigate the effect of changing the accuracy by setting it with


or by setting Absolute tolerance in the Tools-VariableTimeStep dialog.

An important point to note is that this is an absolute tolerance. For quantities such as membrane potential measured in millivolts, 0.001 is small. However, if our simulation contained quantities that have very small absolute values, the absolute tolerance we have specified could be many orders of magnitude greater than these quantities. It is possible to set smaller tolerances for some variables (e.g. calcium concentration); see atolscale in the neuron documentation.

When we are using cvode, we can still use the PickVector or record vector methods (above) to save data to file (Note: in NEURON 5.4 and earlier the record vector methods cannot be used with cvode. An alternative record method in the cvode object is provided in the earlier versions.)


Hines, M. L. and Carnevale, N. T. (2001) "NEURON: a Tool for Neuroscientists", The Neuroscientist 7:123-135.

Andrew Gillies (andrew@anc.ed.ac.uk)
David Sterratt (dcs@anc.ed.ac.uk)
with the assistance of Ted Carnevale and Michael Hines
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