This reference sheet gathers some extra information that may be useful for more advanced users of Nengo. Specifically, there is information on: - User- defined functions - Function inputs (defining a wide variety of functions to drive a network) - More precise control over tuning curves (with preferred direction vectors)
- When creating a function input or defining the function being computed by a decoded origin, you can type in standard expressions for the function.
- For function inputs, the variable you write the function over is always x0, which is time.
- For decoded terminations, you can have multi-dimensional functions, and the variables are x0, x1, ...
- The syntax is typical: +,-,*,/, cos, sin, ln, exp, sqrt, max, and others. The list of all functions registered to the parser is provided when you first create a function input of type 'user-defined function' in the pull-down menu.
For most situations, we want to define inputs that change over time. Here we are basing the example on the network constructed in the Building a Feedforward network (Part 2) tutorial. The methods are the same for all networks.
- For many common types of inputs, we can use the Configure interface.
- Right-click on the second function input in the model and select Configure.
- Go to functions->Constant function
- Right-click on Constant function and choose Replace
- Use the drop-down box to select SineFunction
- Press the right-arrow button twice to see the constructor that allows you to set amplitude and omega (frequency)
- Double-click on amplitude and set it to 10
- Double-click on omega and set it to 5
- Click on Create, OK, and Done.
- Run the simulation and plot the result. You may also wish to add probes for the two function inputs.
- We can also create functions using the script console
- Click on an function input so that it is highlighted in yellow.
- Press Ctrl-P to go to the script console
- Enter the following command:
- This creates the same function as done through the GUI above
- There are a variety of other functions available
- For example, consider multiplying the following two functions together
- Select the first function input, go to the script console, and enter
- Select the second function input, go to the script console, and enter
- Run the simulation and plot the inputs and results
- You can also define your own functions using Python code
- Since this will involve a few lines of code, we can use the script editor (or any text editor)
- Go to View->Open Script Editor
- Choose File->Open->myfunction.py
class MyFunction(ca.nengo.math.impl.AbstractFunction): def map(self,x): t=x if t<0.2: return 0 return -30_t_t+20*t
- This is a basic definition of a Python class inherting from AbstractFunction. It defines the function that is 0 for t<0.2 and -30t2+20t otherwise.
- To use this, close the script editor and return to the script console
- type ârun myfunction.pyâ. This will define the new class. We can now use it in the same way as the other functions.
Setting preferred direction vectors
The preferred direction vectors (a.k.a. encoding vectors) for ensembles are normally randomly chosen from a unit sphere. We may wish to configure these to be more consistent with known neural properties.
- The easiest way to set preferred direction vectors is to click on the 'Advanced' tab when creating a population of neurons. Changing the 'Encoding distribution' slider determines how clustered neurons are along the dimensions of the vector space being represented.
- More fine grained control of encoders can be done with a script or with the script console.
- The following code allows us to explicitly specify encoding vectors. This assumes that you have constructed a population of 200 2-dimensional neurons, and it is selected.
` import random
class MyVectors(ca.nengo.util.VectorGenerator): def
vectors= while len(vectors)
- In this case, the vectors are randomly chosen from the set [1,1],[1,-1],[-1,1],[-1,-1]. This is optimal for multiplication (see the Building a Feedforward network (Part 2) tutorial).
- Note that we have to set the number of neurons so that new neurons are created using the new encoding vectors.