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The Lorenz Chaotic Attractor

This example show how a classical chaotic dynamical system (the Lorenz “butterfly” attractor) can be implemented in a neural population. The dynamical equations for this attractor are:

\[\begin{split}\begin{align} \dot{x}_0 &= \sigma(x_1 - x_0) \\ \dot{x}_1 &= x_0(\rho - x_2) - x_1 \\ \dot{x}_2 &= x_0 x_1 - \beta x_2 \end{align}\end{split}\]

Since \(x_2\) is approximately centered around \(\rho\), and because NEF ensembles are typically optimized to represent values within a pre-defined radius of the origin, we substitute \(x'_2 = x_2 - \rho\), yielding these equations:

\[\begin{split}\begin{align} \dot{x}_0 &= \sigma(x_1 - x_0) \\ \dot{x}_1 &= -x_0 x'_2 - x_1 \\ \dot{x}'_2 &= x_0 x_1 - \beta(x'_2 + \rho) - \rho \end{align}\end{split}\]

As with the previous oscillator examples, an exponential synapse is being used to implement these dynamics, meaning that the neural feedback matrix is:

\[\mathbf{A}' = \tau\mathbf{A} + \mathbf{I}\]

Because the Lorenz attractor has no input signal \(u\), this in turn means that the neural feedback function can be computed as:

\[\begin{split}\begin{align} f_{fdbk}(x) &= \mathbf{A}' x \\ &= \tau\mathbf{A}x + x \\ &= \tau\dot{x} + x \\ &= \begin{bmatrix} \tau\dot{x}_0 + x_0 \\ \tau\dot{x}_1 + x_1 \\ \tau\dot{x}'_2 + x_2 \end{bmatrix} \end{align}\end{split}\]

For more information, see “Chris Eliasmith. A unified approach to building and controlling spiking attractor networks. Neural computation, 7(6):1276-1314, 2005.”

Step 1: Set up the Python Imports

[1]:
%matplotlib inline
import matplotlib.pyplot as plt
from IPython.display import HTML

import nengo

from mpl_toolkits.mplot3d import Axes3D

from anim_utils import make_anim_chaotic

import nengo_fpga
from nengo_fpga.networks import FpgaPesEnsembleNetwork

Step 2: Choose an FPGA Device

Define the FPGA device on which the remote FpgaPesEnsembleNetwork will run. This name corresponds with the name in your fpga_config file. Recall that in the fpga_config file, device names are identified by the square brackets (e.g., [de1] or [pynq]). The names defined in your configuration file might differ from the example below. Here, the device de1 is being used.

[2]:
board = "de1"  # Change this to your desired device name

Step 3: Create the FPGA Ensemble

Create a remote FPGA neural ensemble (FpgaPesEnsembleNetwork) using the board defined above, 2000 neurons, 3 dimensions, and with no learning rate. We will also specify the recurrent connection here.

[3]:
tau = 0.1  # Synaptic time constant

# Lorenz attractor parameters
sigma = 10
beta = 8.0 / 3.0
rho = 28

with nengo.Network(label="Lorenz Attractor") as model:
    # Remote FPGA neural ensemble
    fpga_ens = FpgaPesEnsembleNetwork(
        board,  # The board to use (from above)
        n_neurons=2000,  # The number of neurons to use in the ensemble
        dimensions=3,  # 3 dimensions
        learning_rate=0,  # No learning for this example
        feedback=1,  # Activate the recurrent connection
    )

    # The representational space of the Lorenz attractor is pretty big,
    # so adjust the radius of the neural ensemble appropriately.
    fpga_ens.ensemble.radius = 50

    # Setting `feedback=1` creates a `feedback` connection object
    # with the identity transform. To implement the oscillator, it
    # is necessary to set the transform on the feedback connection
    # using .transform.
    fpga_ens.feedback.synapse = tau  # `nengo.LowPass(tau)`

    # Define the feedback function
    def func_fdbk(x):
        # These are the three variables represented by the ensemble
        x0, x1, x2 = x

        dx0 = sigma * (x1 - x0)
        dx1 = -x0 * x2 - x1
        dx2 = x0 * x1 - beta * (x2 + rho) - rho

        return [tau * dx0 + x0, tau * dx1 + x1, tau * dx2 + x2]

    # Assign the feedback function to the feedback connection
    fpga_ens.feedback.function = func_fdbk
Specified FPGA configuration 'de1' not found.
WARNING: Specified FPGA configuration 'de1' not found.

Step 4: Add Probes to Collect Data

[4]:
with model:
    # The output from the FPGA ensemble
    # (filtered with a 10ms post-synaptic filter)
    output_p = nengo.Probe(fpga_ens.output, synapse=0.01)

Step 5: Run the Model

[5]:
with nengo_fpga.Simulator(model) as sim:
    sim.run(20)
Building network with dummy (non-FPGA) ensemble.
WARNING: Building network with dummy (non-FPGA) ensemble.

Step 8: Plot the Results

[6]:
def plot_results():
    plt.figure(figsize=(8, 12))
    plt.subplot(211)
    plt.plot(sim.trange(), sim.data[output_p])
    plt.legend(["$x_0$", "$x_1$", "$x_2$"], loc="upper right")
    plt.xlabel("Time (s)")

    # Make 3D plot
    ax = plt.subplot(212, projection=Axes3D.name)
    ax.plot(*sim.data[output_p].T)
    ax.set_title("3D Plot")

    plt.show()


plot_results()
../../_images/examples_notebooks_06-chaotic-attractor_12_0.png

The plots above show the decoded output of the oscillator exhibiting the dynamics of a Lorenz “butterfly” attractor. The 3D plot illustrates why this attractor is called a “butterfly” attractor.

While a static plots are informative, an animated figure can be used to show how the Lorenz attractor evolves over time. It may take a moment to generate the animation.

[7]:
def make_anim():
    _, _, anim = make_anim_chaotic(
        sim.data[output_p][:, 0],
        sim.data[output_p][:, 1],
        sim.data[output_p][:, 2],
        ms_per_frame=20,
    )
    return anim


HTML(make_anim().to_html5_video())
[7]:

Step 9: Spiking Neurons

The plots above show the output of the Lorenz attractor network implemented with non-spiking rectified linear neurons. The network can also be simulated using spiking neurons to illustrate the similarities and differences between a spiking and a non-spiking network.

Below, we configure the FPGA neural ensemble to use spiking neurons, run the simulation, and plot the results.

[8]:
with model:
    fpga_ens.ensemble.neuron_type = nengo.SpikingRectifiedLinear()

with nengo_fpga.Simulator(model) as sim:
    sim.run(20)
plot_results()
Building network with dummy (non-FPGA) ensemble.
WARNING: Building network with dummy (non-FPGA) ensemble.
../../_images/examples_notebooks_06-chaotic-attractor_16_2.png
[9]:
HTML(make_anim().to_html5_video())
[9]: