Individual Neurons - Computational Explorations in Cognitive Neuroscience - page 48

sponse of the receiving unit. To see this, we can run the
simulation.

off_cycle (the total number of cycles is controlled
by n_cycles ).
The second line, I_net (in orange/dashed), shows
the net current in the unit. As expected, this shows an
excitatory (upward) current when the input comes on,
and an inhibitory (downward) one when the input goes
off. This line is plotted using its own special vertical (Y)
axis going from ￿1 to 1 (shown in orange), while all the
other lines share the 0￿1 Y axis with the net-input (red).
Note that the axis used by each line is color-coded to the
right of the variable buttons.
The third line, v_m (in yellow/dotted), shows the
membrane potential , which starts at the resting potential
of .15, then increases with the excitation, and decreases
back to rest when the input goes off.
The fourth line, act (in green/dash-dot), shows the
activation value (using the NOISY_XX1 rate-coded ac-
tivation function) that results from the membrane po-
tential. It goes from 0 up to roughly .72 and back down
again. Note that the activation rise trails the net input
by several cycles — this is due to the time it takes for
the membrane potential to reach threshold.
Software Tip: You can click with the left mouse but-
ton on any line in the graph to get the precise numerical
value at a given point.

Press the Run button on the control panel.
You should see that very shortly after the input unit
comes on (indicated by the yellow color), the receiv-
ing unit is activated by this input. To get a better idea
of the precise trajectory of this activation, it is much
more convenient to use the GraphLog , which displays
the information graphically over time, allows multiple
variables to be viewed at the same time, and even al-
lows multiple runs (e.g., with different parameters) to
be compared with each other.

Iconify(minimize) the NetView window.
Software Tip: Iconify using your window manager's
buttons (typically on the upper right-hand corner of the
window), or using the Iconify menu option in the
Object menu at the very left-hand side of the win-
dow. Never use the Object/Close menu option to
iconify — it will destroy the object and cause a crash!

Press View on the control panel, and select
GRAPH_LOG to open up the GraphLog display.
The plots produced by this simulation are much like
that shown in figure 2.9. Only the excitatory and
leak currents are operating here, with their conduc-
tances ( g_bar_e , g_bar_l ) and reversal potentials
( e_rev_e , e_rev_l ) as shown in the control panel.

Use the mouse to verify that the activation rise does
indeed occur right around the threshold value for the
membrane potential (.25).
The fifth line, act_eq (in blue), is the rate-code
equivalent activation value for discrete spiking units.
When an actual rate-code activation function (noisy
XX1) is used, act_eq is just equivalent to act .This
line is not turned on by default in the graph — we will
use it later when we switch to spiking units.
Now we will use some of the parameters in the con-
trol panel to explore the properties of the point neuron
activation function.

Press the Run button on the control panel.
This produces a plot using the current parameters
(figure 2.17). You should see various lines plotted over
200 time steps ( cycles ) on the X axis. Note that the stan-
dard 0 ￿ 1 normalized parameters are used by default
(we can switch to the biological values later).
First, let's focus on the net line (it's the red one,
displayed as a solid line in figure 2.17). This shows
the net input to the unit, which starts out at 0, and then
rapidly jumps to .4, remaining there until around 160
time steps, where it goes back to 0 again. Recall that
this net input is just another name for the total excita-
tory input to the neuron (i.e., net = g e g e ). In this sim-
ulation, the sending unit always sends a g e value of 1
when it's on, and a 0 when off. Later, we will manipu-
late the value of g e ( g_bar_e in the control panel) to
control the magnitude of the net input (it is .4 because
the default value of g_bar_e is .4). The timing of the
input is controlled by the parameters on_cycle and

Take a moment to familiarize yourself with the pa-
rameters (you can click on the label for each parameter
to view a brief description of what it is).
Software Tip: All edit dialogs like the control panel
have at least two buttons across the bottom: Apply
(which applies any changes you have made to actually
set the parameters) and Revert (which reverts to the
previously applied values, which is useful if you have