Individual Neurons - Computational Explorations in Cognitive Neuroscience - page 49

Object Actions LogFile .views View: Object Actions

Display Toggle Update Init Clear

1− 1−

Move/Scan ReScale

0.8−

Graph Labe l s

0.8− 0.6−

New Delete Edit

0.4−

trial
phase_no

0.6−

0.2−

0−

cycle

−X−

0.4−

−0.2−

net

−0.4−

I_net

0.2−

−0.6−

v_m
act

−0.8−

0−

1X 1X

act_eq

1X

0 20 40 60 80 100 120 140 160 180 200

Figure 2.17: The GraphLog display of a run with standard parameters. The solid line ( net ) is red in the actual display, dashed
( I net ) = orange, dotted ( v m ) = yellow, and dash-dot ( act ) = green.

accidentally typed in a value that you didn't want). In
addition, there are various special-purpose buttons that
are specific to a given simulation. For more information
on using these edit dialogs, see appendix A.5.
First, we will focus on g_bar_e , which controls
the amount of excitatory conductance as described ear-
lier. In general, we are interested in seeing how the
unit membrane potential reflects a balance of the differ-
ent inputs coming into it (here just excitation and leak),
and how the output activation responds to the resulting
membrane potential.

Software tip: It is often useful to overlay different
Run s on top of each other in the graph log, which will
happen naturally. When you want to clear the log, press
the Clear button. Also, if for any reason the graph
goes blank or is somehow garbled, pressing the Init
button should fix things.
By systematically searching the parameter range for
g_bar_e between .3 and .4, you should be able to
locate the point at which the membrane potential just
reaches threshold (recall that this is at V m = :25 in the
normalized units). To make it easier to locate the thresh-
old, we will switch from the noisy XX1 activation func-
tion (which has a soft threshold due to the effects of the
convolved noise) to the XX1 activation function (which
has a hard threshold due to the absence of noise).

Increase g_bar_e from .4 to .5 (and press the
Apply and Run buttons to see the effects). Then ob-
serve the effects of decreasing g_bar_e to .3.

Question 2.1 (a) Describe the effects on the neural re-
sponse of increasing g_bar_e to .5, and of decreasing
it to .3. (b) Is there a qualitative difference in the unit
activation ( act ) between these two changes of magni-
tude .1 away from the initial .4 value? (c) What impor-
tant aspect of the point neuron activation function does
this reveal?

Switch the act_fun setting on the control panel
from NOISY_XX1 to XX1 and press Apply .

Question 2.2 (a) To 3 decimal places, what value of
g_bar_e puts the unit just over threshold? Can you
think of a better way of finding this value (Hint: Do you
remember an equation for the equilibrium membrane
potential given a particular set of inputs?) (b) Compute