Biomedical Engineering Reference
In-Depth Information
a
b
c
Figure 18.3
Construction of a generic neural microcircuit model, as used for all computer simulations
discussed in this chapter (only the number of neurons varied).
a)
A given number of neurons
is arranged on the nodes of a 3D grid. 20% of the neurons, marked in black, are randomly
selected to be inhibitory.
b)
Randomly chosen postsynaptic targets are shown for two of the
neurons. The underlying distribution favors local connections (see footnote on this page for
details).
c)
Connectivity graph of a generic neural microcircuit model (for m
=
2, see footnote
below).
Parameters of neurons and synapses were chosen to fit data from microcircuits in
rat somatosensory cortex (based on [5], [15] and unpublished data from the Markram
Lab).
†
†
Neuron parameters
: membrane time constant 30 ms, absolute refractory period 3 ms (excitatory neu-
rons), 2 ms (inhibitory neurons), threshold 15 mV (for a resting membrane potential assumed to be 0),
reset voltage 13.5 mV, constant nonspecific background current
I
b
=
13
.
5 nA, input resistance 1 MX.
Con-
nectivity structure
: The probability of a synaptic connection from neuron
a
to neuron
b
(as well as that
of a synaptic connection from neuron
b
to neuron
a
) was defined as
C
D
2
m
2
,wherem is
a parameter which controls both the average number of connections and the average distance between
neurons that are synaptically connected (we set m
·
exp
(
−
(
a
,
b
)
/
)
2, see [16] for details). We assumed that the neurons
were located on the integer points of a 3 dimensional grid in space, where
D
(
a
,
b
)
is the Euclidean dis-
tance between neurons
a
and
b
. Depending on whether
a
and
b
were excitatory (
E
) or inhibitory (
I
), the
value of
C
was 0.3 (
EE
), 0.2 (
EI
), 0.4 (
IE
), 0.1 (
II
). In the case of a synaptic connection from
a
to
b
we
modeled the synaptic dynamics according to the model proposed in [15], with the synaptic parameters
U
(use),
D
(time constant for depression),
F
(time constant for facilitation) randomly chosen from Gaussian
distributions that were based on empirically found data for such connections. Depending on whether
a
and
b
were excitatory (
E
) or inhibitory (
I
), the mean values of these three parameters (with
D
,
F
expressed
in seconds, s) were chosen to be .5, 1.1, .05 (
EE
), .05, .125, 1.2 (
EI
), .25, .7, .02 (
IE
), .32, .144, .06 (
II
).
The SD of each parameter was chosen to be 50% of its mean. The mean of the scaling parameter
A
(in
=
Search WWH ::
Custom Search