Biomedical Engineering Reference
In-Depth Information
Figure 1
. (
A
) Schematic of ADT neuromorph. Synapses are located at the cross points on the
3-branch dendritic tree and at the soma. Activating soma synapses sets the spike-firing thresh-
old for the integrate-and-fire spike generator, whose integration time constant is determined by
the programmable resistor, R, and fixed capacitor, C, which is discharged whenever a spike is
generated. (
B
) A two-compartment section of dendrite. Each compartment contains a mem-
brane capacitance (C
m
), a membrane resistance (R
m
) that connects to V
rest
, and axial resistances
(R
a
) that connect to adjacent compartments. The excitatory (G
e
) and inhibitory (G
i
) synaptic
conductances, which turn on momentarily when a synapse is activated, pull the compartment
capacitor voltage towards V
excitatory
and V
inhibitory
, respectively. The label GND corresponds to the
common voltage node to which all other nodes are referenced.
3.
HARDWARE SYSTEM
In our ADT neuromorphs fabricated by VLSI, the dendritic branches are
modeled electrically by arrays of capacitors and resistors (Figure 1B) (4,5). The
dendrites are composed of multiple compartments, each with a capacitor, C
m
,
representing a membrane capacitance, and two programmable resistors, R
m
and
R
a
, representing a membrane resistance and an axial or cytoplasmic resistance.
In most of our neuromorph chips (Figure 2), each dendritic branch has sixteen
compartments, with 3-8 branches connected together to form a tree like that
shown in Figure 1A. Every compartment has an excitatory and an inhibitory
synapse formed by MOS (metal oxide semiconductor) field-effect transistors
that enable brief currents into or out of the compartment. Applying an impulse to
the transistor gate turns on a synapse. The resultant "transmembrane" current
depends upon the potential difference across the transistor, and upon the synap-
tic weight, which can be controlled (a) by the conductance of the transistor in
