24-11-2012, 12:03 PM
Computer Electronics Meet Animal Brains
electronics meet animal brain.pdf (Size: 1.03 MB / Downloads: 34)
Until recently, neurobiologists have used
computers for simulation, data collection,
and data analysis, but not to interact
directly with nerve tissue in live, behaving
animals. Although digital computers and
nerve tissue both use voltage waveforms to transmit
and process information, engineers and neurobiologists
have yet to cohesively link the electronic
signaling of digital computers with the
electronic signaling of nerve tissue in freely behaving
animals.
Recent advances in microelectromechanical systems
(MEMS), CMOS electronics, and embedded
computer systems will finally let us link computer
circuitry to neural cells in live animals and, in particular,
to reidentifiable cells with specific, known
neural functions. The key components of such a
brain-computer system include neural probes, analog
electronics, and a miniature microcomputer.
Researchers developing neural probes such as submicron
MEMS probes, microclamps, microprobe
arrays, and similar structures can now penetrate
and make electrical contact with nerve cells without
causing significant or long-term damage to
probes or cells.
INTEGRATING SILICON AND NEUROBIOLOGY
Neurons and neuronal networks decide, remember,
modulate, and control an animal’s every sensation,
thought, movement, and act. The intimate
details of this network, including the dynamic properties
of individual neurons and neuron populations,
give a nervous system the power to control a
wide array of behavioral functions.
The goal of understanding these details motivates
many workers in modern neurobiology. To make
significant progress, these neurobiologists need
methods for recording the activity of single neurons
or neuron assemblies, for long timescales, at high
fidelity, in animals that can interact freely with their
sensory world and express normal behavioral
responses.
Conventional techniques
Neurobiologists examine the activities of brain
cells tied to sensory inputs, integrative processes,
and motor outputs to understand the neural basis
of animal behavior and intelligence. They also
probe the components of neuronal control circuitry
to understand the plasticity and dynamics of control.
They want to know more about neuronal
dynamics and networks, about synaptic interactions
between neurons, and about the inextricable
links between environmental stimuli and neuronal
signaling, behavior, and control.
Salient objectives
The solution to these problems lies in making the
test equipment so small that a scientist can implant
it into or onto the animal, using materials and
implantation techniques that hurt neither computer
nor animal. Recent developments in MEMS, semiconductor
electronics, embedded systems, biocompatible
materials, and electronic packaging
finally allow neuroscientists and engineers to begin
packaging entire neurobiology experiments into
hardware and firmware that occupy less space than
a human fingernail.
DESIGNER NEUROCHIPS
Like their benchtop experimental counterparts,
neurochips use amplifiers to boost low-voltage
biological signals, analog-to-digital converters
(ADCs) to digitize these signals, microcomputers
to process the signals, onboard memory to store
the signals, digital-to-analog converters (DACs)
to stimulate nerves, and software to control the
overall experiment.
Figure 1 shows a neurochip’s basic elements. The
key requirements are that the neurochip be small
and lightweight enough to fit inside or onto the animal,
have adequate signal fidelity for interacting
with the millivolt-level signals characteristic of
nerve tissue, and have sufficient processing power
to perform experiments of real scientific value.
Probes
Building the probes that let a neurochip eavesdrop
on the electrical signaling in a nerve bundle,
group of neurons, or single neuron presents a
daunting task. Benchtop experiments on constrained
animals typically use metallic needles—
often made of stainless steel or tungsten—to
communicate with nerve bundles, micromachined
silicon probes to record from groups of neurons,4
or glass capillaries filled with a conductive ionic
solution to penetrate and record from the inside of
individual neurons. In unconstrained animals, flexible
metallic needles, attached to the animal with
surgical superglue, and micromachined silicon
probes still work. However, replicating the performance
of glass capillaries in flying, swimming, wiggling
animals is a different story entirely.
Several centimeters long and quite fragile, the glass
capillaries that neurobiologists use to probe the
insides of nerve cells typically have tip diameters
smaller than 0.3 microns. They impale neurons even
more fragile than the probes themselves. Neurobiologists
use micromanipulators to painstakingly
and precisely drive single probes into single neurons.
Fortunately, MEMS technology offers a possible
alternative to these glass capillaries. As Figure 3
shows, University of Washington researchers are
developing silicon MEMS probes and flexible interconnect
structures to mimic the performance of glass
capillaries in an implanted preparation.5 Researchers
have already recorded intracellular signals with early
prototypes, and development is ongoing.
A STIMULATING WORLD
Passive neurochips that do nothing more than
record will provide neurobiologists with a wealth
of data. But even now, with the first neurochips
barely in production, neurobiologists are already
calling for designs that stimulate nerve tissue as well
as record from it. Active neurochips will allow stimulus-
response experiments that test models of how
nervous systems control behavior, such as how sensory
inputs inform motor-circuit loops and the logic
or model behind the response.
Indeed, the neurochip project’s long-term goal is
to develop a hardware and software environment in
which a neurobiologist conceives a stimulus-response
experiment, encodes that experiment in software,
downloads the experiment to an implanted
neurochip, and recovers the data when the experiment
concludes. Figure 5 shows a model of integrative
biology in which neurochips play a key part.