Biomedical Engineering Reference
In-Depth Information
2.2 ORGANIZATION OF
NEUROBIOLOGICAL SENSORY
SYSTEMS
To date neuromorphic systems have attempted
to mimic the functionalities of these sensory
organs, among many other neurobiological struc-
tures, such as the cochlea and retina [9] . Except
for a few cases, the performance of most neuro-
morphic architectures falls far short of that of
their biological counterparts in terms of sensing
capabilities and energy efficiency. One of the pri-
mary reasons for this performance gap could be
that the design principles focus mainly and only
on enhancing the system's signal-to-noise ratio
(SNR) by alleviating system artifacts like noise ,
nonlinearity, and sensor imperfections. In biology,
however, noise and nonlinearity play a construc-
tive role where by sensing and signal detection
are in fact enhanced due to these system artifacts.
For instance, it has been shown that the ability of
the paddlefish ( P. spathula ) to localize and capture
plankton is significantly enhanced when different
amplitudes of random noise are intentionally
added to its environment [6] . In biology, learning
and adaptation also play key roled in noise
exploitation by shaping the system signal and
system noise in the frequency domain.
The purpose of this chapter is to describe
some of the important noise exploitation and
adaptation principles observed in neurobiology
and show how they can be used for designing
neuromorphic sensors. The chapter is organized
as follows: Section 2.2 briefly introduces the
organization of a typical neurobiological sen-
sory system and includes a brief overview of the
structure of a neuron, synapses, and different
types of neural coding. Section 2.3 provides an
overview of two noise exploitation principles:
(a) stochastic resonance (SR) and (b) noise shap-
ing. Section 2.4 describes the adaptation mecha-
nisms in neural systems with a focus on plasticity
and learning. Section 2.5 presents a case study
of a neuromorphic acoustic source sensor and
localizer that emulates the neurobiological prin-
ciples observed in the parasitoid fly ( Ormia
ochracea ) . The chapter concludes in Section 2.7
by discussing open problems and challenges in
this area.
The typical structure of a neurobiological sensory
system is shown in Figure 2.2 . The system con-
sists of an array of sensors (mechanoreceptors,
optical, or auditory) that are directly coupled
to a group of sensory neurons, also referred to
as afferent neurons. Depending on the type of
sensory system, the sensors (skin, hair, retina,
cochlea) convert input stimulus such as sound,
mechanical, temperature, or pressure into electric
stimuli. Each of the afferent neurons could poten-
tially receive electrical stimuli from multiple sen-
sors (as shown in Figure 2.2 ), an organization that
is commonly referred to as the sensory receptive
field.
For example, in the electric fish, the electro-
sense receptors distributed on the skin detect a
disruption in the electric field (generated by the
fish itself) that corresponds to the movement
and identification of the prey. The receptive field
in this case corresponds to electrical intensity
spots that are then encoded by the afferent neu-
rons using spike trains [10] .
The neurons are connected with each other
through specialized junctions known as synapses.
While the neurons (afferent or non-afferent)
form the core signal-processing unit of the sen-
sory system, the synapses are responsible for
adaptation by modulating the strength of the
connection between two neurons. The dendrites
of the neurons transmit and receive electrical
signals to and from other neurons, and the soma
receives and integrates the electrical stimuli. The
axon, which is an extension of the soma, trans-
mits the generated signals or spikes to other
neurons and higher layers.
The underlying mechanism of a spike or
action-potential generation is due to unbalanced
movement of ions across a membrane, as shown
in Figure 2.3 , which alters the potential diffe-
rence between the inside and the outside of the
neuron. In the absence of any stimuli to the
 
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