Periodicity Detection and Localization using Spike Timing from the AER EAR.

Abstract

We present a system consisting of a spiking cochlea chip and real-time event-based processing software that is able to discriminate between two sets of sounds based on their periodicity content. The periodicity measurements are computed from the spike timing information of asynchronous output spikes from the binaural spiking-cochlea chip. The chip consists of a matched pair of silicon cochlea with an address event interface for the output. Each section of the cochlea is modeled by a second-order low-pass filter followed by a simplified inner hair cell circuit and a spiking neuron circuit. We show discrimination results using the periodicity measure for 2 classes of sound and preliminary localization results based on a discriminated sound.

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Periodicity detection and localization using spike
timing from the AER EAR
Theodore Yu1, Andrew Schwartz2, John Harris3, Malcolm Slaney4, and Shih-Chii Liu5

1School of Electrical and Information Engineering
University of San Diego
San Diego, USA
teyu@ucsd.edu


3School of Electrical and Computer Engineering
University of Florida
Gainesville, FL, USA
harris@cnel.ufl.edu
2Speech and Hearing Bioscience and Technology Program
Harvard-Massachusetts Inst of Technology
Cambridge, MA, USA
ozydingo@gmail.com


4Yahoo! Research
Santa Clara, CA, USA
malcolm@ieee.org

5Institute of Neuroinformatics
University of Zürich/ETH
Zürich, Switzerland
shih@ini.phys.ethz.ch
Abstract—We present a system consisting of a spiking cochlea
chip and real-time event-based processing software that is able
to discriminate between two sets of sounds based on their
periodicity content. The periodicity measurements are
computed from the spike timing information of asynchronous
output spikes from the binaural spiking-cochlea chip. The chip
consists of a matched pair of silicon cochlea with an address
event interface for the output. Each section of the cochlea is
modeled by a second-order low-pass filter followed by a
simplified Inner Hair Cell circuit and a Spiking Neuron
circuit. We show discrimination results using the periodicity
measure for 2 classes of sound and preliminary localization
results based on a discriminated sound.
I.
INTRODUCTION
Periodicity information is an important alternative to
spectral analysis because of its precision and repeatability.
The biological cochlear filters are highly nonlinear, with
bandwidths and gain changing based on the incoming sound
level [1]. The location of the filter with the peak response
changes, thus it is difficult for a perceptual system to
estimate the spectrum from only the rate profile. Some
authors [2] have suggested that neural circuits in the cochlea
subtract the responses at two nearby locations to mark the
location of the sharp high-frequency cutoff, which is more
stable. But the phase of the signal, as a function of place on
the cochlea, is also changing rapidly at this location, and it is
difficult to know how this subtraction is implemented in a
spiking network.
Instead, in this paper, we investigate the use of just
timing information to recognize and localize sounds. Even
as the cochlear filters change, peak excursions in a 100Hz
cochlear channel due to a 100Hz input occur every 10ms.
The timing information is preserved. Others have used this
information to judge the relative time delay of a sound
between two ears [3] and in this paper we use the same
information to judge pitch and identify sounds. The same
tolerance to imperfect filters useful to the auditory system is
also useful in our silicon models.
Although we have 20 years of experience in designing
silicon cochlea chips, it is only in recent years that we see
cochlea chips that produce asynchronous spike outputs
resembling outputs of the auditory nerve fibres. These spikes
are transmitted using the Address Event Representation
(AER) where each spike carries the identity of the sender.
There are a handful of silicon cochleae with an Address
Event type representation [4][5][6][7]. The AER EAR chip
that we use in this work is an improved version over the
prototype described by Chan et al. [3].
There are a couple of groups that have looked at aVLSI
systems for extracting periodicity in sounds. The
implementation from van Schaik [9] extracted periodicity
information by ANDing the neuron outputs of bandpass filter
channels that are spaced a period apart. The implementation
from Abdalla and Horiuchi consists of an aVLSI chip which
extracts the periodicity information directly from the output
of the microphone [10]. In this work, we extract periodicity
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from spike trains by using a system consisting of a spiking
silicon cochlea (AER EAR) and an event-based software
infrastructure (jAER) by using the spike timing information
from the output spikes of the AER EAR [3]. The jAER
software can process in real-time the spike events from AER
chips/devices [11].
We use this periodicity information to discriminate
between two classes of sounds, harmonic and inharmonic
sounds or noise, independent of the speaker. In addition, this
classification information can be used to selectively localize
an auditory sound that falls into one of the two classes.

Figure 1. The block diagram of circuits of one of the 2 cochleas on the
binaural AER EAR chip.

II.
THE SILICON COCHLEA
Figure 1 shows the basic building blocks in the spiking
cochlear chip. The incoming sound goes through a cascade
of 32 bandpass filters for each cochlea of this chip with a
range of exponentially decreasing cutoff frequencies usually
tuned from around 100 Hz to 1 kHz.
A simplified Inner Hair Cell circuit rectifies and low
passes the output of each bandpass filter before passing it to
a ganglion cell circuit. The cut-off frequency of the Inner
Hair Cell is set around 1 kHz, as in the real Inner Hair Cell.
This low-pass filtering models the reduction in phase-locking
observed on biological auditory nerve fibres at frequencies
greater than 1 kHz. The outputs of the ganglion cell circuits
are transmitted asynchronously on a common digital-address
bus which carries the identity of the channel that produced
the output spike. The time of the spike is coded implicitly in
the event.



Figure 2. Spike responses from a single cochlea across 32 channels (y-
axis) in response to (top) a “hiss” and (bottom) a “coo” from a speaker.
Channel ‘0’ of this chip has a low threshold for spiking because of
transistor mismatch.
III.
PERIODICITY MEASUREMENT
We chose a database of “coo” and “hiss” sounds voiced
by 12 speakers for our experiment. We recorded both the
analog waveform from the microphone and the spike trains
of the cochlea. Figure 2 shows examplar responses of the 32
channels of the cochlea to these sounds as voiced by one
speaker. The periodicity of the spike patterns in the “coo” is
obvious while the spike patterns of the “hiss” do not show
this regularity. To compute the periodicity from the spike
times, we first calculated an all-order histogram of the
interspike intervals (ISIs) of the spikes. Using only a first-
order ISI histogram (that is, taking only the time difference
between adjacent spikes in each channel) will not give the
right period for the fundamental frequency because the low-
frequency channels spike more than once per cycle while the
high frequency channels spike once or none for a cycle. The
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peaks in the histogram will not reflect the period of a cycle
for low-frequency sounds.
For each speaker, we computed the ISI histogram of the
spike responses to the two sets of sounds. We tried different
order ISIs and found that the histogram of ISIs up to 7th order
give noticeable peaks (Figure 3 shows an example).
Including higher than 7th order ISIs does not change the
histogram profile noticeably. The first peak under 1ms
represents the ISIs of spikes from a single cycle and is
ignored. The next peak reflects the pitch of the speaker and
subsequent peaks represent the harmonics of the pitch
frequency.


Figure 3. Histogram computed from 1st to 7th order ISIs of cochlear spikes
from a single speaker voicing a “coo”. Peaks represent the harmonics in
the speaker’s pitch except for the first peak which is due to the ISIs of
spikes within a cycle.

To determine whether the pitch and harmonic frequencies
of harmonic sounds such as “coo” extracted from spike trains
are similar to the pitch information in the analog output from
the microphone, we plotted the fundamental peak from the
FFT of the analog waveform versus the fundamental
frequency computed from the FFT of the ISI waveforms
across all speakers (Figure 4). As seen, these points fall very
close to the unity line, even with the response variations
across the silicon cochlea frequency channels (Figure 2).
Even in the case of a sound file where the speaker had varied
his pitch in time while voicing the “coo” sound, the FFT of
the extracted pitch from the analog waveform and the spike
ISI histogram are almost equal (Figure 5).

IV. DISCRIMINATION
To discriminate between the two classes of sounds, we
select a fundamental frequency that is around 100 to 200 Hz
and we then look for multiples of this base frequency. Each
detected frequency corresponds to the inverse of the period
of a local peak in the ISI histogram. A local peak in the ISI
histogram meets two criteria: 1) it is significant because it
contains a sufficient number of samples to meet a set
threshold population level and 2) it is prominent because it
contains a sufficient number of samples more than
surrounding samples to distinguish it from the neighboring
region. We label a segment of sound as harmonic if its ISI
histogram has a local peak. For each 0.2s segment of the
sound, we classified it as a “coo”, “hiss”, or “undecided”.
Taking the majority of the hits in each of the 3 classes, we
determined if the speaker was voicing a “coo” or a “hiss”.
Using this approach and our database of 12 speakers, the
“coo” was correctly identified in 10 speakers and the same
was true of the “hiss”.

Figure 4. Data set of the 12 speakers (circles) showing that the
fundamental frequency for a steady pitch computed from the FFT of the
analog waveform vs the fundamental frequency computed from the ISI
histogram.


Figure 5. Data set of a speaker showing the correspondence for a time-
varying pitch between the frequency computed from the FFT of the analog
waveform and the frequency computed from the ISI histogram.
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V.
SOUND LOCALIZATION
We were able to use the outputs of the binaural cochlea,
each with input from its own microphone, to predict the
horizontal location of the sound source. When a sound wave
travels from a source to the two microphones, there is a
difference in the travel time to each microphone that is
visible in the recorded waveforms. This time difference can
be seen in the phase difference of the spikes between the two
cochleas.

Figure 6. Localization data where a speaker voicing the “coo” sound
moves continuously in time from the far end of one microphone to the
other microphone and back again. The ITD is computed from the spike
trains of the binaural cochlea.

We fed the outputs of matching channels from each
cochlea chip to a correlation algorithm that counts the
number of occurrences at a range of inter-spike delays. This
algorithm was also implemented in jAER. If the spike
outputs from corresponding channels are exactly in phase
with each other (for example, being fed input from the same
microphone), the algorithm gives a spike at 0us, and have no
output for any other inter-spike delay. For real signals the
data is somewhat noisier, but there is still an observable peak
at 0us delay. When a sound comes from one side of the
microphone pair, the spikes from the chip with the closest
microphone will lead the other, and there will be a peak in
the correlation algorithm’s output at the corresponding delay.
We combine information across frequency channels to
estimate the sound’s location. A naïve approach simply
weights each frequency channel equally. However it is also
possible to use available information about the stimulus, for
example the fundamental frequency and harmonic
composition of the sound as determined by the periodicity
measurement described in Section IV, to assign more weight
to frequencies that are known to be dominated by the
stimulus of interest. This can serve to reduce the effect of
the corruption of phase information by background noise and
competing sound sources.
VI. CONCLUSION
In this work we demonstrated a system that uses the
timing of spike outputs from a binaural silicon AER cochlea
to determine the harmonicity of a sound. We present data
that shows that the harmonicity information in the spike
trains was compatible with the information in the original
analog waveform, even with the variance in the ISIs across
the different frequency channels of the silicon AER EAR.
This information can be used for distinguishing the sex of the
speaker [10] or to distinguish between two classes of sounds.
This harmonicity computation can be combined with a
localization module which uses the interaural time difference
information in the spike trains from the binaural cochlea. The
subsequent system consisting of the silicon AER cochlea and
the jAER program can detect the location of a particular
class of sounds in real-time. This approach is important
because it shows the temporal information can be used for
perceptual experiments, even in the face of imperfect
cochlear filters.
ACKNOWLEDGMENT
We acknowledge Tobi Delbruck for help with jAER, and
the Telluride Neuromorphic Workshop for providing the
infrastructure to execute this project.
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