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Separation of Auditory Evoked Responses to the Right- and Left-Ear
Inputs
S. Kuriki
1
, H. Kurumaya
2
, K. Tanaka
2
and Y. Uchikawa
2
1
Research Institute for Science and Technology, Tokyo Denki University, Inzai, Japan
2
Department of Science and Engineering, Tokyo Denki University, Hatoyama, Japan
1 INTRODUCTION
In the clinical examination of auditory cortical
function, it is desirable to observe the evoked
responses separately to the right- and left-ear inputs
during binaural stimulation. Due to the dominance
of contralateral auditory pathway, it is expected that
the right ear response represents the function of the
left central auditory system and vice versa.
However, the response to the input of the same-ear
side is also included due to the conduction through
ipsilateral pathway. Difficulty exists how to
discriminate the mixed responses in the auditory
cortex into their input channels. From a view point
of basic science, separate observation of the two
ears’ responses to different acoustic stimuli such as
tones and speech sounds may be interesting to study
the hemispheric difference in the auditory function
(Zatorre and Belin, 2001). Here, we developed a
novel method for the separation of evoked responses
based on synchronous and asynchronous averaging
of signals.
2 PRINCIPLE
In the conventional averaging of evoked responses,
recorded signals, which are mixed with biological,
environmental and sensor noises, are averaged with
a trigger time-locked (synchronous) to the stimulus
epochs. This is based on the fact that noise signals
are asynchronous to the stimuli and thus attenuated
by averaging. Transient evoked responses are
usually phasic, being composed of several peaks
changing in polarity, e.g., P1, N1, P2, N2
components, of auditory response. It is expected that
such phasic responses are attenuated in amplitude
after averaging over many epochs if we use such
triggers that are asynchronous to the stimulus epochs.
This attenuation should be maximal when the jitter
of the trigger time extends to the whole response
period that may contain multiple peaks. For the
relevant response to be observed, synchronous
triggers are used besides the ongoing asynchronous
averaging, reducing the noise but not the signal.
We prepared two sets of time series of onset-
triggers in the simulation, where each set had
random onset-to-onset intervals varying in a period
of 500 ms. Figure 1a shows the distribution of the
onset intervals of the first trigger series, where bars
indicate the number of triggers included within time
bins of 50 ms width. The trigger onsets were
distributed uniformly in the 500 ms period, keeping
random intervals. It was confirmed that the second
trigger series prepared also had similar distribution.
(a)
(b)
Figure 1: (a) Distribution of the onset-to-onset intervals of
the triggers used in the simulation and (b) waveforms of
the signals averaged using synchronous and asynchronous
triggers.
As the signal, we used a waveform of auditory
evoked response consisting of several peak
components superposed with white noises. Signal
epochs of 500 ms length were generated repeatedly
at the onset of triggers of the first series. Averaging
Kuriki S., Kurumaya H., Tanaka K. and Uchikawa Y..
Separation of Auditory Evoked Responses to the Right- and Left-Ear Inputs.
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2013 SCITEPRESS (Science and Technology Publications, Lda.)