Biomedical Engineering Reference
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conditions showing a decrease of tactile responses
during the application of a distracter stimulus.
The spontaneous firing rate of trigeminal cells
was not modified after sensory-interference tri-
als compared to control values (0.9±0.9 spikes/s
vs. 1±0.9 spikes/s). In all cases, the latency of the
tactile responses was not altered by the distracter
stimuli. This reduction of the number of spikes
evoked by tactile stimulus (about 20%) in a large
neuronal population of SP5C nucleus represents a
significant decrease of the sensory transmission
when different sensory stimuli occurred at the
same time.
Our results demonstrate a clear tendency to
decrease sensory responses in PSTHs during
sensory-interference. However, changes in the
spiking pattern evoked by the stimulus could not
be studied with this method. The spiking pattern
evoked by the stimulus is crucial in the sensory
transmission because many studies have demon-
strated that different types of sensory plasticity are
elicited according to the presynaptic input pattern.
Thus, we use the wavelet analysis of spike trains
to describe the neural behavior in both time and
frequency domains.
has no well defined dominant periodic activity.
Tactile stimulation of the vibrissa leads to the
neural response with a pronounced peak in the
PSTH (Figure 2A). This suggests the presence of
a rhythm at the stimulus frequency imposed by
the stimulation in the neural response. Indeed,
the distribution of the power in the control condi-
tions (Figure 3B) shows a consistent band at the
stimulus (1 s) period lasting over all recording.
Thus stimulation evokes a stable rhythm in the
neural firing, i.e. the neural firing is function-
ally associated to the stimulus and moreover the
strength of this coupling is pretty constant over
all 30 vibrissa deflections. Sensory distraction
clearly influences the stability of the neural re-
sponse to the vibrissa deflection (Figure 3C and
3D). The power of 1s period rhythm is decreased
and becomes oscillatory, i.e. the peak amplitude
is not persistent in time but instead exhibits a low
frequency oscillation. For instance in Figure 3C
(ipsilateral distraction) the power maxima are at
11 s, 20 s, and 32 s, which corresponds to about
0.1 Hz oscillation. Such oscillation of the spectral
power are explained by the fact that during dis-
traction the neural response to the same vibrissa
deflections is not the same over time but instead
has some variability, i.e. the neuron fires essen-
tially different number of spikes with different
inter-spike intervals to the same stimulus events
along the stimulation epoch. We note that this
dynamical behavior cannot be inferred from the
PSTH (Figure 2). Comparing the spectral power
at 1 Hz in different experimental conditions we
can quantify the degree of influence of the tactile
distraction on the stability (coupling) of the neural
response to vibrissa deflections. However, for this
purpose the wavelet coherence is more suitable.
The wavelet coherence allows quantifying the
level of coupling between two systems analyz-
ing their activity. In our case the stimulus onsets
eventually evoke the firing activity of the neuron.
Then we can speak about unidirectional stimulus
- neural response coupling. To test the effect of
the ipsi- or contralateral sensory-interference in
Wavelet Analysis
The wavelet power spectrum described in the
equation (6) allows us to study the spectral prop-
erties of a spike train and how these properties
change in time. Figure 3 shows a representa-
tive example of the wavelet power spectrum of
neural spike-train in spontaneous condition and
during response to vibrissa deflection in control
conditions and during ipsi- and contra-lateral
distractions.
Spontaneous firing is irregular showing no
repetitive temporal patterns. Thus, maxima of the
spectral power are localized at the spiking events
and do not form long lasting structures (Figure 3A).
Only between 20 s and 25 s we observe a weak
rhythm with period about 0.5 s overlapping with
1.5 s period. Thus spontaneous spiking activity
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