Biomedical Engineering Reference
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3% for contralateral) distraction had no effect on
the neural response. Finally, in considerable num-
ber of cases (27%) distraction at the contralateral
side led to the coherence enhancement. The same
effect for distraction on the ipsilateral side has
been observed in 4% cases only.
chance. The latter can be tested by Monte-Carlo
simulation with surrogate spike trains obtained
by shuffling inter-spike intervals.
Neurons can communicate using time averaged
firing rate (Shadlen & Newsome 1998). However,
precise and reproducible spike timing is also fre-
quently observed in the central nervous system,
raising the likelihood that stimulus elicited firing
patterns encode sensory information (Abeles et
al. 1993; Bair & Koch 1996; Mainen & Sejnowski
1995). Thus, analysis methods studying the
temporal coherence between evoked spikes and
their stimulus may be an important tool to reveal
synaptic plasticity in a neuronal network.
FUTURE TRENDS
Although some rough description of the somato-
sensory loop could be obtained on the basis of
analysis of PSTHs, methods from the rapidly
growing fields of nonlinear dynamics and complex
systems theory can provide new insights on the
underlying complex biological processes (Pavlov
et al., 2006). In this line we have shown how the
wavelet approach for analysis of non-stationary
signals can be adapted to investigation of spike
trains and can be used for quantification of the
functional stimulus-neural response coupling.
The coupling between two signals (i.e. stimulus
and neural response) can be detected by means of
the conventional spectral coherence. However, its
Fourier transform origin limits its practical appli-
cation to the neural spike trains. Here the wavelet
coherence is better suited and can be directly
evaluated from the spike trains. Similarly, we
have studied the stimulus response coherence of
the firing patterns in gracilis nucleus and its time
evolution along stimulation epochs (Castellanos
et al., 2007). This analysis has revealed that the
coherence enhancement can be more relevant in
sensory processing than an increase in the number
of spikes elicited by the tactile stimulus.
For the optimal use of the wavelet coherence
it is desirable: (1) to tune the Morlet parameter k
according to the problem, and (2) to test coher-
ence significance. For the constant frequency
stimulations (1 Hz) we have found that the Morlet
parameter k = 2 allows better resolving stimulus
induced firing rhythm. The coherence significance
affects our (statistical) believe at which extent the
observed functional associations are not due to a
CONCLUSION
The somatosensory cortex may enhance relevant
stimulus, as it occurs in the dorsal column nu-
clei (Malmierca & Nuñez, 1998, 2004) or may
decrease sensory responses in the trigeminal
nuclei when a novel (distracter) stimulus is ap-
plied (sensory-interference). This effect evoked
by distracter stimuli applied outside of the RF
of the recorded cell is observed even when the
distracter stimulus is applied on the other side
of the face. Consequently, these data suggest the
existence of an attentional filter at this early stage
of sensory processing. To evaluate this complex
modulation of sensory responses we have cre-
ated a new mathematical tool to reveal dynamic
changes of synaptic interactions into a neuronal
network.
By analysis of the wavelet power spectra we
have shown that tactile stimulation of the neuron
RF, as it was expected, strongly increases the
spectral power in the stimulus frequency band.
We have further demonstrated that the stimulus re-
sponse coherence was decreased in about 86% for
ipsilateral and in 70% for contralateral distractions
with respect to the coherence measured in control
stimulation. Moreover, the analysis showed that
the coherence decay was larger immediately after
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