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the application of the distracter stimulus and then
the stimulus coherence recovered but without
reaching the value observed in the control condi-
tions. According to previous results about synaptic
plasticity, the decrease in the coherence has an
important implication since it means a decrease
of the temporal correlation between the tactile
stimulus and the evoked spike train.
This decrease can be interpreted as sensory
interference, as it has been previously described
in the SI cortex (Alenda & Nuñez, 2004, 2007). In
addition, it has been observed in the rodent cortex
that deflection of ipsilateral whiskers suppresses
cortical responses evoked by the contralateral
whisker stimulation (Shuler et al., 2001). These
data are consistent with the model where the
evoked sensory activity from ipsilateral whiskers
inhibits cortical responses to the stimulation of its
RF that is located in the contralateral whiskers.
According to this effect, the absence of ipsilateral
whiskers to the somatosensory cortex increases
the RF of the contralateral whiskers (Glazewski
et al., 2007).
Sensory interference was observed as a de-
crease of the spike number elicited by a whisker
deflection as well as a change in the temporal cor-
relation between the stimulus onset and the evoked
spike train (coherence). Both events are crucial
in the sensory transmission since synchronized
stimulus-driven discharges will induce EPSPs
within a narrow temporal window in thalamic
cells, increasing chances of generating spikes
to be further transmitted to the somatosensory
cortex.
Sensory interference evoked by ipsilateral
distracter stimuli could be due to synaptic inter-
action into the trigeminal nucleus. However, the
fact that a contralateral distracter stimuli also
induce sensory interference in SP5C neurons,
strongly suggests that this effect is mediated by
corticofugal projections from the somatosensory
cortex since there are no projections between
trigeminal nuclei or thalamic nuclei of each side
(Killackey, 1973).
Our results can be consistently interpreted
under the assumption that somatosensory corti-
cal neurons receive information from their con-
tralateral RF and also from other RFs located in
the contra- or in the ipsi- lateral side of the body.
We propose that interhemispheric connections
through the corpus callosum are a likely source
of these sensory inputs. Callosal projections in
somatosensory cortex are thought to be roughly
homotopic (Olavarria et al., 1984) although
other studies have indicated that there are few
direct connections between barrel fields (Arm-
strong-James & George, 1988) and that callosal
projections from the barrel field can be highly
distributed over many different contralateral
cortical areas, including SII and the motor cortex
(Welker et al., 1988). Consequently, data suggest
the existence of a complex sensory processing in
the somatosensory cortex that modulates sensory
responses in subcortical relay stations according
to the appearance of novel sensory stimuli.
Resuming, cortical feedback enhances corti-
cally relevant stimuli while decreases other senso-
ry stimuli. This effect has been called “egocentric
selection” (Jen et al., 2002) and play a pivotal role in
the control of the sensory information that reaches
the thalamus and cortex. Our results show that a
distracter stimulus may reduce sensory responses
in trigeminal neurons. For these reasons, a sug-
gested role for the corticofugal feedback system
is that it may contribute to selective attention
(Malmierca & Nuñez, 2007). Selective attention
is defined as the cognitive function that allows
the focusing of processing resources onto the
relevant sensory stimuli among the environmental
information available at a given moment, while
other irrelevant sensory information is largely
ignored. Different studies have shown that, in
addition to enhancing neuronal processing for
attentionally relevant stimuli, selective attention
also involves suppression of the sensory process-
ing of distractive stimuli (Reynolds et al., 1999;
Smith et al., 2000). Thus, corticofugal projections
act as an attention filter that enhances relevant
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