Tactile and Non-tactile Sensory Paradigms for fMRI and Neurophysiologic Studies in Rodents - Dynamic Brain Imaging - page 221

The rat relies heavily not only on its exquisite sense of smell, but
also on its vibrissa (i.e., whisker system). On either side of its face,
the rat has approximately 35 whiskers organized in a grid-like
pattern of rows and columns. The roots of these are long hairs
wrapped in mechanoreceptors sensitive to minute vibrations. The
signals generated by motion of the whiskers are transmitted via
the trigeminal nerve to the ventral posterior lateral nucleus of the
thalamus to whisker-specific regions in the primary somatosen-
sory cortex. The conspicuous anatomical detail of densely packed
clusters of cells, forming a topographic map of the contralateral
whisker pad (86) led to the concept of “barrel” cortex (87) .Dur-
ing exploration of their environment, rats actively make contact
with objects with their whiskers by voluntary head motions and
facial muscle activity. The combined multi-parametric effects of
the amplitude, frequency, and motion of whisker deflections upon
impact provide the sensation of the object.
The whisker stimulation is a difficult model to apply repro-
ducibly inside the magnet primarily because of space constraints.
Nevertheless, our prior approach for whisker stimulation inside
the magnet (25, 62) using Lorentz force to move a small con-
ducting copper wire attached to the whiskers is unfortunately
not easily reproduced identically outside the magnet. A good
alternative is mechanical movement of whiskers (88) , but this
works poorly inside a magnet. Since this method requires a non-
magnetic manipulator which must span the entire length of the
bore, dexterity becomes a problem and, therefore, there is lim-
ited control of the amplitude and/or frequency of the whisker
stimulation. Thus, we developed a non-magnetic whisker stimu-
lation device using air puffs (63) which can be easily used - either
simultaneously or concurrently - with almost any other sensory
stimuli (e.g., forepaw, olfactory, visual), both inside and outside
the magnet, designed specifically for multi-sensory experimental
paradigms. The frequency of whisker stimulation with this device
could range from a few Hz to as much as 40 Hz, which seems to
be the upper limit of naturalistic whisker stimulation (89) .
In the same rat or across a group of rats, whisker stimula-
tion induced multi-modal responses (of BOLD, CBF, and MUA)
in the contralateral S1 BF ( Fig. 10.5) . The induced response
seemed to peak at stimulation frequencies between 8 and 12 Hz
( Fig. 10.6 ) which is within the natural whisking frequency in
rodents (89) . Earlier studies (90) have shown that CBF in the
rat S1 BF increases linearly with higher whisker movement fre-
quencies. However, future studies should investigate the neu-
rophysiologic basis of the increased area of BOLD activation
observed at the higher stimulation frequencies ( Fig. 10.6) .In
summary, unilateral (and bilateral) whisker stimulation with air
puffs resulted in fMRI and neurophysiologic signal increases in

4.1.3. Tactile: Whisker
Stimulation