The frequency dependencies of whisker and forepaw stimuli have not been demonstrated previously in the same animal by fMRI, perhaps due to technical challenges in the experimental setup. This study demonstrates BOLD activations in distinct contralateral somatosensory areas during independent whisker and forepaw stimuli at varying frequencies. In the same subject, stimulation of whisker and forepaw gave rise to significant BOLD signal increases in corresponding S1BF
areas, respectively, with no significant spatial overlap between these regions (). The success of being able to apply both stimuli to the same subject relied on a home-built non-magnetic, air puff stimulator for moving the whiskers. Air puffs allowed precise control of stimulus onset, frequency, and duration thereby facilitating reproducible and consistent stimulation to the same subject in conjunction with forepaw stimulation (). We used this method to stimulate even a single whisker reproducibly in the same subject (, bottom row). It should be noted, however, that prior studies of S1BF
) have shown that the cortical responses (e.g., blood flow or neuronal activity) to stimulation of an individual whisker vs. several rows of whiskers are non-linear, both in terms of magnitude and area of activation.
The BOLD responses in S1BF
were found to be variable with stimulus frequency of each type (). The peak magnitude of the BOLD response was larger with forepaw stimulation (). However the peak area of activation for the two stimuli were quite comparable () which partially agrees with previous assignments by Welker (34
) who showed that 16–20% of the somatosensory cortex is designated each for the limbs and whiskers.
In the S1BF
, the BOLD response magnitude increased linearly with whisker stimulation frequency up to 12 Hz and plateaued above 12 Hz (12 – 30 Hz) (). The area of activated pixels in the S1BF
at the different stimulation frequencies followed a similar pattern (). In agreement with these observations, previous studies have reported that blood flow (35
) increases linearly in the rat somatosensory cortex with whisker movement frequency of 1.5 to 10.5 Hz, whereas the total spike rate (36
) increases linearly with whisker movement frequency of 12 Hz and beyond which the electrical activity is slightly decreased. Since whisker stimulation frequency range of 4–12 Hz correspond to active whisking frequencies (36
), it is expected that maximal responses to whisker movement will be within this range as has been observed by a variety of methods (37
). However it should be categorized that fMRI responses to electrical stimulation of the whisker pad directly may differ from physical movement of whiskers (39
In the S1FL
, the magnitude of the BOLD response was largest at forepaw stimulation frequency of 1.5 – 3.0 Hz, beyond which the response diminished with little or no activity at frequencies higher than 20 Hz (). The volume of tissue activated followed a similar pattern as the magnitude trend (). The impact of the forepaw stimulation frequency on BOLD or related hemodynamic responses has been extensively investigated (3
). Our current results are consistent with these results where the maximal response is detected with stimulation frequencies of 1–3 Hz.
Cortical perception of the physical world relies on the formation of multi-dimensional representation of stimuli impinging on the different sensory systems. Activation studies in experimental neuroscience assume that a sensory stimulus may have very different neurophysiologic outcome(s) when paired with a near simultaneous event in another modality (45
). Before approaching this level of complexity in future fMRI studies, reliable measures must be obtained of the relatively small changes in the BOLD signal and other neurophysiologic markers (electrical, blood flow) induced by different peripheral stimuli. The demonstration of both whisker and forepaw stimuli given to the same subject, which can be applied identically both inside and outside the magnet, may be used in studies of multi-sensory interactions in anesthetized rats, en route
to a rudimentary understanding of the functioning brain where various sensory cues presumably interrelate. This model can also be used for mapping of adjacent somatosensory representation by fMRI and to study cortical reorganization or plasticity (47