Page 89 - Bladder Dysfunction in the Context of the Bladder-Brain Connection - Ilse Groenendijk.pdf

Single subject and group whole-brain fMRI mapping of male genital sensation at 7 Tesla 87
mainly discriminative brain regions. In addition, functional connectivity was assessed between activation clusters for both the genitalia and feet. This is the first study to report on functional connectivity of genital sensation.
Some have described the representation of the genitalia to be positioned in the medial wall below the representation of the feet in S1,4-6 while others describe a more dorsolateral representation between the trunk and leg.7,8 At both single subject and group level, our data clearly indicates that the genitalia are represented dorsolateral of the feet in S1 (Fig. 1 and Fig. 2), similar to what has been reported by previous studies us- ing 3T fMRI.7,8 Animal studies investigating genital representations have also described this location measuring extracellular recordings in primates23 and more recently using cortical microstimulation in rats.24 Despite applying unilateral stimulation to the penile shaft, we observed bilateral activation in S1 irrespective of stimulation side. This corre- sponds well with findings showing that cutaneous regions situated in the midline of the body are represented bilaterally in S1.25 Interestingly, studies using electrical stimulation of the DNP to locate the genitalia in S1 repeatedly demonstrated activation deep in the interhemispheric fissure. Cortical evoked responses elicited by electrical stimulation of the DNP were consistently located beneath those elicited by stimulation of the posterior tibial nerve.4-6 It should be noted, however, that earlier techniques (e.g. EEG/MEG) used to measure brain responses offered poor spatial resolution. In addition, it is known that differences in evoked brain potentials can be observed when comparing electrical to tactile stimuli, further questioning this method when investigating the processing of physiological somatosensory stimuli.26 In the present study, passive tactile stimulation of the medial aspect of the feet served as a control, analogous to electrical stimula- tion of the posterior tibial nerve. We expected to see activation in S1 lateralize in the contralateral hemisphere, as can be seen during stimulation of the left foot (Table 1). During stimulation of the right foot, however, significant yet weaker activation was also observed in the ipsilateral hemisphere (Table 1). Absence of lateralization in S1 has also been demonstrated during tactile stimulation of only the right and not the left hand in right-handed subjects.27 The authors suggested this asymmetry is associated with hand preference and proficiency. Humans not only have a preference for left- or right- handedness, but also left- or right-footedness which can be seen in for instance football players.28 In the present study we did not assess footedness prior to inclusion, however, we suggest ipsilateral activation in S1 during stimulation of the right foot may be the result of right-footedness.
Activation in S1 evoked during tactile stimulation of the feet extended anteromedial along the postcentral gyrus. Beneath this activation cluster, in most single subjects and at group level, activation was also observed deep in the interhemispheric fissure in response to tactile stimulation of the genitalia (Fig. 2; medial-view left hemisphere). At
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