Dissertation

Der Effekt des Alterns auf funktionelle Deaktivierung im somatosensorischen System

Sohns, Jan Martin

2009

Overview

Functional magnetic resonance imaging, based on the BOLD effect, typically involves looking for positive cortex activation during a specific task in a controlled state. Tactile stimulation via peripheral electrical nerve stimulation results in measurable activation of the somatosensory system in fMRI studies. In addition to the positive BOLD signal changes in the contralateral, primary cortex (Figure 25), there is increasing evidence that negative BOLD responses can also occur during this stimulation (Figure 26). Although the general opinion in the literature is that these neuronal BOLD signal changes represent a suppression of neuronal activity, direct evidence for this hypothesis in different age groups is rare. For the somatosensory system, it has long been possible to carry out quantitative measurements of sensory function during simultaneous activation and stimulation. In the past, this system was very well characterized and represented using fMRI technology.With this study, we were able to demonstrate the effect of aging on functional deactivation in the somatosensory system in connection with ipsilateral NBR and neuronal inhibition during unilateral sensory stimulation. We selected a younger and older group of subjects in order to be able to observe and understand possible potential aging processes. For this reason, we took measurements of positive and negative BOLD responses in the somatosensory system as well as other important cortical areas during unilateral electrical stimulation of the right median nerve at a frequency of 40 Hz. In addition, we determined the perception threshold CPT of the left index finger during right-sided electrical stimulation of the median nerve as a quantitative measurement of sensory function. Furthermore, we had the subjects actively compare their fingers on the hand where we had previously performed the passive stimulation during the FT. The FT showed that the older subjects did not generally show a worse hemodynamic response, but at least an equivalent response compared to the younger ones (Figures 18 to 22). As previously suspected, significant increases in the BOLD signal (PBR) were observed in contralateral, primary (Figure 25) and bilateral, secondary, somatosensory areas (Figure 27) in the younger and older subjects, whereas a decrease in the BOLD signal (NBR) in the ipsilateral, primary, somatosensory cortex (Figure 26) during electrical median nerve stimulation was more pronounced mainly in the younger subjects. In contrast to the older subjects, these NBRs correlated very closely with an increase in the CPT of the contralateral, left and unstimulated index finger (Figures 43 and 44). This finding supports the hypothesis that the ipsilateral NBR reflects a functional and effective inhibition in the somatosensory system, which is largely lost in old age. Active motor-induced tasks using FT showed that the older subjects, compared to the younger ones, do not necessarily activate hemodynamically less, but in some cases more clearly (Figures 20 and 21), expanding and undifferentiated cortical areas with in some cases higher maximum values. There is a change from lateralized, inhibited and differentiated activities in the younger subjects in mainly primary and secondary sensorimotor cortex areas to a compensatory recruitment of additional cortex areas with a reduction in inhibition in older age in order to possibly ensure the same outcome under controlled stimulation as in younger age.Based on our results and those of previous studies, we hypothesize that electrical stimulation of the median nerve leads to reduced transcallosal inhibition of ipsilateral sensorimotor cortex areas in older age. This suggests that older age may play an important role in processing in the human cortex in relation to various peripheral stimuli and should be increasingly taken into account when interpreting fMRI data with BOLD signals, especially in studies that find increased activation of cortical areas in older subjects and deactivation in younger subjects.

• Publisher: ProQuest Dissertations & Theses
• Subject: Health sciences / Medical imaging / Medical research / Scanners
• ISBN: 9798308114178