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DISCUSSION
In the present study DS subjects showed lower SCL and higher heart rates, compared with age matched, normal subjects. Also, DS subjects showed a decrease in finger plethysmogram amplitude during exposure to auditory stimuli, but no other autonomic changes. Though more subjects showed abnormal SSRs to auditory stimuli, they were not significantly more than those who did habituate.
A lower SCL (higher skin resistance) is correlated with lower sympathetic sudomotor activity suggesting that baseline electrodermal responsivity is lower in DS as was described previously as part of the OR (5, 6). Some of the present DS subjects also showed abnormal SSRs to repeated auditory stimuli, with 10 subjects showing no habituation, 7 subjects showing no SSR, and 8 subjects showing the expected response, i.e., habituation.
The higher resting heart rate in DS and the reduction in finger plethysmogram amplitude (i.e., cutaneous vasoconstriction) (9) during auditory stimuli suggest that cardiovascular sympathetic tone is not lower, but possibly exaggerated in DS, which may be related to the hyperkinetic behaviour which is frequently associated with this syndrome (10). Also, our previous preliminary results (11), showed that the sole difference in the HRV spectrum components between DS and NS was that the HF component peak power was higher in DS. This suggests that activity in the HF band is well within the RSA (Respiratory Sinus Arrhythmia), vagal band of activity. However, this does not suggest any change in cardiac sympathetic-vagal balance.
Hence in DS, there appears to be selective reduction of sympathetic sudomotor activity with either no change or an increase in cardiovascular sympathetic activity. This is in keeping with the accepted idea that different subdivisions of the sympathetic nervous system may be active differently (12). The inadequate sympathetic activity in some subdivision (i.e., sudomotor) in DS may explain the inability of these subjects to focus attention. Electrodermal activity has already been reported to be abnormal in subjects who lose their ability to attend to and recognize familiar information (13). Hence the sudomotor dysfunction in DS may be related to the inability of these persons to focus attention and learn normally, especially since recognition and recall are essential to learning and memory (14).
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