The effects of stereotyped behaviour on orienting and tonic cardiac activity.

The relationship of cardiac activity to stereotyped behaviour was studied in profoundly mentally retarded persons, using both inter-group comparisons. The first intra-group comparison involved examining the relative magnitude of an orienting response to an auditory stimulus during the occurrence or nonoccurrence of stereotyped body rocking. While no differences were found between periods of rocking and non-rocking when behavioural measures of orienting were used, smaller changes in post-tone heart rate were observed during periods of stereotyped rocking. A second intra-group comparison found that stereotyped behaviour was associated with a significant increase in heart rate but no change in heart rate variability when compared to periods of no stereotyped activity. Finally, using an inter-group comparison, individuals who displayed stereotyped body rocking exhibited higher tonic heart rates and lower heart rate variability during periods of no stereotyped behaviour than was observed in matched control subjects. This result suggests that stereotyped behaviour may be correlated with reduced vagal tone.