A rational strategy for the automated detection of artifacts in heart period data is outlined and evaluated. The specific implementation of this approach for heart period data is based on the distribution characteristics of successive heart period differences. Because beat-to-beat differences generated by artifacts are large, relative to normal heart period variability, extreme differences between successive heart periods serve to identify potential artifacts. Critical to this approach are: 1) the derivation of the artifact criterion from the distribution of beat differences of the individual subject, and 2) the use of percentile-based distribution indexes, which are less sensitive to corruption by the presence of artifactual values than are least-squares estimates. The artifact algorithms were able to effectively identify artifactual beats embedded in heart period records, flagging each of the 1494 simulated and actual artifacts in data sets derived from both humans and chimpanzees. At the same time, the artifact algorithms yielded a false alarm rate of less than 0.3%. Although the present implementation was restricted to heart period data, the outlined approach to artifact detection may also be applicable to other biological signals.

John Hughlings Jackson believed that with the phylogenetic and ontogenetic development of the nervous system, higher neural systems re-represent and elaborate on lower brain mechanisms.' According to this view, the functional contributions of lower mechanisms are not bypassed or replaced by higher systems. Rather, lower mechanisms continue to participate in adaptive functions, serving as the basic substrates through which newer re-representative systems achieve expression. Consistent with this suggestion is the fact that decerebrate animals retain many behavioral capacities, including righting, locomotion, eating, grooming, sexual responses, and escape and aggressive behavior^.^-^ In addition, decerebrate animals have demonstrated both habituation and associative learnir~g.~-~ Similarly, decerebrate humans also retain many of the adaptive capacities of the normal neonate. In the absence of the cerebral hemispheres, anencephalic and hydranencephalic infants demonstrate typical infantile reflexes, pleasure and aversive reactions, and ingestive behaviors, including the stereotyped oro-facial acceptance and rejection responses characteristic of normal infants.*"O Consistent with animal studies, habituation and associative learning have also been reported in the decerebrate human Taken together, these findings document the remarkable behavioral capacities of brainstem circuits.The appearance of grooming behaviors in the decerebrate animal clearly indicates that many of the neural substrates for grooming responses are organized at lower levels of the neuraxis.z Although the grooming behaviors of the decerebrate have not been systematically characterized, these responses often appear somewhat fragmentary or poorly directed.' One significant consequence of decerebration, however, is the appearance of extensor rigidity that can severely interfere with the expression of adaptive behaviors. This and other acute effects of decerebration may obscure the adaptive potential of brainstem circuits and confound interpretation of the decerebrate state. Since decerebrate rigidity partially resolves with time, one approach to this problem has been to maintain decerebrated animals beyond the immediate postsurgical period. Significantly, the chronic decerebrate has been reported to show a dramatically enhanced behavioral repertoire relative to the acute decerebrate state. An additional approach has been to employ immature animals in decerebration studies, since denervation phenomena and decerebrate rigidity are much diminished in this Consequently, the behavioral features of the immature decerebrate may provide a less confounded view of brainstem f~n c t i o n s .~.~ ' This work was supported in part by USPHS grant no. M H 25630 to G. G. Berntson. 350 BERNTSON er al.: BRAINSTEM SYSTEMS 351 FIGURE 1. Neuraxial level of decerebration. Brain transections for all animals were located within the shaded zone. (Brain diagram drawn from sagittal plane, 0.2 mm, of the 39-day-old rat brain, based on Sherwood, N. M. & P. S. Timiras. 1970. A Stere...