Phasic T-Wave Amplitude and Heart Rate Changes as Indices of Mental Effort and Task Incentive

The sensitivities of T-wave amplitude (TWA) and heart rate (HR) to manipulations of the mental effort (ME) and incentive associated with performance of the backward digit span task were investigated. The experimental task was so chosen because it could be administered without requiring subjects to speak during its performance, thus minimizing the intrusion of respiratory and motor artifacts. The ME required during task performance was manipulated by varying the number of digits presented, creating Hard and Easy trials. The incentive for subjects to perform well was varied by the threat of the delivery of a noxious noise stimulus, which was ostensibly contingent upon adequate performance on some (Test) but not other (Practice) trials. TWA attenuation and HR acceleration were greater on the Hard as compared to the Easy Trials, and on the Test as compared to the Practice trials. No interactions between the ME and Incentive factors emerged for either cardiac variable. These results suggest that the effort and the incentive of subjects during mental work may be indexed reliably by phasic changes in TWA and HR. The conjoint use of these two cardiac measures may also elucidate the involvement of the two autonomic branches during task performance.