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We examined the relationship between tonic—a correlate of self-regulatory functioning—and phasic cardiac vagal
activity (indexed by heart rate variability; HRV) during a selective attentional task with varying levels of load.
Participants detected a target letter among letter strings superimposed on either fearful or neutral face distractors. Letter
strings consisted of six target letters under low load and one target letter and five nontarget letters under high load.With
fearful distractors, lower tonic HRV was associated with phasic HRV suppression, suggesting an autonomic stress
response under both low and high load. In contrast, higher tonic HRV was associated with phasic HRV enhancement,
suggesting greater self-regulatory effort under low load and an absence of phasic HRV suppression under high load. The
current research suggests that tonic cardiac vagal tone is associated with the ability to flexibly adapt autonomic responses.