Heart rate variability biofeedback (HRVBF) is a promising
anxiety-reducing intervention that activates the autonomic nervous
system through paced breathing, using feedback from heart rate
variability (HRV). Many studies have reported the anxiety-reducing
effects of HRVBF; however, reports on the effectiveness of training have
been negative. Furthermore, the effects of training and the underlying
brain activity changes remain unclear. This study aimed to examine the
anxiety-reducing effects of HRVBF training and related brain activity
changes, by randomly assigning participants, employing an active control
group, and measuring anxiety-related attentional bias using the
emotional Stroop task and EEG. Fifty-five right-handed healthy young
students with anxiety were randomly assigned: finally, 21 to the HRVBF
group and 19 to the control group. Both groups performed 10 training
sessions of 20 minutes per day within three weeks and were assessed
using HRV, EEG, attentional bias, and the State-Trait Anxiety
Inventory-JYZ (STAI-JYZ), before and after training. Results showed
increased HRV in the HRVBF group compared with the control group, but no
differences in EEG, attentional bias, and STAI-JYZ. Further analyses
revealed that participants with higher pre-training HRV achieved higher
heart rhythm coherence in HRVBF training and had reduced attentional
bias. This study suggests that individuals with higher HRV are more
likely to be proficient in HRVBF training and benefit from its
anxiety-reducing effects. The findings contribute to an understanding of
participant selection and modification of HRVBF training protocols for
non-responders.