The respiratory phase causally modulates the readiness potential amplitude
Previous research has shown that the respiratory phase correlates with both voluntary action timing and readiness potential (RP) amplitude, but whether this relationship is causal or merely correlational remains unclear. Here, we experimentally manipulated breathing patterns to test if the respiratory phase causally influences the neural preparation for voluntary movement. Participants performed self-initiated button presses under four conditions: breathing in (BI), breathing out (BO), normal breathing (NB), and breath-holding (BH). Electroencephalographic recordings revealed that RP amplitude was significantly more negative during exhalation compared to inhalation and during breath-holding compared to normal breathing. These neural differences occurred without corresponding changes in behavioral measures (e.g., waiting times or retrospective timing judgments), indicating that respiratory effects on RP were not associated with altered action timing. These findings demonstrate that the respiratory phase causally modulates cortical motor preparation. We further propose that the brain optimizes voluntary action execution during respiratory phases when breathing-related motor activity is minimal, thereby revealing respiration as a fundamental organizing rhythm for voluntary behavior.