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• The conditional entropy technique enabled simultaneous analysis of heart-brain rhythms. • Directional coupling information C (heart→brain) and C (brain→heart) differentiated internally and externally operative attention. • Attention states were examined using simultaneous recording of EEG and ECG signals. • A modified Posner's spatial orienting task used to assess A I and A E attention. • Internal and external attention states investigated using identical stimulus design. Heart and brain interactions mediate human cognition. This investigation identifies heart-brain interactions during internally operative attention (A I) and externally operative attention (A E). A I attention involves short term memory, whereas A E attention deals with automatic and transient response to objects in the external world. A modified Posner's spatial orienting task used to differentiate A I and A E attention. Heart and brain rhythms recorded in fourteen healthy participants. Functional coupling from heart-to-brain (C h e a r t → b r a i n) and brain-to-heart (C b r a i n → h e a r t) time series derived using an information domain approach based on conditional entropy. The experimental results showed that low-frequency power of heart rate variability (HRV-LF) and sympathovagal balance (LF/HF ratio) during A E significantly increased compared with that for A I. Furthermore, the information flow from heart-to-brain increased and decreased form brain-to-heart during A E as compared to A I. Also, opposite trend in relationship noted between coupling index (C i → j) and HRV-LF during A I and A E attention. The conditional entropy technique enabled simultaneous analysis of heart-brain rhythms to identify heart-brain interactions during A I and A E attention. [ABSTRACT FROM AUTHOR]