When interacting with the complex and rapid change environment, human brains often face the challenge that not only abundant external, sensory information but also sophisticated internal information need to be processed in real time with limited cognitive resources. Attention shifts have been demonstrated as an effective approach to dynamically allocate the brain’s processing power towards more relevant aspects of incoming sensory information, such as specific spatial locations, sensory modalities or features, etc. However, how the brain switches between external and internal attention has not been fully understood. In this work, 20 human subjects performed four different attention tasks, including visual, auditory, audio-visual and mental arithmetic, while scalp electroencephalogram (EEG) signals were recorded. With similar auditory and visual stimuli given simultaneously across all tasks, the dynamic switching of brain state between internal (mental arithmetic) and external (visual, auditory and audio-visual) attentions were studied. We found that the delta and theta oscillations exhibited increased power in a wide range of brain areas, especially in the frontal and occipital regions, during the mental arithmetic task. The changes in the delta and theta power were accompanied by enhanced functional connections (FCs) within the frontal area but reduced FCs between the frontal area and other regions. Such changes were frequency specific, as we didn't find the same trend for alpha oscillations. These results suggest that enhanced oscillatory activities of relatively lower frequencies across a wide range of brain areas, as well as associated reconfiguration of FCs, may be a feature of dynamic switching of attention towards internal processing versus the incoming sensory information.