A soft handoff of attention between cerebral hemispheres.

Each cerebral hemisphere initially processes one half of the visual world. How are moving objects seamlessly tracked when they traverse visual hemifields? Covert tracking of lateralized objects evokes a difference between slow-wave electrophysiological activity observed from contralateral and ipsilateral electrodes in occipitoparietal regions. This event-related potentials (ERP) waveform, known as contralateral delay activity (CDA) [1, 2], is sensitive to the number of objects tracked [1, 2] and responds dynamically to changes in this quantity [3]. When a tracked object crosses the midline, an inversion in CDA polarity revealed the dropping of the object's representation by one hemisphere and its acquisition by the other. Importantly, our data suggest that the initially tracking hemisphere continues to represent the object for a period after that object crosses the midline. Meanwhile, the receiving hemisphere begins to represent the object before the object crosses the midline, leading to a period in which the object is represented by both hemispheres. Further, this overlap in representation is reduced if the midline crossing is unpredictable. Thus, this process is sensitive to observer expectations and does not simply reflect overlapping receptive fields near the midline.