Tracking visible and occluded targets: changes in event related potentials during motion extrapolation.

Previous research suggests that there is overlap between the systems used for tracking visible and occluded moving targets. Both visually derived and stored velocity information provide input to the tracking system when the target is visible, but after occlusion the system receives its input solely from stored velocity representations. We used EEG to investigate the transition to purely memory guided tracking after the initial period of occlusion. Participants fixated while they covertly tracked targets moving horizontally rightward at velocities of 12 or 20 degrees. The targets were either continuously visible or disappeared for a short interval mid trajectory. Similar positive event related components were recorded over the right occipitoparietal region in both tasks, providing further evidence that the same neural systems are involved in tracking both visible and occluded targets. Furthermore, when the target was visible, the development of this positivity was linked to the spatial location of the target, peaking earlier in the 20 degrees condition. However, in the occluded condition the positive deflection began around 200 ms following the onset of occlusion and was not modulated by target velocity or location. This indicates a neural response after occlusion is cortically registered, consistent with the hypothesis that there is increased reliance on memory guided tracking at this point.