  Humans can covertly track the position of an object, even if the object is
temporarily occluded. What are the neural mechanisms underlying our capacity to
track moving objects when there is no physical stimulus for the brain to track?
One possibility is that the brain 'fills-in' information about imagined objects
using internally generated representations similar to those generated by
feed-forward perceptual mechanisms. Alternatively, the brain might deploy a
higher order mechanism, for example using an object tracking model that
integrates visual signals and motion dynamics. In the present study, we used
EEG and time-resolved multivariate pattern analyses to investigate the spatial
processing of visible and imagined objects. Participants tracked an object that
moved in discrete steps around fixation, occupying six consecutive locations.
They were asked to imagine that the object continued on the same trajectory
after it disappeared and move their attention to the corresponding positions.
Time-resolved decoding of EEG data revealed that the location of the visible
stimuli could be decoded shortly after image onset, consistent with early
retinotopic visual processes. For processing of unseen/imagined positions, the
patterns of neural activity resembled stimulus-driven mid-level visual
processes, but were detected earlier than perceptual mechanisms, implicating an
anticipatory and more variable tracking mechanism. Encoding models revealed
that spatial representations were much weaker for imagined than visible
stimuli. Monitoring the position of imagined objects thus utilises similar
perceptual and attentional processes as monitoring objects that are actually
present, but with different temporal dynamics. These results indicate that
internally generated representations rely on top-down processes, and their
timing is influenced by the predictability of the stimulus.
