Mental processes that are initiated without any input from outside world may differ from those related to the direct external stimulation. Specifi cally, a question arises whether memory traces for previously imagined items may differ from memory traces for perceived items. The aim of our study was to test this issue, using the event-related potentials (ERP). It is well documented that remembered old items elicit more positive-going ERPs than correctly judged new items. This so-called “old/new effect” indexes neural activity associated with correct retrieval of information about a prior event. In this study the old/new effect was investigated separately for previously imagined and perceived items. In the fi rst part of the study, word labels of common objects were presented. Half of them were followed by a colour picture of the corresponding object. The other half was followed by a black screen, signalling to the participants to mentally visualize the object. In the second part, the participants discriminated between new words, words corresponding to previously perceived or imagined pictures. Correctly identifi ed old items were associated with more activity than correctly judged new items over the left parietal areas. Importantly, this statistically signifi cant old/new effect, representing the recollection index, was greater for imagined than for perceived items. Thus brain representations of imagined items might be more detailed and precise than those of real objects.
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