Traces left on visual selective attention by stimuli that are not consciously identified.

Briefly presented information, even if unidentifiable, may speed or delay required responses to following events. It has been assumed that this priming of the motor system may occur without affecting attention to priming and following stimuli. In contrast to this notion, the present study reports that such unidentified stimuli have effects on a physiological indicator of the attentional system. A lateral posterior electroencephalogram component was evoked by laterally presented relevant shapes, reflecting shifts of attention to those shapes. This component was absent, however, when the relevant shape was preceded by a similar shape at the same location, even if this shape was completely masked by metacontrast. The attentional shift evidently became unnecessary in this situation. Thus, unidentifiable information may leave some trace for attention-controlled selection of the following event.