Selective listening to stimuli with certain physical features, such as pitch or location, is reflected by an event-related brain-potential (ERP) component called the processing negativity (PN). PN may reflect a matching or comparison process between the sensory inflow and the hypothesized 'attentional trace'. The latter is an actively formed and maintained neuronal representation of the physical features defining the relevant stimulus. According to this matching theory of selective attention, the better the sensory input corresponds to the attentional trace the longer the stimulus is processes (as reflected by a larger and longer PN). This theory is supported by the experiment in which the relevant and irrelevant stimuli were separated from one another in pitch. It was found that even the irrelevant stimuli elicited PN, and further that this PN was larger the smaller the pitch separation between the eliciting stimuli and the relevant stimuli. The largest PN was elicited by the relevant stimuli. The present study finds analogous results when the relevant and irrelevant stimuli are separated in spatial origin rather than in pitch.