The human brain represents numbers along a mental number line, whose spatial nature was confirmed by studies of patients with visuospatial neglect. Here we describe a neural signature of neglect for the left "number space" by using a task that does not require manipulation of numbers. Patients were asked to discriminate an infrequent ("one" or "nine") from a frequent spoken number word ("five"). P3b brain waves, elicited by infrequent stimuli and indexing cognitive processing, were delayed to targets on the left of the number line ("one") compared to targets on the right ("nine"). The delay of P3b is thus a neural signature of the disorder of representational space.