Event-related potential effects of deviant stimuli were investigated in a visual discrimination task. The stimuli (two angles within a frame) were either frequent (Standard) or one of two types of infrequent deviant (Deviant 1 or Deviant 2) stimuli. In comparison to the Standard stimuli, for Deviant 1 the two angles differed in their orientation, whereas for Deviant 2 the angles were identical but the frame was thicker. In Condition 1 the subjects counted the number of Deviant 1 stimuli. Of the 13 subjects, 12 did not detect the fact that some of the frames were thicker in this condition (i.e., for the Deviant 2 stimuli in Condition 2). The task in Condition 2 was the same (i.e., the target was Deviant 1), but the subjects were instructed about the thicker frame of Deviant 2 stimuli. In Condition 3, Deviant 2 stimuli became the targets. In comparison to the Standard, Deviant 1 elicited two posterior negative waves in the 120-180 and 240-300 ms latency ranges respectively. In addition, when Deviant 1 was the target stimulus (Conditions 1 and 2), this stimulus elicited the N2 and the P300 as well. In contrast, the irrelevant Deviant 2 elicited no such waves. In the target position (Condition 3), Deviant 2 elicited the second posterior negativity, the N2 and the late positivity. The earlier negativity is considered to be a correlate of processes connected to the automatic detection of the deviant features, whereas the later negativity is considered to be related to attentive processes, i.e., this wave is considered to be a member of the family of processing negativities.