IS THE SOMATOSENSORY N250 RELATED TO DEVIANCE DISCRIMINATION OR CONSCIOUS TARGET DETECTION

Effects of attention to, and probability of sudden changes in, repetit ive stimuli on somatosensory evoked potentials (SEP) were studied. Low - (30 Hz) and high-frequency (140 Hz) vibratory stimuli were delivered in random order to the middle finger of the left hand with different presentation probabilities in different blocks. Also ignore conditions were administered. In the ignore conditions, the probability had no e ffect on SEPs. However, when the standard stimuli were omitted, the '' deviants'' elicited small N140 and P300 deflections not observed in re sponse to deviants when standards were also present. In the attention conditions, deviant stimuli (targets) elicited large N250 and P300 def lections which increased in amplitude with a decreased target probabil ity. However, when subjects counted infrequently presented ''deviants' ' alone (standards omitted) the enhanced N140 and the P300 with shorte ned latency were elicited, but no N250 wave could be found. At the ips ilateral side, a distinct N200 deflection was seen which could be the N250 with a shorter latency because of an easier task (detection inste ad of discrimination). The results might be interpreted as suggesting that the somatosensory N250 is related to conscious detection of targe t stimuli.