Event-related brain potentials (ERPs) elicited by novel stimuli during sentence processing.

Event-related brain potentials (ERPs) have proven to be particularly sensitive indices of the brain’s response to unexpected, surprising, or deviant stimuli (for reviews see Donchin et al., 1978; Picton et al., 1979). One of the more prominent of these ERPs following “surprising” stimuli has been a late positive wave called the P3, P300 or late positive complex (LPC). Among other hypotheses, it has been proposed that the P300 may reflect the resolution of prior uncertainty (Sutton et al . , 1965; Ruchkin and Sutton, 1978) and the task-relevant surprise value of the stimulus (Donchin et al., 1978). However, in some situations stimuli having no assigned task relevance, when presented infrequently and unexpectedly, have also been found to elicit enlarged late positive waves in the 200-500 ms range (Ritter et al., 1968; Roth, 1973; Courchesne et al., 1975; Squires et al., 1975; Roth et al., 1976; Snyder and Hillyard, 1976; Courchesne et al., 1978; Squires et al., 1977). These ERPs vary in waveform, amplitude, and scalp distribution, and it is not clear how many varieties of late positive waves may exist. Nonetheless, the general consensus has been that an unexpected event is typically followed by a P300 complex (often in association with an earlier N2 or N200 and a subsequent slow wave). In contrast, recent experiments on ERPs to words in sentences have indicated that not all unexpected events are associated with a P300 complex (Kutas and Hillyard, 1980a,b). Thus, while a late parietal positivity followed an unexpected change in the size of the printed word at the end of a sentence, a large centro-parietal negative wave (N4m) followed semantically deviant words a t the ends of these sentences. The present study was designed to find out whether physically deviant visual stimuli that were even more aberrant in the context of these sentences would be associated with a P30o-like wave or with an N400-like wave. To this end we presented a new group of subjects with the same set of 160 sentences used in previous studies (Kutas and Hillyard. 1980a,b), but now 25% of them were completed at random with slides of complex, unrecognizable abstract drawings (Courchesne et al., 1975). The ERPs to these novel slides were compared with ERPs to the final words of the other sentences, all of which were semantically appropriate and of the expected size.