Latency of P300 : An Index of Task Complexity?

The P300 latency was significantly longer in the LD-1 and in the rhyme tasks than in the size-decision task and was longest for targets in the LD-2, LD-3, and semantic tasks.The latency did not differ significantly between the LD-1 and the rhyme tasks or among LD-2, LD-3, and the semantic tasks. Although, in general, the order in which the P300 in the different tasks peaked was congruent with the a priori determined level of processing, the correlation was not perfect. The significantly shorter latency to word-targets in LD-1 (where the nontargets were illegal nonwords) than in LD-2 (where the nontargets were pseudowords) supports Balota and Chumbley’s (1984) suggestion that the rejection of illegal nonwords (as well as the acceptance of high frequency words) is based on their orthographic familiarity rather than a deeper process of lexical search. Yet, the difference between the latency to word-targets in LD-1 and targets in the size task suggests that although the execution of both tasks was based on a shallow visual analysis, distinguishing words from illegal nonwords was more demanding than distinguishing targets on the basis of their size. Hence it appears that the P300 latency, like RTs, does not reflect the level of processing required to recognize the target but rather the complexity of the process and the decision time. This may also account for the absence of a significant difference between the P300 obtained in the semantic and the lexical decision tasks, which required distinguishing words from pseudowords (LD-2 and LD-3), that is, it could not be based on familiarity or pure phonological grounds, as was possible in the LD-1 and the rhyme tasks. The level of processing seems to be better reflected in the amplitude of P300, to which we now turn.