The Amplitude of P300: An Index of Levels of Processing?

The P300 amplitude was equally high in the LD-1 and the size tasks, significantly higher than in all other tasks.

Furthermore, it gradually decreased from the rhyme to the LD-2, LD-3, and semantic decision tasks. This variation in amplitude cannot be accounted for by the probability of the target because it was similar across tasks. It also cannot be explained by the nature of the target stimuli because the order of the amplitudes did not seem to reflect such factors. For example, although the frequency of the abstract-word targets was higher than that of concrete-word targets and of pseudowords, they elicited a lower P300 amplitude. Moreover, the amplitude of the P300 elicited by the physically outstanding targets in the size task (which were twice as large as all other stimuli) was equal to that in LD-1 where all the targets were words, equally in size with the nontarget stimuli. This suggests that the amplitude of the P300 may have captured the similarly shallow processes required to distinguish words from illegal nonwords or target stimuli that were physically larger than the nontargets. It may also have captured the increasingly deeper processes induced by the different tasks from the rhyme to the semantic decisions. Although this interpretation is tempting, it is obviously not the only one possible. A different factor that may account for the variation in the amplitude of the P300 in the different tasks is differential jitter in the latency of single trials. It is possible that for simple visual discriminations the decision time was about the same across the single trials. On the other hand, it is conceivable that in more difficult tasks the time required for discriminating between targets and primes varied across words. Consequently, the average decision-related ERP should have lower amplitude (and a larger duration) in the deeper than in the shallower tasks. For example, as is evident in Figure 9, the P300 was considerably broader in the semantic task than in the size or LD-1 tasks. This possibility is supported by the larger variance across subjects in the P300 latency for the LD-3 and semantic tasks than for the size and LD-1 tasks. Hence, the alternative interpretation is that the amplitude of the P300 in different tasks, like its latency, is (inversely) correlated with their complexity.

Whether the P300 variation across tasks reflected only task complexity or also, at least indirectly, the level of processing induced in each task, its pattern of variation supports our a priori distinction between the tasks. Consequently, we can now analyze the ERPs elicited by nontarget stimuli which, “unmasked” by the P300, ⁵may have better reflected the neural activity associated with each type of process.