DISCUSSION

The present study was designed to explore the time course of processing visually presented words, as reflected by the neural electrical activity elicited while reading words at different task-induced levels of processing. An oddball paradigm was used in which the distinction between targets and nontargets was based on either visual, phonologic, or semantic processes. In addition we introduced a rhyme task in which we assumed the need for phonetic processing in task performance. We focused mainly on the ERPs elicited by nontargets for which the negative waveforms were relatively “unmasked” by the robust P300 that is typically observed in response to targets. Four negative potentials distinct in latency and scalp distribution were discerned, each associated with a different level of processing: (1) one peaked at a latency around 170 msec (N170) over the occipito-temporal areas and distinguished between orthographic and nonorthographic stimuli in the size-detection task; (2) the second peaked at a latency around 320 msec (N320) over midtemporal areas, was larger at left than at right hemisphere sites,and distinguished between pronounceable and nonpronounceable letter strings in the rhyme detection task; (3) the third peaked around 350 msec (N350) over left fronto-temporal regions and distinguished between phonologically legal and phonologically illegal orthographic patterns in a lexical decision task; (4) the fourth peaked around 450-msec latency (N450) over left fronto-central regions and distinguished between meaningful and meaningless phonologically legal orthographic patterns in a semantic decision task. A detailed examination of each of these negative potentials and their interpretation will be deferred until after discussing the late positive potential elicited by the target stimuli.

Figure 9. Across-subjects average of P300 to target stimuli in the six tasks (size decision, lexical decision-1 (LD-1), rhyme decision, lexical decision-2 (LD-2), lexical decision-3 (LD-3), and semantic decision).