Visual Processing (Size) Task

The ERPs elicited by the five stimulus types in the sizediscrimination task revealed two distinct categories ofresponses. One included all the three types of ortho-graphic stimuli (words, pseudowords, and nonwords);the other included the two types of nonorthographicstimuli (symbols and forms) (Figure 1). This differencebegan in the latency range of a negative wave peakingat T5 and T6 around 170 msec (N170) and lasted forabout 600 msec, throughout the stimulus exposure time(Figure 1A). The initial statistical evaluation of this pat-tern compared the mean amplitude elicited between140 and 600 msec by each Stimulus Type (words,pseudowords, nonwords, symbols, forms) at four poste-rior sites (OM1/2, O1/2, PO3/4, T5/6) on each Hemi-sphere (left, right). The analysis of variance (ANOVA) showed that the stimulus type and the site effects were significant (F(4, 92) = 29.3, MSE □= 6.6, p□< 0.0001, GG epsilon = 0.87, and F(3, 69) = 28.3, MSE□= 6.2, p<0.0001, GG □epsilon = 0.53, respectively), whereas the hemisphere effect was not (F(1, 23) < 1.00). Post hoc univariate ANOVAs revealed that the mean amplitudeselicited by words, pseudowords, and nonwords did notdiffer among themselves (F(2, 46) < 1.00), nor did themean amplitude elicited by symbols differ from thatelicited by forms (F(1, 23) < 1.00). The average meanamplitude of the three orthographic stimuli was sig-nificantly less positive than the average of the meanamplitude of the two nonorthographic stimuli (F(1, 23)= 81.1, MSE = 0.5, p < 0.0001) (Table 2). As revealed bysignificant interactions, the difference between ortho-graphic and nonorthographic stimuli was larger at theleft than the right hemisphere sites (2.01 and 1.65 μV,respectively) (F(4, 92) = 7.69, MSE = 0.37, p < 0.001, GGepsilon = 0.56) and larger at the PO (2.26 μV) and O(1.91 μV) sites than at the T (1.73 μV) and OM (1.43 μV)sites (F(12, 276) = 14.3, MSE = 0.21, p < 0.0001, GGepsilon = 0.37). No other interactions were significant.Because no differences were found among the threeorthographic stimulus types or between the two nonor-thographic stimulus types, for the subsequent statisticalanalyses the responses to the five stimulus types weregrouped into two distinct categories: orthographic stim-uli including words, pseudowords, and nonwords (252stimuli) and nonorthographic stimuli including stringsof alphanumeric symbols and strings of forms (168 stim-uli) (Figure 1B).A series of point-by-point t tests comparing thewaveforms elicited by orthographic and nonorthog-raphic stimuli showed that the difference betweenthe two categories became significant (p < 0.01) at140 msec at T5 (left hemisphere) and at 210 msec at T6 (right hemisphere). Because the latency rangeof the N170 wave (140 to 200 msec) was the earliesttime window where the responses to orthographicand nonorthographic stimuli differed (Figure 1B), andbecause previous studies suggested that the N170 isthe earliest informationalspecific ERP component elic-ited by visual stimuli (Bentin, Allison, Puce, Perez, &McCarthy, 1996; George, Evans, Fiori, Davidoff, & Renault,1996), we focused the analysis on the influence of stimulus-type category on N170 latency, amplitude, and topography.

Table 1. Summary of the Experimental Design

Level of Processing	Task	Nontarget Stimulus Type (N)	Target Stimuli (N)
Visual/orthographic processing	Size decision	Concrete words (84) Pseudowords (84) Illegal nonwords (84) Alphanumeric symbols (84) Forms (84)	Double-sized stimuli (16 of each type)
Phonetic processing	Rhyme decision	Concrete words (84) Pseudowords (84) Illegal nonwords (84)	Stimuli rhyming with vitrail Concrete words (16) Pseudowords (16)
Lexical/phonological processing	Lexical decision LD-1 LD-2 LD-3	Illegal nonwords (84) Pseudowords (84) Concrete words (84)	Concrete words (16) Concrete words (16) Pseudowords (16)
Semantic processing	Semantic decision	Concrete words (84) Pseudowords (84) Illegal nonwords (84	Abstract words (32)

Figure 1. ERPs in the vis-ual/orthographic task. (A)ERPs elicited by nontargetstimuli (strings of forms,strings of alphanumeric sym-bols, words, pseudowords, andnonwords) at lateral posteriorsites (T5, T6). (B) ERPs col-lapsed across orthographicstimuli (words, pseudowords,and nonwords) and nonortho-graphic stimuli (strings of al-phanumeric symbols andstrings of forms) in the vis-ual/orthographic task. TheN170 wave was largest at thelateral posterior sites T5 andT6 and peaked around 170-msec latency. The negativepeak at about 600 msec isprobably the “off” response ofthe stimuli (which lasted onthe screen for 500 msec).