Modulation of semantic processing using word length and complexity: an ERP study

INTERNATIONAL JOURNAL OF PSYCHOPHYSIOLOGY International

Journal

of Psychophysiology

19 (1995) 233-246

Modulation of semantic processing using word length and complexity: an ERP study Marc E. Pratarelli

*

Department of Psychology, Fort Hays State University, 600 Park Street, Hays, KS 67601-4099, USA Received

10 January

1994;

revised 28 February 1995; accepted 2 March 1995

Abstract

The objective of this study was to assess changes in semantic priming in two experiments which separately manipulated word length and the location of a semantic incongruence within spoken compound words. All words began 1500 ms after the presentation of related, partially unrelated, or totally unrelated picture-primes. Using a picture of a dogbone as an example, either the spoken words DOGBONE, WISHBONE, DOGHOUSE, or MAILBOX could occur as targets (underscores indicate semantically unrelated parts). In practice, only one of the four possible targets occurred with each prime. Subjects made speeded responses by pressing one of two buttons (related/unrelated) while event-related brain potentials (ERPs) were collected from various scalp locations. The behavioral analyses of response time and accuracy were sensitive to changes in word length, but inconclusive for semantic relatedness. However, repeated-measures ANOVAs of peak amplitude, latency, and mean area amplitude of the N400 ERP revealed significant effects of word length and semantic priming. Comparison of related compounds with partially unrelated word-initial violations of context (e.g., WIsHBONE) revealed diminished activity following the peak of the N400. Comparison of partially unrelated word-final contextual violations (e.g., DOGHOUSE) revealed an N400 effect delayed by approximately the mean length of the word-initial component. These results demonstrate the dynamic sensitivity of the cortical semantic processor. Keywords:

Semantic processing; ERP; N400; Auditory word processing

1. Introduction

Semantic priming occurs when the processing of one stimulus 61) facilitates the subsequent processing of a second stimulus (S2) if the two stimuli are semantically related, e.g., CAT-DOG (Meyer and Schvaneveldt, 1971; Neely, 1976). In psycholinguistic studies, the facilitation effect is recorded in terms of faster behavioral response

* Corresponding author. Oklahoma State University,

At: Department of Psychology, Stillwater, OK 74705, USA.

0167-8760/95/$09.50 0 1995 Elsevier SSDZ 0167-8760(95)00015-l

Science

times CRT) to S2 in semantically related pairs, and longer RTs to S2 in unrelated pairs. More recently, psychophysiologists using the event-related brain potential (ERP) technique, have demonstrated that a late occurring ERP is also sensitive to the degree of contextual facilitation Sl provides towards the recognition of S2 (Bentin et al., 1985,1993; Pratarelli, 1994; Rugg, 1984). When elicited by time-locked presentations of semantically unrelated items, this late occurring ERP is characterized by its negative polarity, a broad parietal scalp distribution, and peak ampli-

B.V. All rights reserved

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Journal of Psychophysiology 19 (1995) 233-246

tude between 200 and 600 ms following stimulus onset. While certain differences exist between studies, its sensitivity to changes in the semantic context is consistent with that of the N400 ERP first reported by Kutas and Hillyard (1980a,b) following the presentation of sentence-final unrelated words (e.g., “I take my coffee with cream and dog”). Together, the complementary behavioral and electrophysiological measures have resulted in many new insights into the nature of psycholinguistic mechanisms in cognitive processing, specifically, the time course of activation of the semantic processor. In the present study, the objective was to demonstrate that the semantic processor could be temporally modulated by manipulating its activation with simple and complex spoken words, and within complex words, by the specific location of an incongruent component. Under normal circumstances the process of spoken word recognition involves the sensory decoding of the acoustic-phonetic input, thereby enabling the access, search, and retrieval of an appropriate lexical item from memory (Dell, 1986; Elman and McClelland, 1984; Klatt, 1989; Marslen-Wilson and Welsh, 1978; Marslen-Wilson, 1987). Spoken word recognition takes place as the acoustic information becomes available in a left-to-right fashion (Marslen-Wilson, 1989; Taft and Forster, 1976). An early stage in word recognition is the process of lexical access. Lexical access takes place as processing units (e.g., phonemes, morphemes, syllables, or words) are matched with their respective mental representations in lexical memory. While a model of spoken word recognition must explain how listeners decompose the acoustic signal into linguistically and psychologically salient processing units, a model of lexical access must explain how items in memory become candidates for matching on the basis of the processing units that are chosen. This logic applies to all words. Five years ago, Marslen-Wilson (1989) noted that fifteen years of research have shown that listeners perceive speech in a smooth and continuous fashion, not in broken discontinuous chunks that we refer to as words, morphemes, etc. Moreover, the speech input is recognized when contextually appropriate about 200 msec into the utter-

ance. Marslen-Wilson (1987) has also shown that the initial 200 msec of sensory information is often quite insufficient for identifying a spoken word. How then is semantic information accessed as early as 200 msec if the form-based representation is not yet complete? Presumably, as long as there is some sensory information, partial information can be used, especially with prior context to select a set of likely candidates from the mental database. This process has been referred to as matching on the basis of “goodness-of-fit” (Marslen-Wilson, 1989, p. 5). Applying the goodness-of-fit model, one could hypothesize that the position of the semantic information within a word which allows the listener to confirm a bestfit, should be the location at which the semantic processor should be most active. This might also be referred to, loosely, as the point of recognition, i.e., the recognition point for discriminating a unique word should equate with activation or retrieval of the best-fitting candidate. One issue in spoken word recognition that has received only scant attention has been the study of the differential processing of complex words. Generally, simple words can be considered short in length, while increasing word length adds to the complexity of an utterance. An important consideration in dealing with complex words is the determination of how much of a word needs to be processed before a unique lexical match is viable. This is commonly the case with polysyllabic, polymorphemic, or compound words (e.g., BANANA, SUBJECT, or SCHOOLHOUSE). The processing of such items is further complicated in that certain word-initial components themselves appear to have independent status in memory (Marslen-Wilson and Zwitserlood, 19891, e.g., school and house both have general identities independent of the specific concept schoolhouse. Taft and Forster (1976) showed that the wordinitial syllable in polysyllabic words like FALLING, and the initial component of a compound word like DOGBONE, mediate lexical access. The stems in the above examples (FALLor DOG-), thus, appear to have preferential status in the lexicon to the extent that the frequency of occurrence of these word-initial components

M.E. Pratarelli/ International Journal of Psychophysiology 19 (1995) 233-246

affects the time needed to recognize the entire item. Word stems of higher frequency of occurrence speed the process while those of lower frequency take longer. Given the disposition of the lexical processor to operate on word segments during the access of complex items, like the above, what effect does the processing of the stem, or any components within the word, have on the later semantic processing? A clearer picture of the time course of activation of the semantic processor would yield an answer to such questions. Although speech is inherently a temporal phenomenon that would appear to benefit from a serial processing scheme, it does not rule out the possibility that each stage of processing (i.e., encoding, access, retrieval, or semantic processing) may begin prior to the completion of preceding stages. In fact, various models advocating some form of functional parallel processing among different stages have provided explanations that are consistent with several performance measures (Marslen-Wilson and Welsh, 1978; Marslen-Wilson, 1987; see Marslen-Wilson (1989) for a review). The N400 ERP has been described by many to be a sensitive index of semantic processing (cf., Kutas and Van Petten, 1988). Therefore, from an electrophysiological perspective, the fact that N400 activity may begin as early as 200 ms into an utterance of 400-500 ms in total duration, suggests that 1) semantic processing of the stimulus begins immediately as information becomes available to the processor, and 2) all preceding processing stages must continue to function and feed the semantic processor as long as new information is becoming available. Although this logic seems intuitively straight forward, little is actually known about the dynamic temporal processing characteristics of the semantic processor. Behavioral studies, of course, are constrained by a single time point of measurement, i.e., a reaction time provides very little insight into the ongoing dynamic processing of words. However, the ERP method can tell us a great deal about the dynamics of a process that has reached its peak activation and continues to be sensitive to new and possibly unexpected information.

235

One method of testing the modulation of semantic processing without using sentences is with spondaic (compound) words. The unique feature of producing a legitimate word by concatenating two legitimate shorter words, e.g., DOGBONE from the words DOG and BONE, makes it possible to use either of the components to systematically alter the location of a possible change in meaning. For example, by establishing an initial semantic context in an Sl-S2 paradigm, where Sl is always a photograph and S2 is always a word, subjects could be presented with trials in which the word target was either perfectly related to the picture prime (e.g., DOGBONE-DOGBONE) or unrelated in three possible ways. To a picture whose most appropriate verbal referent is DOGBONE, one can alter the nature of an unrelated trial either by presenting a completely unrelated word such as ik?AILBOX, or by modifying either of the individual components, e.g., WISHBONE or DOGHOUSE (the underscore is used to highlight the unrelated components). Subjects instructed to respond to all spoken words as being absolutely related or unrelated to the preceding picture will judge all three of the latter cases to be unrelated. Although such a paradigm has a strong semantic component, cross-modal repetition priming must contribute, in part, to the morphology of any N400 priming effects elicited. Nevertheless, by combining behavioral and electrophysiological techniques in the analysis of these semantic discrepancies, we can assess both absolute and intermediate degrees of contextual facilitation and semantic processing. Specifically, we ask whether the semantic processor operates on whole words in an all-or-none fashion, or whether it is an ongoing mechanism that is modulated on a moment-to-moment basis according to changes within words. To answer these questions, two experiments were conducted. The first experiment was used to establish a benchmark for the effects of complexity related to word length. Semantic violations by short monosyllabic words and long compound words will be compared to examine differences in waveform morphology attributable to word length alone. All unrelated trials in Experiment 1 were completely incongruent with their primes. The

236

M.E. Pratarelli / International Journal of Psychophysiology 19 (1995) 233-246

focus of the second experiment was to examine differences in waveform morphology attributable to the placement of the semantic incongruence within long words. In both experiments, the Sl-S2 (picture-word) priming paradigm, similar to that used by Carr et al. (19821, and Pratarelli (19941, was used to elicit semantic priming effects that could be observed with ERP measures. Such priming studies have demonstrated the efficacy of using nonlinguistic stimuli to establish context as effectively as with words.

2. Materials

and methods

2.1. Subjects Twenty-four subjects between the ages of 18 and 28 years (mean = 21.6 years; SD = 2.46 years) were solicited from the student population at the University of California, San Diego, with an advertisement in the local university newspaper; subjects were paid to be participants in the study. At the outset, four subjects made similarity judgements to validate the final stimulus set. Ten other subjects gave imageability (Paivio, 1971) and familiarity-of-use ratings for all stimuli. The remaining 10 subjects subsequently participated in both experiments for which they were paid five cents ($0.05) per correct response; the two present experiments were conducted as part of a larger series of experiments which required two visits to the laboratory. These two experiments consumed approximately half of each visit, or a total of 90 minutes. Despite the nature of their participation in the study, all subjects were screened on the basis of having normal visual acuity, no appreciable hearing loss in the speech frequencies (250-8000 Hz), English as the first language, no history of neurological disorders or trauma, and right-handed with no familial sinistrality. Subjects were also screened for geographic residency during their pre-collegiate school years (K-12). All subjects were raised in the same southwestern region of the United States. This was done in order to reasonably assure that subjects had similar exposure to the English language, i.e., in terms of word usage.

2.2. Stimuli In Experiment 1, short and long words were presented in separate blocks as targets preceded either by a semantically related or unrelated photograph. All trials were self-initiated by the subject. Fifty-two short monosyllabic words (e.g., FOOD, BIRD, DOG, TRUCK, etc.) divided equally between related and unrelated trial types were randomly ordered and presented as block one to half the subjects, and as block two to the other half. All stimuli were balanced for wordfrequency using the norms of Francis and Kucera (1982). For the long word items, 74 compound words were equally divided between related and unrelated trial types. These included items such as: SEASHELL, NOTEBOOK, COWBOY, FREEWAY, etc. Four independent judges rated all picture-word pairs for Sl-S2 relatedness. Another ten subjects were given Paivio’s instructions (Paivio, 1971) for making imageability ratings of each word target. Subjects listened to each word and scored it on the basis of imageability and familiarity-of-use using a scale of 1-7. An analysis of variance (ANOVA) did not reveal any significant differences between trial types. All target words (S2) were generated using the carrier phrase “Say the word . .. .. again” by a coached speaker of the same regional English dialect as the subjects who participated as listeners. The carrier phrase was used to insure a constant stress and intonation across trials. The speaker was coached to produce an emotionally neutral tone, as well as to generate stimuli having equivalent meter and intensity. The speech stimuli were recorded inside a sound attenuated test suite using a high-quality head-mounted low-impedance microphone (Sony model SM-1Oa) with a fixed 10 cm microphone-to-mouth distance. All samples were recorded onto magnetic tape, passed through a high quality amplifier, and then manually gain-adjusted to +3 dB prior to digitization. Samples were then digitized at a 16 kHz sampling frequency (12-bit resolution) using a bandpass of 75 Hz to 7.5 kHz. A waveform editor was used to extract each word from the carrier phrase and to measure its duration. Word onsets

ME. Pratarelli /International

Journal of Psychophysiology 19 (1995) 233-246

were trimmed to within 3 ms of the initial phoneme. Picture primes 61) were presented as 35mm slides taken of the actual objects or events. Objects and events were photographed such that each displaced approximately 75% of the slide area. During presentation, pictures displaced approximately 18 degrees of visual angle in either the vertical or horizontal dimensions (depending on the natural orientation of the item within the photograph). Items that were small enough to be handled manually were photographed on a light blue background to minimize distractions. Larger items (e.g., CRUISESHIP) where photographed in such a way as to minimize peripheral distractors in the foreground and background. 2.3. Procedure Subjects were fitted with a stretch-forming electrode cap (Electra-Cap, Intl.) which had two midline and ten lateral Ag/AgCl electrodes positioned over specific brain areas of interest. Recording sites included International lo/20 system sites Cz, Pz, F7, F8, 01, and 02 (Jasper, 1958). Nonstandard recording sites were positioned laterally over areas 22 and 41 (Brodmann’s), and Wernicke’s (including a righthemisphere homologue). Two additional electrodes were positioned 1) in the infra-orbital region of the left eye, and 2) at the right outer canthus to record vertical and horizontal eye movements respectively. These data were later used for purposes of artifact rejection of individual trials. Subjects were instructed about the importance of minimizing artifacts during their session. Electroencephalographic (EEG) recording (Grass Model 7P511J amplifiers) of all electrode sites was performed continuously to magnetic tape using a 200 Hz sampling frequency and 0.01-100 Hz bandpass filter. The subjects were seated in a comfortable reclining chair in front of a dull white screen positioned inside a copper-shielded test suite. A loose-fitting high quality headphone (Sennheiser model #HD-410SL) was used to present the spoken target stimuli. Subjects were then given a three-button response box to position in their lap.

237

By random assignment, half the subjects were instructed to respond “related” with their left hand and “unrelated” with their right; the other half used the reverse configuration. The third button was used to indicate that a trial had been missed, unattended, or otherwise an accurate response was not possible. The specific instructions to the subjects given during a lo-20 trial practice set was to look directly forward at the center of the display screen, initiate a trial with any button press, and attempt to maintain a regular 5-10 second inter-trial-interval. Two seconds after the initiating button press the visual picture-prime was presented for 1000 ms and then removed. The subjects then prepared to respond as quickly and accurately as possible to the target word presented another 500 ms after the slide was removed from the screen. 2.4. Data analysis Mean behavioral reaction times (RT) and response accuracies (RA) for both trial types were calculated for each subject in both blocks of trials. RT and RA data, and short and long word blocks were analyzed using four separate one-way analyses of variance (ANOVA). For the ERP data, the average peak amplitude relative to a 100 ms prestimulus baseline, the average peak latency relative to stimulus onset, and the mean area amplitude were analyzed in three contiguous analysis windows using each subject’s grand-average. The analysis windows were preselected based on the results of a fivesubject pilot study. These included 50-150 ms, 150-300 ms, and 300-1000 ms windows loosely corresponding to the observed locations of NlOO, P200/N200, and P300/N400 ERPs. ERP data from lateral electrode sites were analyzed separately from midline sites. Separate three-way and two-way repeated measures ANOVAs were used with the following designs: Sl-S2 relatedness (2) by hemisphere (2) by electrode site (5) for lateral sites, and Sl-S2 relatedness (2) by electrode site (2) for midline sites. Geisser-Greenhouse corrected values are presented in all tests having greater than two degrees of freedom.

M.E. Pratarelli / International

238 Table 1 Summary

of reaction

times and accuracies

Trial-type

RT(SE)

Related short words Unrelated short words Related long words Unrelated long words

681t63.4) 7OlC76.2) 932C46.0) 929C64.2)

(ms)

Journal

in Experiment

RAISE)

1

(% correct)

97.5(0.8) 98.6CO.7) 94.6C1.4) 99.5(0.4)

Artifact rejection of individual trials was performed off-line by eliminating all trials exceeding a 100 PV peak-to-peak threshold. However, a visual inspection of each subject’s waveforms was used to verify the integrity of the threshold. In the few cases where the 100 PV window was too lenient, a more stringent threshold was used. The behavioral data for these trials were excluded from the behavioral analyses. The mean rejection rate across all conditions and both experiments was 7% with a range from 3 to 13%.

3. Experiment 1

of Psychophysiology

19 (1995) 233-246

reported are from the larger data set corresponding to the lateral electrode sites. Generally, the midline analyses were redundant with the lateral analyses. Super-average waveforms generated by averaging together all 10 individual grand-averages, separately for semantically related and unrelated trial types, are illustrated in Figs. 1 and 2 for short and long words respectively. Apart from the early Nl potential, for which no trial type differences were found, Fig. 1 reveals differential processing of related versus unrelated short word targets beginning at about 200 ms. The peak negativities in the unrelated targets condition (dashed lines) occur between 350 and 455 ms in post-central recording sites, and between 450 and 680 ms in pre-central sites. These negativities are consistent with previous N400 research using auditory word targets, and hence, will be referred to as the N400 semantic priming effect.

LEm

RIGHT HEMISPHERE

HEMISPHERE

3.1. Results The mean RTs and RAs and their standard errors are listed in Table 1. No significant main effects of trial type in either short or long word blocks were found. However, a post-hoc analysis testing the mean RTs for short versus long words was significant (F[1,9] = 25.21, p < 0.0007); predictably, short words were responded to faster than long words. The mean RT difference between short and long words pooled across trial types was 240 ms. No significant effects were observed in the RA scores, probably because all subjects performed at ceiling levels. From the acoustic analyses performed on the word stimuli, the mean difference between durations of short and long words was 380 ms.

ERP effects Other than electrode site, no significant main effects were found within the early 50-150 ms analysis window. Therefore, only the results from middle and late analysis windows will be reported. In addition, the ANOVA results to be

SHORT MONOSYLLABLE -

RELATED cal=SuV/side

WORD TARGETS

- --UNRELATED lOOms/division

: Ed0sm:,loa ‘, I. x.2 464 Fig. 1. ERP effects to short monosyllable target words that were either semantically related (solid lines) or semantically unrelated (dashed lines) to their picture primes. As in all Figures illustrating ERPs, the calibration bar denotes stimulus onset and 5 pV positive and negative polarity; each tic mark on the time base denotes 100 ms.

239

M. E. Pratarelli/ InternationalJournalof Psychophysiology19 (1995) 233-246

LEFT HEMISPHERE

RIGHT HEMISPHERE

LONG COMPOUND WORD TARGETS

--RELATED cal=SuV/side

---UNRELATED lOOms/division

Fig. 2. ERP effects to long compound target words that were either semantically related (solid lines) or semantically unrelated (dashed lines) to their picture primes.

Significant effects of semantic priming were found in the 150-300 ms window for measures of negative-peak amplitude (F[1,91= 5.97, p < 0.031, negative-peak latency (F[1,9] = 8.92, p < 0.02), and mean area amplitude (F[1,9] = 12.62, p < 0.006). Results from the late 300-1000 ms window revealed significant effects of negative-peak amplitude (F[1,9] = 35.3, p < 0.00021, negativepeak latency (F[1,9] = 10.0, p < 0.011, and mean area amplitude (F[1,9] = 43.3, p < 0.0001). Moreover, a significant hemisphere by electrode interaction for both negative peak amplitude and mean area amplitude (F[4,36] = 10.8, p < 01 and F[4,361 = 4.73, p < 0.003 respectively) is consistent with previous findings using linguistic tasks, left hemisphere sites tend to be i.e., anterior more negative than right anterior sites; no such relationship existed between left and right posterior sites. The ERP results for the block of long words shown in Fig. 2 are generally consistent with those produced by short words, with a few specific exceptions. The differential negativities between related and unrelated targets at post-

central sites WL/WR and 01/02 appear, once again, to begin at about 200-250 ms. However, the pre-central sites now reveal an additional negative potential, following the Nl, and peaking at about 250 ms. This N250 appears in both related and unrelated ERPs and is not sensitive to semantic priming. Instead, the differential negativity in pre-central sites begins later at about 350-400 ms, and for unrelated targets peaks variably between 500-700 ms. Statistically, only negative-peak amplitude was sensitive to trial type differences in the 150-300 ms window (F[1,9] = 5.75, p < 0.04). The results for the late 300-1000 ms window were more definitive, however. The ANOVAs using negative-peak amplitude and mean area amplitude were both significant (F[1,91= 19.92, p < 0.002, and F[1,9] = 18.38, p < 0.002 respectively). The only significant interaction in the late window was for electrode by trial type (F[4,36] = 4.39, p < 0.005). This effect is summarized by the graph in Fig. 3. The interaction is notably due to the increase in negativity seen in post-central sites in the semantically unrelated trial type; this is consistent with previous evidence showing a parietal maximum for the N400 semantic priming effect.

MEAN N4 PEAK AMPLITUDE FOR RELATED AND UNRELATED LONG WORDS -7 -6 -5 i

UNRELATED ----- RELATED

.

ANTERIOR < ..---.--------... > POSTERIOR SCALPDISTRIBUTION

Fig. 3. Scalp distribution of N400 for related and unrelated long words in the anterior-posterior dimension. Note the predominantly posterior distribution of the N400 consistent with previous N400 studies.

M. E. Pratarelli /International

240

Journal of Psychophysiology 19 (1995) 233-246

RIGHT HEMISPHERE

LEW HEMISPHERE

the early vs. late analysis window (F[4,36] = 12.18, p < 0.003). This effect represents differences in the location of maximum N400 activity attributable to semantic priming in short versus long words. Fig. 5 summarizes the interaction seen most clearly at parietal sites. In the early 200-400 ms window, the semantic priming N400 effect is greater for short words than for long words. The reverse pattern of effects is seen in the late 500-700 ms window, i.e., the N400 semantic priming effect is greater for long words and smaller for the short words. 3.2. Discussion

DIFFERENCE -

WAVES FOR SHORT AND LONG WORDS

LONG WORDS 4=5uV/side

Fig. 4. A comparison long word conditions tions of the N400.

- - -SHORT

WORDS

lOOms/divisioo

of difference waves from short word and showing comparable posterior distribu-

Fig. 4 presents difference waves for short and for long word conditions; these were generated by subtracting the related ERPs from the unrelated ERPs for each subject, and then computing a new super-average. The N400 negativities reflected in the differential activation patterns clearly demonstrate two distinct morphologies related to the semantic priming effects for short words (dashed lines) and for long words (solid lines). Particularly notable are the effects at post-central sites WL/WR where N400 is often reported. The peak differential processing for short words is seen early peaking at about 350 ms, whereas the peak differential processing for long words is seen later peaking at about 600 ms. These subtle differences were exposed by reanalyzing the ERP data using two narrower analysis windows (200-400 ms and 500-700 for short and long word N400s, respectively). A three-way ANOVA testing the effects of word length (2) by electrode site (5) by analysis window (2) revealed significant main effects for all three independent variables. However, the important result is the significant interaction between word length and

The ERP results demonstrate the sensitivity of the putative cortical semantic processor to manipulations of word length. The morphology of the present N400 effects further demonstrate that word length does not appear to affect the onset phase of the semantic processor, but rather its peak activation and overall duration. That is, the onsets for both short and long words are virtually indistinguishable as one might expect if indeed word recognition begins to make viable “best-fits” after about 200 ms. It is particularly noteworthy that the peak of the N400 effect, in long words,

N4 PEAK FOR SHORT AND LONG WORDS IN THE EARLY AND LATE ANALYSIS WINDOWS

WORDS ___-_ SHORT LONG WORDS

EARLY WINDOW

LATE WINDOW

Fig. 5. Peak N400 activity for short and long words as analyzed in an early analysis window and a late analysis window. The data correspond to the effects at post-central sites (pooled across WL/WR) where the peak amplitudes for N400 were found.

M.E. Pratarelli/Intemational

Journal of Psychophysiology 19 (1995) 233-246

was delayed by nearly the same amount of time that distinguishes the behavioral reaction times for short and long words. The pattern of N400 semantic priming effects for both short and long words also maintained the post-central maximum seen in previous studies. Another notable result is the lack of significance in the behavioral RTs in light of such provocative ERP effects. This will be addressed in the general discussion, however. Most importantly, the N400 effects for the long word conditions can serve as a benchmark for further examination of the effects of word length and complexity on the N400 semantic priming effect. These results further suggest that caution should be taken when constructing stimulus sets, and when comparing N400 class ERPs elicited to words - if stimuli in different conditions have not been counterbalanced on the basis of word length. The second experiment was designed to further assess the dynamic temporal processing characteristics of the cortical semantic processor. Specifically, the initial and terminal components of compound words were used to modulate the degree of absolute semantic relatedness between the picture primes and their word targets. By manipulating the specific location of the semantic incongruence within a long compound word, the sensitivity of the semantic processor to subtle changes in context can be evaluated more thoroughly. Such is the focus of Experiment 2.

4. Experiment

2

4.1. Materials The same subjects, recording and data analysis procedures reported for Experiment 1 were used

Table 2 Summary

of reaction

times and accuracies

in Experiment

in Experiment 2; a different developed, however.

241

set of stimuli

were

Stimuli Three types of long (compound) words were tested using the same related versus unrelated forced-choice response task. The first category included semantically related compound words similar to the related trials used in Experiment 1; that is, the pictures and words referred to the same concept (e.g., TOOLBOX-TOOLBOX). A total of 74 semantically related picture-word pairs were used in the present experiment; these were divided into two separate bins (37 each) to be used as contrasts for each of the two unrelated conditions. The two remaining types of compound word trials composed the two partially related conditions; both will be referred to as “unrelated” because they do not absolutely match their respective picture primes. One group of 37 semantically unrelated compound words had violations placed in the word-initial location. For example, to a picture of DOGBONE, subjects would hear and respond to the word WISHBONE; similarly, to a picture of a PINECONE, subjects would respond to the word SNOWCONE. During their practice trials, subjects became keenly aware of the partial relatedness of each unrelated trial. Nonetheless, their instructions were to respond “related” only to word targets that were perfectly related to their picture prime. The second type of semantically unrelated compound words (n = 37) had the semantic violations placed in the terminal position, e.g., to the picture of FOOTPRINT, subjects heard the word FOOTSTOOL and would correctly make the “unrelated” response; similarly, to the picture of SKIBOAT, subjects responded to the word SKI LIFT.

2

Trial-type

RTCSE) (ms)

RAISE)

Related long words Unrelated (word-initial viol.) Related long words Unrelated (word-final viol.)

961C66.6) 968C59.8) 966C67.8) 984C46.9)

97.9CO.7) 98.9CO.6) 96.1CO.9) 9830.7)

(% correct)

M.E. Pratarelli / International Journal of Psychophysiology 19 (1995) 233-246

242

LEFT HEMISPHERE

RIGHT HEMISPHERE

ERPs TO WORD INlTML. VIOLATIONS -

RELATED cal=SuV/side

- - -UNRELATED lOOms/divisioo

Fig. 6. ERP effects to semantically related compound words (solid lines) and unrelated compound words where the semantic violation was placed in the work initial position (dashed lines).

4.2. Results

ANOVAs revealed significant main effects of trial type for negative peak amplitude (F[1,9] = 16.29, p < 0.003) and mean area amplitude (F[1,9] = 17.61, p < 0.0021, but only a marginally significant effect for negative peak latency (F[1,9] = 4.23, p < 0.069). As Fig. 7 shows, the peak latency of the late negativity for unrelated trials is approximately 600 ms at post-central sites. Although both semantically unrelated conditions revealed significant ERP effects in the late analysis window, their morphology appears somewhat different from the benchmark condition developed in Experiment 1. Therefore, difference waves were computed for both of the semantically unrelated conditions (word-initial and word-final violations), and contrasted with the difference waves from the benchmark (long words) condition in Experiment 1. Fig. 8b contrasts the differential negativities from the benchmark condition (solid lines) and the word-initial violations condition (dashed lines) at midline sites Cz and Pz where the effects were most pro-

LEFT HEMISPHERE

RIGHT HEMISPHERE

The mean RTs and RAs and the standard errors are listed in Table 2. Separate ANOVAs were used for word-initial unrelated and word-final unrelated contrasts with related trials. No significant behavioral effects of semantic priming were found. ERP effects Fig. 6 illustrates the ERP effects for the unrelated condition (dashed lines) in which the wordinitial component violated the established context. No significant effects were observed in the 150-300 ms window for any of the three dependent measures. However, the ANOVAs for the late window revealed significant trial-type effects in negative peak amplitude (F[1,9] = 44.01, p < 0.0001) and mean area amplitude (F[1,9] = 12.93, p < 0.0061, but not for negative peak latency. Fig. 7 illustrates the ERP effects for the unrelated trials (dashed lines) where the word-final component violated the established context. The

ERPs TO WORD FINAL VIOLATIONS -

RELATED cal=SuV/side

- --UNRELATED lOOms/divisioo

Fig. 7. ERP effects to semantically related compound words (dashed lines) and unrelated compound words where the semantic violation was placed in the word final location (dashed lines).

M. E. Pratarelli /International

ERF’ DIFFERENCE

(a)

Benchmark

Journal of Psychophysiology I9 (1995) 233-246

WAVES

vs. Word-final

Violations

cz;-

(b)

Benchmark

vs. Word-initial

Fig. 8. Comparison of differences the ERP difference waves for the the word-final violations, and (b) tion versus word-initial violations notable differences at Cz and Pz onsets of the different processing within long words.

Violations

at midline sites between (a) benchmark condition versus the same benchmark condiof semantic context. The between (a) and (b) are the related to context violations

nounced. Clearly, although the differential negativity begins at the same time for both, the two begin to diverge about loo-150 ms later. The differential negativity in the word-initial violations condition begins in the same manner as the benchmark condition, but the negativity is diminished after about 150 ms in response to the related word-final component. Nevertheless, the negativity is diminished, but not fully eliminated. Similarly, Fig. 8a contrasts the differential negativities of the benchmark condition (solid lines) with the word-final violations (dashed lines) at midline sites. In this case, the differences are observed earlier rather than later in the epoch. Owing to the related word-initial component, the differential negativity corresponding to the word-final violation onsets between 400-450 ms, some 200-250 ms later than the benchmark ERPs. Since each of these differential negativities correlate well with semantic priming, all can be considered morphological relatives of the conventional N400 semantic priming effect. The ERP data for the difference waves at all lateral sites

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were reanalyzed using two narrower analysis windows in order to test the observed differences in each of their N400 class effects. An early 200-400 ms window and a late 400-600 ms window were used and the data for each contrast submitted to separate ANOVAs. The first ANOVA assessing the different N400 effects for the benchmark and the word-initial violation condition revealed a significant effect of condition (F[1,9] = 12.17, p < 0.04) in the later window. This result supports the visual observation that differences in the amount of negativity exists following the peak activity. Similarly, the second ANOVA assessing the different N400 class effects for the benchmark and the word-final violation conditions revealed a significant effect of condition in the early window (F[1,9] = 47.07, p < 0.001). Collectively these results support the visual observations from Fig. 8 that differences in the amount and onset of negativity exists depending on the placement of the semantic violation within the long word. 4.3, Discussion The N400 semantic priming effect shifted temporally as a function of the location of the semantically unrelated part of the compound word. In comparison to the fully unrelated benchmark condition in Experiment 1, the condition with the semantic violation in the word-initial position produced an N400 of about the same overall duration. Also by comparison with the benchmark, the condition with the semantic violation in the terminal position of the word produced an N400 effect that began later but ended at about the same time. Therefore, the main similarity between the three semantically unrelated long word conditions is that each of the respective N400s end at the same time around 800-900 ms post-stimulus onset. In contrast, the main diffeerewes between the same three conditions is the late onset of N400 in the word-final violations, and the diminished but continuing N400 activity following the peak in the word-initial violation condition. This relationship between the specific location of the semantic violation within a word, and the

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Journal of Psychophysiology 19 (1995) 233-246

onset or amplitude of the N400 priming effect demonstrates the dynamic sensitivity of the N400 ERP. Furthermore, even though the later part of a word may indeed be related to the context, the N400 is dampened, but never loses sight of the fact that the word, as a whole, is still unrelated. Experiment 1 demonstrated that the semantic processor was differentially sensitive to short versus long words that violated the context established by the picture prime. Generally, the N400 priming effect peaked earlier and usually ended earlier for short words than for long words. This is the effect of word length alone. Experiment 2, however, demonstrated that the semantic processor was differentially sensitive to the placement of the violation and to the degree of unrelatedness within the word. This is the effect of word complexity. In general, these results are consistent with previous N400 semantic priming effects elicited in the auditory modality (Bentin et al., 1993; Domalski et al., 1991; Holcomb and Neville, 1990; Pratarelli, 1994;). In terms of the behavioral data, however, the failure to find differences in mean RTs, i.e., a semantic priming effect, is not consistent with many previous behavioral and electrophysiological studies. However, the group means for related and unrelated conditions are in the proper direction. Related trials in Experiment 2 were faster than their unrelated counterparts. Two factors may account for the failure to find significant behavioral effects. First, many studies use more than ten subjects to assess behavioral effects, and second, the long 1500 ms stimulus onset asynchrony may have contributed as well. More importantly, the pattern of significant ERP effects without significant behavioral effects demonstrates the superior sensitivity of the ERP technique for investigating subtle changes in the dynamic response of the semantic processor. Differences between the present N400 semantic priming effects and those from previous studies are probably attributable to differences in word length or the specific task demands required of subjects. For example, Bentin et al., (1993) have recently shown that the N400 is affected by task instructions. In their study, subjects were instructed to memorize a series of words

and then recognize them later during testing. In a separate condition, subjects were asked to simply count the number of nonwords presented in a lexical decision task. Their N400s differed on the basis of task effects. However, they used spoken words ranging from 310 to 790 ms. The present results would suggest that the difference between their short and long words would produce slightly different N400 morphologies analogous to those seen presently in Experiment 1. In fact, Bentin et al. note that “an N400 may be elicited at the recognition point of each word, but variability across stimuli can produce latency jitter and consequently a prolonged negativity in the average ERP.” Similarly, one could anticipate further modulation of the ERP if subjects were asked to respond positively whether the first or second component of the compound matched the picture. However, in keeping with Bentin et al.‘s findings, such effects should not displace the main N400 effect. Another question that arises in view of Bentin et al.% findings concerns the nature of a “recognition point.” The absence of differences in the onset of N400 activity between short and long words in Experiment 1 would suggest that a common recognition point exists for all words regardless of length. That is, as long as the initial phonetic input is incongruent with the subject’s expectations based on the most appropriate word (i.e., context), the semantic processor, as indexed by the N400, begins immediately the process of recognizing the violation. On the other hand, the “recognition point” may be related to the peak latency of the N400 rather than its onset. Indeed, this explanation is more consistent with the behavioral evidence in that short words are recognized faster than long words. Alternatively, one must further define whether recognition refers to the point at which a unique word is retrieved (Marslen-Wilson, 1975), or to the recognition that a semantic violation has occurred. Kutas and Van Petten (1988) have pointed out that all words produce an N400, but that words which violate context more than others will produce larger effects. Therefore, it is feasible that some part of the N400 is related to MarslenWilson’s unequivocal recognition of an auditory

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stimulus, and that some other part of the N400 is related the later semantic analysis. The present data from Experiment 2 suggest that the two can be morphologically dissociated. A closer inspection of Figs. 1, 2, 6, and 7 reveals the differences between the two parts of the N400. That is, semantically related words, whether short or long, always generated the conventional negativity peaking at 400 ms. Unrelated trials not only surpassed the amplitude of the related words’ N400, but produced an additional peak with a unique latency related to the processing of the semantic violation. This latter peak latency of the N400 corresponding to violations of context can be attributed to “recognition” of the violation, rather than to the recognition of the word itself. The unrelated condition in Experiment 2, in which the semantic violation is placed in the terminal position of the word, best demonstrates the dissociation between the word recognition N400 component, and the N400 component related to the processing of the negativity (Fig. 7). In fact, Fig. 7 reveals not two, but three late negativities. The first of these late negativities, referred to earlier as N250, has a predominantly frontal distribution and is most likely the N2 ERP component discussed by NZittinen and others (N&&en and Picton, 1986). The latter two negativities have nearly identical posterior scalp distributions, suggesting they may have a common source. This provides evidence in support of the logic behind the following interpretations of N400. The preferred interpretation given the present findings is that the semantic processor may be instrumentally involved in the initial recognition of a word. The same processor, however, continues to operate as long as new phonetic input is available. Moreover, the operation of the semantic processor can be modulated by the degree to which the phonetic “bit” of information currently being analyzed matches the previously established context. The implication is that the N400 can not be seen as an “all or none” response to each word, but rather as a dynamic ongoing process with potentially several interactive levels. These may include sensitivity to morphemes, words, phrases, sentences, and possibly even the processing of discourse itself.

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Acknowledgements This study was supported by a Grants-in-aidof-research from Sigma Xi to the author, as well as support from the University of Southern California. The author wishes to thank Drs. Marta Kutas, Donald Cooper, Lex Merrill, and the Neuropsychology Lab at the Salk Institute for Biological Studies for their assistance during different phases of the study. Also, the comments of three anonymous reviewers on earlier versions of this manuscript are greatly appreciated.

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