On the Locus of the Semantic Satiation Effect:
Evidence from Event-Related Brain Potentials
University of Pennsylvania
Sonja I. Kotz
The Max Planck Institute of Cognitive Neuroscience
Phillip J. Holcomb
The present study sought to determine whether semantic satiation is merely a byproduct of adaptation or satiation of upstream, non-semantic perceptual processes, or whether the effect can have a locus in semantic memory. This was done by measuring event-related brain potentials (ERPs) in a semantic word-detection task involving multiple presentations of primes and critical related and unrelated words in three experiments involving visual (Experiment 1) and auditory (Experiments 2a,b) stimuli. Primes varied in their type case (Experiment 1) or pitch (Experiment 2b) in order to discourage sensory adaptation. Prime satiation and relatedness of the primes to the critical word had interacting effects on ERP amplitude to critical words, particularly within the time-window of the N400 component. Because numerous studies have indicated a role for the N400 in semantic processing, modulation of the N400 relatedness effect by prime satiation (with little or no contribution from perceptual adaptation) suggests that semantic memory can be directly satiated rather than the cost to semantic processing necessarily resulting from impoverishment of perceptual inputs.
When a word is repeatedly produced or perceived, many people experience what has become known as the semantic satiation effect, a subjective and temporary loss of the meaning of that word. Early research by adherents of the introspectionist school analyzed the phenomenal qualities of semantic satiation (e.g., Severance & Washburn, 1907). More recent research has shown that such repetitive priming inhibits performance in semantic verification tasks (Smith, 1984; Smith & Klein, 1990).
Three significant recent studies have sought to answer important questions concerning the locus and nature of the semantic-satiation effect. Balota and Black (1997) examined semantic satiation in younger and older adults in a series of semantic verification experiments. Their basic paradigm required participants to make speeded judgments about whether two visually presented words were semantically related (e.g., ROYALTY -- QUEEN) or not (e.g., ROYALTY -- BOX) after 2, 12, or 22 visual presentations of one of the members of the pair. They found evidence of semantic satiation, defined as a diminution of the difference in mean reaction time and accuracy between responses to related and unrelated pairs. Specifically they reported that: (A) the relatedness effect decreased with more repetition in younger participants (particularly for related pairs of relatively low strength of association, e.g., ROYALTY -- DUKE), while older participants did not yield reliable evidence of such satiation; (B) such satiation did not depend on overt pronunciation of the satiated item (Experiment 2); (C) satiation did not occur in a phonological task requiring rhyming judgments (Experiment 3), implying that the effect was truly semantic and not sensory or perceptual in nature; and (D) that satiation was mediated by semantic relatedness of specific items and not by general satiation of semantic memory. In sum, the results of Balota and Black suggest that semantic satiation is truly semantic, and that such satiation may reflect some type of inhibitory process that diminishes with age.
There is, however, at least one significant methodological concern about the experiments reported by Balota and Black (1997) that render their results somewhat uncertain. Specifically, they operationalized semantic satiation as the difference in mean reaction times (and accuracies) between responses to related and unrelated word pairs, with a decrease in this difference implying satiation. Unfortunately, this is a somewhat ambiguous metric of satiation, because related and unrelated pairs required different judgments (i.e., related versus unrelated), rendering a direct comparison of the corresponding reaction times somewhat problematic (e.g., due to a possible differential influence of response bias). So, the results of Balota and Black (1997), while tantalizing, are not definitive.
In another recent study Pilotti, Antrobus, and Duff (1997) examined the issue of whether the primary effect of semantic satiation is really on semantic information processing, or whether the primary effect is on functionally prior sensory processes. Their idea was that a decrement in the efficiency of the processing of the meaning of an auditorally presented word could result from satiation, inhibition, or adaptation of an acoustic system whose outputs feed into a semantic system. In the latter case, if the inputs to the semantic system are of relatively poor quality, then the outputs of the semantic system might be poorer as well.
Pilotti et al. tested this hypothesis by having subjects listen to words on a series of trials with each trial consisting of either 3 or 30 presentations of a category prime (e.g., flower) followed by a pair of exemplar terms drawn from either the prime category (e.g., rose–tulip) or another category (e.g., shrimp–lobster). Subjects indicated by button-press whether the words in the target pair were drawn from the same or different categories. The critical manipulation was intended to influence satiation of sensory acoustic codes required for word recognition, specifically whether the presentations of the category prime on a given trial were all in the same voice, or whether they were in different voices. In fact, when all presentations of the prime were in the same voice, they found evidence of satiation. Specifically, they found that massed prime repetition slowed responses to word pairs drawn from the same category as the prime relative to responses to word pairs drawn from a category different from the prime, but only when all the primes were presented in the same voice (i.e., there was no satiation when the primes were presented in different voices).
Like the results of Balota and Black (1997), the results of Pilotti et al. (1997) are suggestive, but not definitive. One concern in the multiple-speaker condition is that switches among different voices might have induced subjects to pay more attention to the acoustic properties of the primes than to their meanings. Such a diversion of attention may have diminished semantic priming in the multiple-speaker condition. Furthermore, this tendency might have been exacerbated by the experimenters’ use of a between-subjects design in which the two groups could have adopted differential strategies for the single- and multiple-speaker conditions (e.g., attending to the meaning of the primes or attending to the sound of the primes). Moreover, the results of Pilotti et al. contradict those of Balotta and Black (1997, Experiment 3), who found no evidence of phonological satiation in a rhyme-verification task, although the phonological codes tapped by the Balotta and Black task may have been different from the acoustic codes presumably satiated in the Pilotti et al. experiment.
The third recent study, by Frenck-Mestre, Besson, and Pynte (1997), utilized a different approach to studying semantic satiation. Instead of measuring the effects of massed word repetition on measures of performance such as mean reaction time and accuracy, they looked for neurophysiological correlates of satiation. Specifically, they measured event-related brain potentials (ERPs) during a satiation task.
ERPs are obtained by measuring a participant’s electroencephalogram (EEG) using electrodes attached to the scalp. The EEG can be measured while the subject performs a task involving the occurrence of discrete events such as stimuli or responses. Segments of EEG following or preceding occurrences of a class of discrete events are averaged. The resulting ERP waveform reflects electrical brain activity correlated with the occurrence of members of that class of events. The amplitude, latency, and scalp topography of the various deflections (i.e., components) of the ERP can provide important evidence about the neural mechanisms underlying a variety of cognitive processes (Hillyard & Picton, 1987; Rugg & Coles, 1995).
Frenck-Mestre et al. (1997) focused on the N400 component, a negative deflection in the ERP waveform that typically peaks approximately 400 milliseconds after stimulus onset (Kutas & Hillyard, 1980). The N400 component, likely a complex of related subcomponents (e.g., Holcomb, Kounios, Anderson & West, 1999; Kounios, 1996), was of particular interest to these investigators, because it has been shown to be sensitive to semantic factors in a variety of tasks (for reviews, see Kutas & van Petten, 1994; Kounios, 1996; Osterhout & Holcomb, 1995). Specifically, N400 is typically of larger amplitude to words (or pictures) that do not fit their semantic context (e.g., "He put cream and sugar in his dog/coffee", where dog elicits a larger N400 than coffee). Although a definitive theory of the N400 does not yet exist, a number of researchers have argued that this component reflects a mechanism that attempts to integrate a piece of information into its semantic context (e.g., Kounios, 1996; Osterhout & Holcomb, 1995), hence the larger amplitude in response to a word such as dog that does not fit well.
Frenck-Mestre et al. reasoned that if semantic satiation were truly semantic, then satiation should diminish the magnitude of the N400 effect (defined as the difference in N400 amplitude for words that fit and do not fit their context). In their task, a category word was presented visually, followed by either the number 3 or 30 which indicated the number of times the word was to be pronounced. This number was decremented once per second to indicate the rate of pronunciation. Then, following a warning stimulus, another (exemplar) word was presented visually, the subject’s task being to judge whether that word was a member of the preceding category. Subjects were signaled 1200 milliseconds after target-word onset to press one of two buttons to indicate this judgment, the enforced response delay minimizing the contribution of neural motor activity to the ERP.
Frenck-Mestre et al. found the standard N400 effect of relatedness, namely, greater negativity (from 300-600 milliseconds) in response to target words that were not drawn from the preceding category than to target words that were drawn from the preceding category. They also found an effect of category repetition (throughout most of the time-course of processing) such that 30 presentations of the category resulted in an ERP to the target that was less negative (i.e., more positive) than when the category had been presented only 3 times. However, they found no interaction between the effects of relatedness and category repetition on target N400 amplitude. In other words, prime satiation apparently did not diminish or otherwise modulate the N400 relatedness effect. Their conclusion was that if the N400 is, in fact, a neural correlate of some aspect of semantic processing, then satiation does not have its effect on any aspect of semantic processing related to the N400.
This surprising result provides a piece to the puzzle of semantic satiation. However, because of the experimental procedure used by Frenck-Mestre et al., at least one more important piece still remains to be found. Their procedure involved overt verbal repetition of the category prime, followed by category verification of a visually presented target exemplar. This change in stimulus modality from primes to target could be the reason that category repetition did not interact with the effects of category–exemplar relatedness. Specifically, if there are modality-specific semantic representation and processing systems, then in the Frenck-Mestre et al. experiment category repetition could have satiated a semantic system that receives phonological or proprioceptive articulatory inputs and left relatively unaffected another semantic system which receives visual orthographic inputs (and which was subsequently tested). The implication of this analysis is that if the prime and target had been presented in the same modality, then category satiation might have diminished the size of the N400 relatedness effect, thereby providing evidence that semantic satiation does have an effect at the level of semantic integration. Consistent with this interpretation, Holcomb and Neville (1990, 1991) reported that auditory and visual N400s, while similar in some respects, nevertheless have somewhat different scalp distributions and time-courses, indicating that the neural and cognitive processors associated with the auditory and visual N400s are not identical.
The experimental paradigm adopted here did not require participants to make any overt responses to prime or target words, thereby minimizing the contribution of decision and response processes to target ERPs. The procedure involved the presentation of a continuous stream of words varying in case (Experiment 1) or pitch (Experiment 2b). The task was to view or listen to each word and press a button immediately upon detection of a word that referred to a part of the body (e.g., hand). These body-part words constituted approximately 5 percent of all items, and were included only to ensure that participants attended to all the words and analyzed them for meaning. The primary focus of this study was on the non-body-part words. These consisted of primes and critical items drawn from a variety of semantic categories. A critical word could follow either 1 or 15 presentations of a prime that was either semantically related or unrelated to the critical item. ERPs were averaged separately for critical items following low or high satiation of primes that were related or unrelated to the critical items.
Experiment 1: Visual Satiation
Experiment 1 implemented the paradigm just described in a semantic verification task involving visual presentation of repeated primes (in varying case) and critical related/unrelated words. It was predicted that if semantic satiation is indeed semantic, then the ERP difference between related and unrelated critical words (i.e., the N400 priming effect) should be significantly smaller following massed presentations of the prime than following a single presentation of the prime.
Figure 1. A schematic of a typical trial in Experiment 1. The trial begins with the presentation of the prime word (e.g., dog) in the center of the screen and is followed by either a related or unrelated Low Satiation Critical Item (e.g., Pet or Car). The prime then repeats 14 times with occasional interrupting target items (e.g., Arm) and ends with either a related or unrelated High- Satiation Critical Item (e.g., CAT or TABLE).
Participants. Sixteen student volunteers (9 female, mean age: 20.3 years, range: 18-22) served as participants. All were right-handed (4 had at least one left-handed relative in the immediate family) native speakers of English.
Stimulus materials and procedure. The critical word stimuli for this study were formed from 80 triplets of semantically related exemplar words (e.g., dog/cat/pet, truck/car/bus, table/chair/desk). All words were between three and eight letters in length. The first member of each triplet was designated the prime while the second and third members were designated as critical items. These 80 related triplets were then rearranged to form 80 triplets of semantically unrelated words (e.g., dog/car/table, truck/cat/desk, table/pet/bus). From the above related and unrelated triplets, four stimulus lists were formed such that within each list each critical item occurred only once, but across the four lists each critical item occurred in each of the four conditions of the experiment (see below).
A trial consisted of 15 presentations of a prime word with intervening critical words after the first (low satiation) and 15th repetition of the prime (high satiation). Fifty percent of critical words following both low- and high-satiated primes were semantically related to their primes and 50 percent were unrelated. A typical trial proceeded as follows (see Figure 1): (1) a prime word (e.g., dog) was presented for 100 ms; (2) 700 ms later, either a semantically related (e.g., cat) or an unrelated critical word (e.g., car) was presented for 100 ms; (3) 700 ms later, the prime word was presented again for 100 ms; (4) the prime word was repeated 13 more times (100 ms on, 700 ms off); and (5) 700 ms after the offset of the last repetition of the prime word a final related (e.g., pet) or unrelated (e.g., table) critical word was presented for 100 ms. Seven hundred ms after the offset of the high-satiated critical word, the prime for the next trial was presented.
To help ensure that participants processed all words to a relatively deep semantic level, in addition to the critical stimuli listed above, occasional words requiring a behavioral response were randomly interspersed throughout each stimulus list (n = 67, probability of occurrence on any given trial = .045). These items were all from the category of body parts (e.g., finger, head, hand – note that none of the prime or critical items were from this category). Participants were told to read all words presented, but to indicate when a word was a body part by rapidly pressing a single button (no response was required for primes and critical items). To help ensure that participants read each word, especially the repeating prime words, stimulus words randomly changed case between (A) all upper-case letters, (B) all lower-case letters, (C) and first letter upper-case, with the remaining letters in lower-case (e.g., DOG, dog, Dog). This manipulation was intended to minimize sensory satiation or adaptation to the physical features of the prime, and to discourage participants from using simple physical features to determine that the current item was a repeating prime and not a body part.
An IBM PC-AT compatible computer was used for stimulus presentation. All stimuli were displayed as white letters at the center of a 20-inch computer monitor and subtended between .75 and 1.75 degrees of horizontal vertical angle and .5 degrees of vertical visual angle. Participants were run in blocks of 20 trials punctuated by two to three minute breaks. A five-trial practice block preceded the actual experiment.
EEG recording procedure. Subjects were seated in a comfortable chair, and an elastic cap (Electro-Cap International, Eaton, OH) containing 13 tin electrodes was fitted to their scalp. Four additional electrodes were attached over the left and right mastoids (the right recorded actively, the left serving as a reference for all other sites), below the left eye (for monitoring vertical eye movements and blinks) and to the right of the right eye (for monitoring horizontal eye movements). The scalp sites included seven standard International 10-20 system locations (O1, O2, F7, F8, Pz, Cz and Fz) and six nonstandard locations including: Wernicke's area and its right-hemisphere homolog (30% of the interaural distance lateral to a point 13% of the nasion-inion distance posterior to Cz: WL and WR), left- and right-temporal (33% of the interaural distance lateral to Cz: TL and TR) and left and right anterior-temporal (one-half of the distance between F7 or F8 and T3 or T4: ATL and ATR). All impedances were maintained below 5K ohm. The 16 active electrodes were interfaced to a Grass Model 12 amplifier system (bandpass: .01 to 100 Hz, 60 Hz notch filter) and the EEG was digitized (200 Hz, 12-bit resolution) continuously on-line throughout the experiment by a second IBM PC-AT type computer. Averaging was performed off-line after the experimental run.
Data analysis. Average ERPs to critical stimuli were formed separately for each of the experimental conditions, which included the two levels of prime-satiation (Low/High), two critical stimulus types (Related/Unrelated) and 16 electrode sites, using trials that were free of ocular and amplifier saturation artifact. The ERPs formed in the above manner were digitally lowpass filtered at 20 Hz and quantified by calculating the mean amplitude in each of three time bands (200-400 ms, 400-600 ms, and 600-800 ms) using the average of the points in the 100 ms prestimulus period as a baseline.
Each of the three mean amplitude measures were submitted to separate repeated-measures analysis of variance (BMDP2V) employing the correction described by Geisser and Greenhouse (1959) where appropriate. Lateral and midline sites were analyzed in separate ANOVAs. The factors utilized were Satiation (High versus Low) and Relatedness (Related versus Unrelated). For lateral-site analyses, the other factors employed included Hemisphere (left versus right) and Anterior--Posterior (Frontal versus Anterior--Temporal versus Temporal versus Temporal--Parietal versus Occipital), while for the midline-site analyses there was a single additional factor of Anterior--Posterior (Frontal versus Central versus Parietal).
Behavioral data. Participants detected words in the body-part category with a mean accuracy of 95 percent and a mean reaction time of 653 ms.
ERPs: Qualitative characterization. The grand average ERPs to the critical words from 13 scalp sites are plotted in Figures 2a (Low-Satiation) and 2b (High-Satiation) for both of the two critical-word types (Related and Unrelated). As can be seen, the waveforms anterior to the Temporal-Parietal (Wernicke’s) Left/Right and parietal midline sites are characterized by an early, central–anterior negativity (N1) with a peak latency at approximately 120 ms. This N1 was followed, at most sites, by a larger positivity (P2) with a peak latency of 200 ms at anterior sites and 220 ms at posterior sites. At the most posterior lateral sites (O1/2) a somewhat different pattern of early ERP components was apparent. Here, the ERPs started with an early P1 peaking near 100 ms, followed by an N1 with a peak latency near 200 ms, a P2 with a peak between 225 and 250 ms and an N2 with a small peak near 300 ms (the latter being especially notable for High-Satiation critical items).
At all sites, there was a relatively large negative-going component with a peak latency between 300 and 400 ms (N400). In the Low-Satiation condition, the critical-word N400 was followed at anterior sites by a broad, slow extended negativity that eventually crossed into the positive range near the beginning of the next stimulus epoch (800 ms), at central and temporal sites by a positivity with a peak between 500 and 600 ms, and at the most posterior sites by a relatively flat baseline-hugging response lasting until the onset of the next word. Following the N400, critical words following high prime satiation produced a positive dip around 500 ms at anterior sites, but then went on to produce a substantial negativity near 600 ms. This anterior negativity is reminiscent of the contingent negative variation (CNV, Grey-Walter et al., 1964). At central and temporal sites, critical words following high prime satiation produced a post-N400 positivity at 500 ms and a subsequent negativity near 600 ms. Finally, at the most posterior sites, critical words in the high prime satiation condition produced a large positivity at 500 ms followed by a slow return to baseline near the beginning of the next word’s epoch.
Figure 2.Plotted in this figure are the grand-mean critical-item ERPs for Related critical words (solid lines) and Unrelated critical words (dotted lines) for the Low Satiation condition (Panel a) and High Satiation condition (Panel b) from the 13 scalp sites of Experiment 1. Time goes from left to right on the x-axes. Stimulus onset is the vertical calibration bar and the 100 ms period prior to this was used as a baseline for the purpose of averaging. The y-axis shows voltage, with negative values plotted up (according to ERP convention). Electrode sites at the bottom of the figure are from the back of the head, those at the top of the figure are from the front of the head, those at the right side of the figure are from the right side of the head, and so forth. The difference between the Related and Unrelated ERPs for the 400-600 ms time window is shaded. Abbreviations: O1/2 - occipital left and right; WL/R - Wernicke's left and right; TL/R - temporal left and right; ATL/R - anterior–temporal left and right; F7/8 - frontal left and right; Pz - parietal midline; Cz - central midline (i.e., vertex); Fz - frontal midline.
200-400 ms. The difference between the ERPs to Related and Unrelated words was not statistically reliable in the 200-400 ms epoch, nor were the interactions of the Relatedness factor with the Satiation factor, though there was a trend for the Unrelated critical items to be more negative than the Related critical items in the Low-Satiation condition and the reverse to be true for the High-Satiation condition (Relatedness X Satiation interaction, midline: F[1,15] = 3.67, p < .075). There was, however, a Relatedness X Anterior--Posterior interaction at the lateral sites (F[4,60] = 4.44, p < .03). This was due to the presence of a slightly larger negative-going response for Unrelated than Related critical items at the back of the head and the reverse trend at the front (i.e., Related critical items yielding ERPs that were more negative). The only other significant differences in this epoch were that critical words in the Low-Satiation condition tended to produce ERPs that were more negative-going than critical words in the High-Satiation condition across the scalp at lateral sites (i.e., main effect of Satiation, lateral: F[1,15] = 5.34, p < .035) and at more posterior midline sites (Satiation X Anterior--Posterior interaction: (F[1,15] = 7.94, p < .004).
400-600 ms. Figure 2 indicates that Related and Unrelated critical items produced their biggest differences in the 400-600 ms epoch (main effect of Relatedness, midline: F[1,15] = 7.53, p < .015; lateral: F[1,15] = 3.93, p < .066) with Unrelated critical items producing significantly more negative-going ERPs than Related critical items. Of most interest, however, was the presence of a Relatedness X Satiation X Anterior--Posterior interaction (midline: F[2,30] = 6.47, p < .009; lateral: F[4,60] = 3.91, p < .045). This interaction indicated that the Relatedness effect was larger for the critical words in the Low- than the High-Satiation condition, especially at central and anterior sites.
600 to 800 ms. There were no discernable Relatedness effects in the 600-800 ms epoch. However, there were differences between ERPs to critical words in the Low- and High-Satiation conditions, especially at the more anterior sites (midline, Satiation X Anterior--Posterior interaction: F[2,30] = 23.14, p < .0001), with High-Satiation critical items producing ERPs that were more negative-going.
To summarize the most important results from Experiment 1, though the Relatedness X Satiation interaction achieved only marginal significance in the earliest epoch (200-400 ms), this interaction was significant and more pronounced in the subsequent 400-600 ms epoch that captured most of the N400 activity. This interaction was such that the Relatedness effect was larger for critical items following low levels of prime satiation than following high levels of prime satiation, meaning that prime satiation decreased the magnitude of the Relatedness effect on the critical-item ERP. Moreover, this pattern of results occurred even though the physical features (i.e., case) of the repeated prime varied across presentations, presumably discouraging sensory satiation or adaptation (cf., Pilotti, et al., 1997) and encouraging sustained attention to the meaning of each stimulus word, as required by the experimental paradigm.
The present results also differed from those of Frenck-Mestre et al. (1997), who found no interaction (i.e., approximate additivity) between the effects of prime satiation and relatedness. This suggests that either the use of spoken primes or the modality shift from primes to critical items incorporated into the design of Frenck-Mestre et al. prevented prime satiation from exerting a discernable effect on semantic processing.
Experiment 2a: Auditory Satiation
Experiment 2a essentially replicated the design and procedure of Experiment 1, except that the stimuli were presented auditorally. The main issue was whether the interaction between the effects of the Relatedness and Satiation factors obtained in the Experiment 1 generalizes to the auditory modality. This is important because, as pointed out above, prior attempts to find satiation with spoken primes have met with mixed success.
Participants. Sixteen student volunteers (11 female, mean age: 18.9 years, range: 18-22) participated. All were right-handed (7 had at least one left-handed relative in the immediate family) native speakers of English.
Stimuli and procedure. The stimuli were the same as in Experiment 1 with the exception that all were presented binaurally via headphones (Sony model MDR-S30). Prior to the experiment, the words from Experiment 1 were spoken by a female native English speaker, recorded on analog tape, and then digitized at 16 KHz (12 bit resolution, 7 KHz lowpass filter) on a PC computer. The files containing each word were edited using a visual waveform editor so that the precise onset of the word was aligned with the beginning of each file. This assured accurate time-locking for recording ERPs. These files were stored on a stimulus computer and played via a 12-bit D/A converter using the same timing parameters used in Experiment 1. Note that although spoken words differed in their duration (mean: 544 ms, min: 138ms, max: 775 ms) the time between successive word onsets was the same as in Experiment 1 (i.e., 800 ms).
Behavioral data. Participants detected words in the body-part category with an accuracy of 93.4 percent with a mean reaction time of 754 ms.
ERPs: Qualitative characterization. The grand-average ERPs at 13 scalp sites from Experiment 2A are plotted in Figure 3a (Low-Satiation) and 3b (High-Satiation) for the two types of critical words (Related and Unrelated). As can be seen, the waveforms anterior to O1/2 in both plots are characterized by an early, central--anterior maximum positivity (P1) with a peak latency at approximately 60 ms. This P1 was followed, at most sites, by a negative going wave (N1) with a peak latency near 120 ms. The N1, in turn, was followed by a P2 with a peak latency between 170 (Low-Satiation condition, Figure 3a) and 300 (High Satiation condition, Figure 3b) ms. Note that at many sites the P1-N1-P2 complex stayed entirely on the positive side of the baseline (especially in the Low-Satiation condition). Also note that just after 800 ms this complex of early components could again be seen, but time-locked to the onset of the next word. At the occipital sites there was very little activity prior to 200 ms.
Figure 3. Plotted in this figure are the grand-mean critical-item ERPs for Related words (solid) and Unrelated words (dotted) from the Low Satiation condition (Panel a) and High Satiation condition (Panel b) from the 13 scalp sites of Experiment 2a. All else is as in Figure 1.
Following the early components, there was a large negative-going wave which peaked at approximately 400 ms (N400) in the Low Satiation condition (Figure 3a), and at approximately 500 ms in the High Satiation condition (Figure 3b). As in Experiment 1, the High Satiation condition also produced a much more negative response (CNV) at anterior sites between 400 and 800 ms than occurred in the Low Satiation condition. However, in Experiment 2a, this anterior negativity occurred in the same temporal window as the N400 instead of in a later temporal window as occurred for visually presented words in Experiment 1.
200 to 400 ms. In the Low Satiation condition, critical words yielded ERPs that tended to be more negative going than those in the High Satiation condition (midline main effect of Satiation: F[1,15] = 15.41, p < .0014), especially over the left hemisphere at lateral sites (Satiation X Hemisphere interaction: F[1,15] = 6.67, p < .02). The critical Relatedness X Satiation interaction did not approach significance in this time window (all F’s < 2.3).
400 to 600 ms. In this epoch, Unrelated critical words were more negative-going than Related ones (main effect of Relatedness, midline: F[1,15] = 15.21, p < .0014; lateral: F[1,15] = 16.48, p < .001), and this difference tended to be larger over the frontal and central sites along the midline (Relatedness X Anterior--Posterior interaction, midline: F[2,30] = 8.11, p < .0055). Critical-word ERPs in the High Satiation condition also tended to be more negative than Low Satiation items, but only over the temporal/central/frontal sites (Satiation X Anterior--Posterior interaction, midline: F[2,30] = 37.92, p < .00001; lateral: F[4,60] = 17.53, p < .00001). At more posterior sites, High Satiation critical words were actually more positive than Low Satiation items, and this trend was larger over the left than the right hemisphere (Satiation X Hemisphere X Anterior--Posterior interaction: F[4,60] = 3.85, p < .04). Finally, and most importantly, the Relatedness effect was larger in the Low than the High Satiation condition (Relatedness X Satiation interaction, midline: F[1,15] = 6.96, p < .019; marginal interaction at the lateral sites: F[1,15] = 3.83, p < .069). In addition, while the Relatedness effect tended to be larger in the Low than the High Satiation condition at lateral sites, this effect was larger at more temporal--anterior sites over the left hemisphere, and at the most posterior sites over the right hemisphere (Relatedness X Satiation X Hemisphere X Anterior--Posterior interaction: F[4,60] = 3.43, p < .04).
600 to 800. Unrelated critical items continued to be significantly more negative than Related critical items (Relatedness effect, midline: F[1,15] = 30.17, p < .0001; lateral: F[1,15] = 19.46, p < .0005), and this difference was larger towards the front of the head along the midline (Relatedness X Anterior--Posterior: F[2,30] = 12.46, p < .0006) and at anterior-temporal/temporal/parietal sites in the lateral analyses (Relatedness X Anterior--Posterior: F[4,60] = 5.44, p < .02). Critical words in the High Satiation condition also continued to be more negative than in the Low Satiation condition (main effect of Satiation, midline: F[1,15] = 15.18, p < .0014; lateral: F[1,15] = 11.54, p < .004). However, at the parietal-midline and occipital-lateral sites, this trend was not apparent. Rather, at these sites, items in the High Satiation condition were actually more positive-going than in the Low Satiation condition (Satiation X Anterior--Posterior interaction, midline: F[2,30] = 37.05, p < .00001; Satiation X Anterior--Posterior interaction, lateral: F[4,60] = 10.66, p < .0006). And finally, the difference in the Relatedness effect between the two Satiation conditions did not extend to this epoch (i.e., a nonsignificant Relatedness X Satiation interaction, F < 1).
The most important result from Experiment 2a was that there was a significant interaction between the Relatedness and Satiation factors during the 400-600 ms epoch, though not during the 200-400 and 600-800 ms epochs. More specifically, during the time-window encompassing most of the activity of the N400 component, greater prime satiation diminished the magnitude of the Relatedness effect on ERP amplitude. Hence, Experiment 2a not only replicates this basic finding from Experiment 1, but it also demonstrates that it generalizes to the auditory modality.
Experiment 2b: Auditory Satiation with Random Pitch Manipulation
Experiment 2b was designed to examine the possible role of sensory/acoustic adaptation or satiation in producing semantic satiation (see Balota & Black, 1997; Pilotti et al., 1997). This was done by means of a slight modification of the design of Experiment 2a whereby the pitch of the voice speaking the stimulus words was artificially varied from presentation to presentation simulating changes in the gender or age of the speaker. According to the logic of Pilotti et al., this manipulation should discourage adaptation or satiation at the level of sensory processing. Furthermore, in contrast to the procedure of Pilotti et al., the present procedure also induces participants to attend closely to the meaning of all the words, because a body-part target word could occur at any time in during the continuous stream of stimuli.
Participants. Sixteen student volunteers (7 female, mean age: 19 years, range: 18-21) served as subjects. All were right-handed (4 had at least one left-handed relative in the immediate family) native speakers of English.
Stimuli and procedure. In Experiment 2b, the stimuli were the same as in Experiment 2a with the exception that repeated prime words were played back to participants at one of three randomly changing frequencies across the 15 presentations of each word: 12 kHz, 16 kHz or 20 kHz. This manipulation had the effect of changing the perceived gender or age of the speaker from male (12 kHz), to female (16 kHz) to that of a child (20 kHz). For example, if the prime word was "dog", then a participant might hear the first presentation of "dog" in a normal female voice (16 kHz), the second in a male voice (12 kHz), and the third in a child's voice (20 kHz), and so forth. Critical words and body-part words were also randomly presented at one of the three frequencies. All else was identical to Experiment 2a.
Behavioral data. Participants detected words in the body-part category with an accuracy of 96.4 percent with a mean reaction time of 744 ms.
ERPs: Qualitative characterization. The grand average ERPs from 13 scalp sites in Experiment 2b are plotted in Figure 4a (Low Satiation) and 4b (High Satiation) for both of the two critical-item types (Related and Unrelated Words). As can be seen, the waveforms anterior to O1/2 in both plots are characterized by an early, central--anterior maximum positivity (P1) with a peak latency at approximately 60 ms. This P1 was followed, at most sites, by a negative-going wave (N1) with a peak latency near 120 ms. The N1, in turn, was followed by a P2 with a peak latency between 170 (Low Satiation condition, Figure 4a) and 250 (High Satiation condition, Figure 4b) ms. Note that at many sites the P1-N1-P2 complex stayed entirely on the positive side of the baseline, especially in the Low Satiation condition. Also note that just after 800 ms this complex of early components could be seen again, but time-locked to the onset of the next word. At the occipital sites, there was very little activity prior to 200 ms.
Figure 4.Plotted in this figure are the grand-mean critical-item ERPs for Related words (solid) and Unrelated words (dotted) from the Low Satiation condition (Panel a) and High Satiation condition (Panel b) from the 13 scalp sites of Experiment 2b. All else is as in Figure 1.
Following the early components, there was a large negative-going wave which peaked near 400 ms (N400) in the Low Satiation condition (Figure 4a) and between 500 and 600 ms in the High Satiation condition (Figure 4b). Similar to their visual counterparts (Experiment 1), the High Satiation condition also yielded a much more negative response (CNV) at anterior sites between 400 and 800 ms than did the Low Satiation condition. However, as in Experiment 2a, in Experiment 2b this anterior negativity occurred in the same temporal window as the N400 (instead of a later one for visual words).
200 to 400 ms. Critical items in the Low Satiation condition tended to be more negative-going than in the High Satiation condition, but only at more anterior sites (Satiation X Anterior--Posterior interaction, midline: F[1,15] = 5.82, p < .02), and only over the left hemisphere (Satiation X Hemisphere interaction: F[1,15] = 4.61, p < .05). At the most posterior midline site and at right-hemisphere lateral sites, Low Satiation critical words actually yielded a more positive waveform than occurred for High Satiation items. The critical Relatedness X Satiation interaction did not approach significance (all Fs < 1.65).
400 to 600 ms. In this epoch, Unrelated critical items were more negative going than Related items (main effect of Relatedness, midline: F[1,15] = 18.87, p < .0006; lateral: F[1,15[ = 9.07, p < .009). Unlike Experiment 2a, this effect did not vary significantly across the scalp. Critical words in the High Satiation condition also tended to be more negative than in the Low Satiation condition, especially over the temporal/central/frontal sites (Satiation X Anterior--Posterior, midline: F[2,30] = 38.17, p < .00001; lateral: F[4,60] = 13.81, p < .0006). At more posterior sites, critical items in the High Satiation condition were actually more positive than Low Satiation Condition, but only over the left hemisphere (Satiation X Hemisphere X Anterior--Posterior interaction: F[4,60] = 3.44, p < .04). Finally, the Relatedness effect was larger for Low than High Satiation critical words, but only over more anterior sites (Relatedness X Satiation X Anterior--Posterior interaction, midline: F[2,30] = 9.12, p < .002; marginally significant interaction at lateral sites: F[4,60] = 3.29, p < .07). Note that in Figure 4b, there was very little evidence of a Relatedness effect in the High Satiation condition at anterior sites (i.e., compared to the Low Satiation condition, Figure 4a).
600 to 800. Unrelated critical words continued to be significantly more negative than Related words (main effect of Relatedness, midline: F[1,15] = 14.78, p < .0016; lateral: F[1,15] = 35.57, p < .00001). High Satiation critical words also continued to be more negative than Low Satiation ones (main effect of Satiation, midline: F[1,15] = 8.42, p < .011; lateral: F[1,15] = 13.46, p < .0023). However, at the most posterior sites this latter trend was not apparent (e.g., occipital). For example, as in Experiment 2a, at the midline-parietal site High Satiation items were actually more positive-going than Low Satiation ones (Satiation X Anterior--Posterior interaction, midline: F[2,30] = 64.69, p < .00001; Satiation X Anterior--Posterior interaction, lateral: F(4,60) = 11.78, p < .0007). Most importantly, the difference in the Relatedness effect for Low and High Satiation critical words was not significant (Relatedness X Satiation interaction, Fs < 1).
Experiment 2b yielded an interaction between Satiation and Relatedness (and a topographic factor) during both the 200-400 and the 400-600 ms time windows, the latter capturing most of the activity of the N400 component. This replicates the central result from Experiments 1 and 2a. Furthermore, Experiment 2b yielded this result in spite of the fact that an important sensory characteristic of the stimuli (i.e., pitch) was randomly manipulated. This supports the notion that semantic satiation is not merely a byproduct of sensory satiation or adaptation.
The present study focused on neurophysiological correlates of semantic satiation with the goal of determining whether such satiation can be linked with processes known to be associated with semantic information processing. In particular, the aims were: (A) to ascertain whether prime satiation influences the effect of relatedness on ERPs, particularly the N400, an ERP component understood to play a significant role in semantic processing (cf., Frenck-Mestre, et al., 1997), and (B) to ascertain whether such effects on the N400 are a mere byproduct of sensory satiation/adaptation (as claimed by Pilotti, et al., 1997), or whether they result from direct effects of prime satiation on downstream semantic processing (as argued by Balota & Black, 1997).
Experiments 1, 2a, and 2b all found interacting effects of prime Satiation and Relatedness on ERP amplitude, particularly in the N400 time-window. Moreover, these effects were found both when the primes and critical items were presented in the visual modality (Experiment 1), and when the primes and critical items were presented in the auditory modality (Experiments 2a and 2b). These results contrast with those of Frenck-Mestre, et al. (1997) who found approximately additive effects of prime satiation and prime-target relatedness on ERP amplitude in the N400 time-window when the primes and critical items were presented in different modalities. Overall, the existence of within-modality satiation effects on the N400 coupled with the previous failure to find such effects in a cross-modality paradigm (Frenck-Mestre, et al., 1997), suggests the existence of modality-specific semantic representation or processing systems that can be selectively influenced.
The other major finding was that there were still interacting effects of Satiation and Relatedness on ERP amplitude in the N400 epoch when the primes were presented visually in different cases (Experiment 1), and when the primes were presented auditorally at different pitches (Experiment 2b), these manipulations being intended to minimize sensory satiation as a cause of downstream effects on semantic processing. These findings support the notion that semantic satiation is not a mere byproduct of sensory or perceptual adaptation. These results agree with Balota and Black’s (1997) conclusion that semantic satiation is not phonological in nature, though they disagree with the findings of Pilotti, et al. (1997), who found obliteration of behavioral semantic satiation effects when the primes were presented in a variety of voices. However, as explained in the introduction, neither of the latter two behavioral studies should be taken as definitive by themselves, both because behavioral concomitants of semantic satiation have proven to be fragile, and because specific features of their experimental procedures and designs may have limited the sensitivity and generality of their experiments. Because the current study rests on this prior foundation, it sidesteps the problematic aspects delineated above. Nevertheless, it should be kept in mind that the conclusion that satiation can occur at the semantic level of processing does not imply that semantic processing cannot be impaired by satiation or adaptation of upstream perceptual processes.
A final question concerns how the present data can be reconciled with the view that the N400 reflects a semantic integration mechanism. Specifically, if semantic satiation reduces the magnitude of the N400 effect, and if the N400 reflects a semantic integration mechanism, does this imply that satiation actually reduces the amount of work necessary to integrate a critical item into its preceding semantic context? In other words, do these data imply that satiation makes it easier (rather than more difficult) to process the critical item? Not necessarily, as there is an alternate interpretation that is more consistent with the notion that semantic satiation inflicts a cost on processing. In particular, the present results could simply mean that less integrative activity occurs after satiation, possibly because satiation causes refractoriness or adaptation in the semantic system thereby rendering it temporarily less responsive, perhaps by weakening the context into which the critical item is to be integrated. So, if there is little or no semantic context remaining after satiation, there will not be much integrative activity, hence a smaller N400 effect. This weakening of the semantic context with repetition may be analogous to a stabilized retinal image seeming to disappear.
In conclusion, the present results support the notion that semantic satiation is, in fact, semantic in nature, rather than having a primarily sensory or perceptual locus. Furthermore, semantic satiation appears to occur within modality-specific semantic systems rather than in an amodal representation or processing system.
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