Psychonomic Bulletin & Review
1996.3 (2). 249-253
Semantic priming in visual word recognition: Activation blocking and domains of processing PENELOPE RUTH CHIAPPE and MARILYN CHAPNIKSMITH University of Toronto, Toronto, Ontario, Canada and DEREK BESNER University of Waterloo, Waterloo, Ontario, Canada The fact that letter search on a prime eliminates the typically robust semantic priming effect in lexical decision is often attributed to the "shallowness" of the prime-processing task. In three experiments we investigated this claim by using two different "shallow" prime-processing tasks: letter search and color identification. Consistent with previous reports, lexical decisions to semantically related targets were not facilitated when subjects searched the prime for a probe letter. In contrast, semantic priming was observed following a color discrimination task on the prime. Wesuggest that a levels-of-processing interpretation is not an adequate framework for understanding these data. Instead, a domain-specific processing account is offered in which explicit processing at the letter level (as in letter search) makes demands on resources (e.g., activation) that drives processing at the semantic level. This competition is resolved by establishing a temporary activation block at the lexicalsemantic interface, which results in the elimination or attenuation of semantic priming. In contrast, global judgment of color is viewed as a domain that does not make demands on the resources that drive the visual word recognition machinery. There is therefore no need for an activation block, and semantic priming is not prevented. When a skilled reader is presented with a familiar word, does that word invariably get semantically processed? To investigate this question, researchers have frequently used the semantic priming paradigm, in which an observer is presented with two words-a prime and a target. Typically, processing of the target is easier when the pair of words is semantically and/or associatively related (e.g., cat-dog) than when the words are unrelated (e.g., nursedog). This facilitation has been found in a number oftarget processing tasks, including lexical decision, naming, categorization, and perceptual identification (see Neely, 1991, for a review). The classical interpretation of these data (see, e.g., Collins & Loftus, 1975; Neely, 1991; Posner & Snyder, 1975a, 1975b) is that activation of the node(s) in memory corresponding to the prime initiates a fast-acting spread of activation to the memory nodes of semantically related words. Hence, presentation of the prime word cat results in activation of the underlying representations of both cat and dog. Consequently, less additional activation is required to fully process the target word dog when it is presented, allowing subjects to identify it more rapidly.
This work was supported by Natural Sciences and Engineering Research Council of Canada Grants OGPINOl2 to M.C.S. and A0998 to D.S. We thank Hiroto Miyoshi for programming assistance and Colin MacLeod for comments. Correspondence should be addressed to D. Besner, Psychology Department, University of Waterloo, Waterloo, ON, Canada N2L 3GI (e-mail:
[email protected]).
In contrast, the notion that semantic-level processing is an inevitable consequence of processing the word is called into question by several studies in which semantic priming has been shown to depend on the way in which the prime is processed. Whereas processing of a related target is typically facilitated following the "reading" of a prime, semantic priming does not occur if task instructions require surface-level processing of the prime. For example, semantic priming is eliminated when subjects are required to indicate whether a prime word has an asterisk beside it (Smith, Theodor, & Franklin, 1983), whether a prime contains a particular letter (Henik, Friedrich, & Kellogg, 1983; Smith et al., 1983), or whether the prime is in upper- or lowercase letters (Kaye & Brown, 1985). Similarly, semantic priming of a target is eliminated if subjects are required to perform a letter search of the prime to determine if there are any repeated letters (Smith et al., 1983). After reviewing some of this literature, Neely (199 L p. 292) concluded that "the overall pattern is one of semantic priming being eliminated when subjects process the prime to a very shallow level." However, a major problem with this account is that a different pattern is seen when word recognition is assessed in the context of the Stroop task (see MacLeod, 1991, for a review). For example, identifying the ink color as "green" takes longer when the stimulus is the color word blue than when the stimulus consists ofsolid squares. This fact suggests that the lexical status of the word is being processed despite the fact that it is logically irrelevant to the task. The further obser-
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Copyright 1996 Psychonomic Society, Inc.
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vation that the magnitude of interference depends on the meaning of the words suggests that processing ofthe irrelevant word extends to the semantic level (DalrympleAlford, 1972; Klein, 1964). Hence, there is a contradiction concerning the conditions when semantic activation occurs during word recognition depending on whether performance is measured in the semantic priming paradigm or in the Stroop task. Both letter search and color naming can be classified as involving nonsemantic processing in the sense that the task requires explicit processing of a surface feature of the stimulus. Nevertheless, the intrusion of the meaning ofthe word on color identification performance suggests that processing surface-level features such as color does not prevent the word from being processed semantically. In contrast, the absence of semantic priming in lexical decision following letter search ofthe prime suggests that semantic processing of the prime is restricted in some way. One way to think about this apparent contradiction is in terms of domain-specific processing (see, e.g., Allport, 1980a, 1980b). Explicit processing at the letter level may make demands on resources, such as activation, that are specific to the word recognition system. If this activation is limited, one way to utilize it for the explicit letter-processing task is to prevent it from being used for other purposes, such as activating semantics. This idea can explain why letter search on the prime can lead to an absence of the typically robust semantic priming effect in lexical decision (see, e.g., Stolz & Besner, in press). In contrast, the kind of color judgments that are required in the Stroop task and its many variants can be understood as relying on resources that are specific to color processing in that they need not make any demands on the resources that drive the visual word recognition machinery. A stronger case could be made for the applicability of the domain-specific processing account of this problem if it also held in a situation in which the target task was held constant and the prime-processing task was varied. We therefore investigated the domain-specific processing account more directly in the context of the semantic priming paradigm, as well as comparing its predictions with the "shallow processing" account offered by Neely (1991). Subjects made a lexical decision to the target in three experiments. In Experiments 1 and 2 subjects performed a letter search on the prime, and in Experiment 3 they made a color decision about the prime. If, as Neely has suggested, shallow-level processing of the prime is a sufficient condition to prevent semantic priming, then neither letter search nor color decision on the prime should facilitate the processing ofa subsequent, semantically related target. In contrast, the domain-specific processing notion outlined above predicts that semantic priming will not be seen following letter search on the prime, but will be seen following a color decision on the prime.
EXPERIMENT 1
Method Subjects. Forty-eight students from the University of Toronto, Scarborough Campus, served as subjects, receiving either course credit or $7.50 for their participation. Materials and Apparatus. Each subject was presented with 288 stimulus pairs, 144 word-word pairs and 144 word-nonword pairs. Nonwords were created by changing one letter of a real word. Half the word pairs (72) were semantically/associatively related (e.g., breadbutter) and half were unrelated, formed by randomly recombining the related word pairs. The 288 stimulus pairs were divided into 3 blocks of 96, with each stimulus type appearing an equal number of times within each block. Across subjects, each target word was preceded equally often by a related and an unrelated prime. Each subject received a different random ordering of stimuli. The experimental stimuli are available from the authors. An additional set of 32 word-nonword and 32 word-word pairs (half related and half unrelated) was prepared for practice. None of the practice words were used as experimental stimuli. The 64 practice pairs were divided into 2 blocks of 32, with each stimulus type appearing an equal number of times within each block. An IBM-compatible computer and a Compal MMIOI color monitor were used to present stimuli and record responses. All stimuli were horizontally centered on the computer screen and appeared 10.7 em from the top of the screen. On each trial a white asterisk appeared in the center of the screen for I sec. Targets were presented in white against a black background. Half the primes appeared in red and half in blue. Prime color was counterbalanced across subjects, with each prime appearing an equal number of times in each color, and with each color prime preceding an equal number of word and nonword targets. All primes were presented with a probe letter in the same color replicated above each letter ofthe prime word; for example, qqqqqqq antique The probe letter was present in the prime on half the trials, and all serial positions were probed equally often. Procedure. Subjects decided whether the specified letter was present in the prime and then decided whether the target was a real English word. Both prime and target presentation were response terminated, with a 100-msec interstimulus interval (lSI) between offset of the prime and onset ofthe target. Subjects indicated ifthe prime contained the specified letter by pressing either a Yes or a No key on the left side ofthe keyboard with their left hand, and then indicated if the target was a word or not by pressing a Yes or a No key on the right side ofthe keyboard with their right hand.
Results and Discussion Mean reaction times (RTs) for correct responses were calculated using a trimming procedure whereby RTs for each subject in each condition that were either larger or smaller than the mean by 2.5 SD were eliminated. This procedure was used in all the experiments reported here. Mean RT for the letter search on the prime was 1,345msec, with 2.4% errors. Mean RTs and percentage of errors for lexical decisions to the target are presented in Table 1. A repeated measures analysis of variance (ANOVA) of the latency data revealed no reliable priming effect [F(I,47) = 2.17, MSe = 918,p > .15], with only 19 of the 48 subjects responding more rapidly to related than to unrelated targets. Hence, these data replicate the typical absence of semantic facilitation in RT following a letter search on the prime (Henik et aI., 1983; Smith, 1979; Smith et aI.,
ACTIVATION BLOCKING AND SEMANTIC PRIMING
Table 1 Mean Reaction Time (RT, in Milliseconds) and Percentage of Errors (% E) for Lexical Decision to Targets Following Letter Search of the Prime in Experiment 1 Target
RT
%E
Unrelated Related Difference
709 700 9
2.6 1.9 .7
1983). This conclusion is qualified by a small (.7%) reduction in errors in the related condition [F(I,47) = 5.13, MS e = 0.00023,p < .05], but is not considered further, given that no evidence of priming following letter search of the prime was observed in Experiment 2.
EXPERIMENT 2 Experiment 2 provided a replication of Experiment 1, but included an additional variable-the letter search task was interrupted prior to completion on 25% of the trials. On interrupted trials, subjects did not make an overt response to the prime before making the lexical decision to the target. Letter search was interrupted prior to the subject making a response in an attempt to shed some light on the nature ofthe processing that prevents semantic priming. If whatever is responsible for preventing the occurrence of semantic priming is ballistic in nature, acting in an all-ornone fashion as soon as letter search is initiated, or if it depends on the set adopted by the subject, then no semantic priming will occur regardless of whether or not letter search is interrupted. On the other hand, ifthis anonymous process builds up incrementally as letter search proceeds, then facilitation on interrupted trials may be observed.
Method Subjects. Forty-eight undergraduate students from the University of Toronto, Scarborough Campus, received either course credit or $7.50 for their participation. Materials and Apparatus. The stimuli and apparatus were the same as those in Experiment I. Procedure. The procedure was the same as that in Experiment I, except that the letter search was interrupted on 72 of the 288 trials, 24 preceding nonword targets, 24 preceding related-word targets, and 24 preceding unrelated-word targets. Subjects were given 48 practice trials, of which 16 contained interrupted letter searches. Subjects were told to simply get ready for the target if the prime disappeared before they had responded to it. On each trial a white asterisk appeared in the center of the screen for I sec. On 216 of the 288 trials, the procedure followed that of Experiment I in that presentation of the prime was terminated by a keypress response indicating that the letter search had been completed. The target then appeared 100 msec after termination of the prime. On the 72 interrupted trials, the prime was displayed for 400 msec, with the target appearing 100 msec later. In both cases, the target remained on the screen until a lexical decision had been made, and the next trial began after a I-sec pause.
Results The uninterrupted letter search on the prime averaged 1,488 msec and 3.7% errors. Mean RTs and percentage oferrors for lexical decisions to the targets are presented in Table 2.
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Lexical decisions to targets following the two types of prime presentation (completed vs. interrupted letter search) were examined separately. When subjects completed the letter search task (as they did in Experiment 1) there was no evidence of priming in either RT (F < 1) or errors [F(I,47) = 2.23, MSe = 0.00040, p > .10]. Similarly, there was no evidence ofsemantic priming following the aborted letter search of the prime in either RT (F < I) or error data [F(l,47) = 1.18, MSe = 0.0012, P > .10]. Inclusion of both conditions in a single 2 X 2 repeated measures ANOVA revealed no main effect of priming in either the RT or error data (F < I in both cases). Although lexical decisions were both faster and more accurate when subjects were able to complete the letter search task [F(l,47) = 62.93, MSe = 22,413,p < .0001, and F(I,47) = 6.10, MS e = 0.0053, P < .02, respectively], this appears to be a global effect that did not affect related and unrelated targets differentially. These results extend those seen in Experiment 1, as well as others in the literature. The failure to observe semantic priming on interrupted trials suggests either that there is a set effect operating that is induced by the requirements of the letter search task or that whatever anonymous process that is operating occurs sufficiently quickly that it is not deterred by premature termination of the letter search stimulus (see also Logan, 1985). We turn now to Experiment 3, in which subjects were required to make a color decision about the prime.
EXPERIMENT 3 Subjects in this experiment were required to identify the color of the prime prior to making a lexical decision to the target. One group ofsubjects made an easy color discrimination (red vs. blue), and a second group of subjects made a more difficult color discrimination between two different shades of blue. I Although both letter search and color decision involve surface-level analysis of the prime, letter search may prevent semantic priming whereas color discrimination may not. One reason is that letter search and lexical decision involve within-domain (linguistic) processing, whereas color decision and lexical decision involve different domains ofprocessing. Semantic priming may only be prevented when processing occurs in the same domain and different levels (letter vs. semantic levels) compete for resources such as activation.
Table 2 Mean Reaction Time (RT, in Milliseconds) and Percentage of Error (% E) for Lexical Decision to Targets Following Letter Search of the Prime in Experiment 2 Search Condition Completed
Interrupted
Target
RT
%E
RT
% E
Unrelated Related Difference
748 748
2.0 1.4 .6
919 920 -1
3.9 4.6 -.7
o
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Failure to find semantic priming in the present experiment would be consistent with Neely's (1991) conclusion that semantic activation does not occur when subjects are explicitly directed to process nonsemantic features of the prime. In contrast, evidence of priming following both easy and difficult versions of the color discrimination task would be consistent with the suggestion that semantic priming is dependent on something other than the general level at which the prime is processed.
Method Subjects. Sixty-four undergraduate students from the University of Toronto received either $7.50 or course credit for their participation. Thirty-two subjects were assigned to the easy color discrimination condition and 32 to the difficult color discrimination condition. Materials. Materials were the same as those in Experiments I and 2. Procedure. Subjects made a color decision about the prime followed by a lexical decision to the target. Halfthe primes appeared in red and half in blue (easy condition) or in different shades ofblue (difficult condition). Subjects made prime responses by pressing the indicated keys on the left side of the keyboard using their left middle and index fingers. For subjects in the easy condition, a red sticker was placed on the Z key and a blue sticker on the X key. For the difficult condition, a dark blue sticker was placed on the Z key and a lighter blue sticker on the X key. The prime remained on the screen until a response occurred; 100 msec after prime offset, the target appeared in the same location. Subjects indicated if the target was a valid English word by pressing the keys marked Yes or No on the right side of the keyboard with the right hand. Prior to beginning the experiment, subjects were given two 32-trial practice blocks.
Results and Discussion Decision times about the color ofthe prime were considerably faster in the easy condition (683 msec) than in the difficult condition (866 msec), with comparable error rates in the two conditions (1.4% and 1.7%, respectively). Ease of prime processing also affected the overall speed of target processing. As can be seen in Table 3, lexical decisions were reliably faster following the easy color decision [F(I,62) = 4.82, MSe = 63,043,p < .04]. Accuracy was not affected by color decision difficulty [F(I,62) = 1.05, MS e = 0.0006,p > .10]. More importantly, Table 3 shows that overall, lexical decisions to targets were faster following related primes (680 msec) than following unrelated primes (703 msec). Reliable priming was evident in both RT [F(1,62) = 8.05, MS e = 2,181,p < .01] and error rates [F(I,62) = 7.83, MS e = 0.0002, P < .01]. This facilitation did not depend on difficulty level; the magnitude of the priming effect was comparable in the difficult (26 msec) and easy (20 msec) conditions. The results ofthis experiment contrast with the results from the letter search conditions in Experiments 1 and 2. Lexical decisions are facilitated by related primes when the prime-processing task consists ofa color judgment but not when the prime task requires explicit, letter-level analysis. GENERAL DISCUSSION The departure point ofthe present paper is the repeated observation that letter search on the prime eliminates semantic priming in visual
Table 3 Mean Reaction Time (RT, in Milliseconds) and Percentage of Errors (% E) for Lexical Decision to Targets Following Color Judgment on the Prime in Experiment 3 Color Decision Easy Color Target Unrelated Related Difference
Difficult Color
RT
%E
RT
%E
653 633 20
2.9 1.4 1.5
753 727 26
1.8 1.6 .2
word recognition. Neely (1991), among others, has suggested that this result can be understood in terms of a levels-of-processing analysis; "shallow" processing of the prime prevents the activation of its semantic features and hence eliminates semantic priming. However, as noted in the introduction, one difficulty with this general levels-ofprocessing analysis is that it is inconsistent with data from the Stroop task. The standard finding in the Stroop task is that identifying the ink color ("shallow" processing) does not prevent the activation of semantic features from the task-irrelevant word. This contrast was examined further in the present experiments by combining two different nonsemantic prime-processing tasks with a semantic priming manipulation in the context of lexical decision. The results were straightforward. Whereas letter search on the prime eliminates semantic priming, color decision on the prime does not. Hence, a general levels-of-processing account does not provide an adequate explanation of these data. 2 One account of semantic priming appeals to the general framework (but not the specific one.') of the interactive activation model (see, e.g., McClelland, 1987), in which there are distinct levels of representation corresponding to letters, words, and semantics. Presentation of a prime such as cat results in activation at the letter level, word level, and semantic level. Dell and O'Seaghdha (1991), as well as Stolz and Besner (in press), have suggested that semantic priming can be explained by the assumption that lexical-level activation for items like cat serves to activate features at the semantic level that overlap with the features that belong to related targets like dog. Activation Blocking Stolz and Besner (in press) have also suggested that the explicit processing at the letter level during letter search of the prime competes for resources shared by semantic-level activation. -The word recognition system resolves this problem by the establishment of a temporary activation block at the lexical-semantic interface. Evidence consistent with this locus of the activation block can be seen in Friederich, Henik, and Tzelgov's (1991) observation that letter search of the prime does not eliminate repetition priming for words in lexical decision, and Besner and Stolz's (1995) observation that letter search on the prime eliminates semantic priming but not morphemic priming. Ifthe activation block were situated prior to the lexical level, neither repetition priming for words nor morphemic priming would be expected. Finally, an activation block at this level is functional in this context because interactive activation between the letter and word level is beneficial to the letter identification processes needed for the letter search task This activation blocking process is presumed to be unconscious, given that there is no phenomenal awareness of it. It also remains to be seen what boundary conditions can be identified. Nevertheless, there are grounds for supposing that an activation blocking process is quite general in its action (see Besner & Stolz, 1995; Borowsky & Besner, 1993; Buchanan & Besner, 1993; Smith & Besner, 1993; Stolz & Besner, in press; Stolz & Neely, 1995). A remaining issue concerns why there is semantic priming following color judgments. The domain-specific processing hypothesis advanced in the introduction assumes that a global color discrimination makes no demands on the word recognition machinery. There is therefore no reason to deploy an activation block in the visual word recognition system.
ACTIVATION BLOCKING AND SEMANTIC PRIMING
Conclusions The review and experiments reported here suggest that the effects of different types of prime-processing tasks on semantic priming in lexical decision are understood better in terms of domain-specific operations than in terms of a general "levels-of-processing" account. Within-domain processing (such as letter search of the prime followed by lexical decision to a target) utilizes a temporary activation block at the lexical-semantic interface in the word recognition system so as to resolve competition for common resources shared by letter-level and semantic-level processes. Global 4 color discrimination belongs to a separable domain that can be executed without making demands on the resources that drive visual word recognition. Semantic priming following color discrimination can therefore be observed.
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NOTES I. An easy and a difficult discrimination were employed so as to head off the objection that it is the difficulty of the discrimination about the prime that dictates how much semantic priming is seen, rather than the type of discrimination. Granted, color discrimination, even when difficult, is still considerably faster than letter search, suggesting that it is easier. Nonetheless, there is just as much semantic priming in the difficult color discrimination as there is in the easy color discrimination (26 vs. 20 msec) despite the fact that the difficult color discrimination leads to significantly slower processing of both prime and target. If simple difficulty of the discrimination about the prime were critical, we would have expected to see a smaller priming effect for the difficult color discrimination than for the easy color discrimination. 2. In the one previous study that examined the facilitation of target processing following a color decision about the prime (Irwin & Lupker, 1983, Experiment 3), subjects either named or categorized a target immediately after naming the color of the prime. Facilitation ofre1ated targets was found in the categorization task but not in the naming task. The reason(s) for this are obscure at present. 3. Note that the interactive activation framework is problematic here because (I) there is no within-level spreading activation, and consequently it is in conflict with the claim that "semantic" priming reflects spreading activation within the semantic system (see, e.g.. Neely, 1991); and (2) its within-level inhibition assumption is difficult to reconcile with the effect of semantic priming if this inhibition completely kills off competitors at the lexical and semantic levels (see Stolz & Besner, in press, for a few simple assumptions that reconcile semantic priming and the interactive activation framework). 4. Note, however, that some forms of color judgments are associated with less extensive word processing. For example, Besner and Stolz (1995) reported that the magnitude of the Stroop effect decreased more when subjects identified the color of a single letter in a word than when all the letters in the word were the same color. We favor the view that some aspect of the local-global contrast is responsible for this, rather than the processing of color per se. (Manuscript received February 6, 1995; revision accepted for publication January 17, 1996.)
