Tracking the acquisition of orthographic skills in developing readers: Masked priming effects

Journal of Experimental Child Psychology 97 (2007) 165–182 www.elsevier.com/locate/jecp

Tracking the acquisition of orthographic skills in developing readers: Masked priming effects Anne Castles

a,b,*

, Chris Davis b,c, Pauline Cavalot b, Kenneth Forster d

a

c

Macquarie Centre for Cognitive Science, Macquarie University, Sydney, NSW 2109, Australia b Department of Psychology, University of Melbourne, Parkville, Vic. 3010, Australia MARCS Auditory Laboratories, University of Western Sydney, Penrith South DC, NSW 1797, Australia d Department of Psychology, University of Arizona, Tucson, AZ 85721, USA Received 2 August 2006; revised 25 January 2007 Available online 3 April 2007

Abstract A masked priming procedure was used to explore developmental changes in the tuning of lexical word recognition processes. Lexical tuning was assessed by examining the degree of masked form priming and used two different types of prime–target lexical similarity: one letter different (e.g., rlay fi PLAY) and transposed letters (e.g., lpay fi PLAY). The performance of skilled adult readers was compared with that of developing readers in Grade 3. The same children were then tested again two years later, when they were in Grade 5. The skilled adult readers showed no form priming, indicating that their recognition mechanisms for these items had become finely tuned. In contrast, the Grade 3 readers showed substantial form priming effects for both measures of lexical similarity. When retested in Grade 5, the developing readers no longer showed significant one letter different priming, but transposed letter priming remained. In general, these results provide evidence for a transition from more broadly tuned to more finely tuned lexical recognition mechanisms and are interpreted in the context of models of word recognition.  2007 Elsevier Inc. All rights reserved. Keywords: Orthographic processing; Lexical processing; Word recognition; Masked priming; Reading development

*

Corresponding author. Fax: +61 3 9850 6059. E-mail address: [email protected] (A. Castles).

0022-0965/$ - see front matter  2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jecp.2007.01.006

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Introduction Children’s capacity to recognize written words develops with remarkable speed. By the end of the second year of high school, adolescents reading English will typically recognize more than 80,000 words (Adams, 1990). How does this phenomenal capability develop? It is widely accepted that, at a basic level, beginning readers must learn a set of correspondences between the letters or graphemes of written words and the phonemes of spoken words, broadly referred to as alphabetic skills. However, to become accurate and efficient readers, at least of a deep orthography such as English, children ultimately need to acquire a rapid and flexible word recognition system that allows immediate identification of both regular and irregular words. It is this orthographic stage of word recognition that is typically seen as being the hallmark of skilled reading (e.g., Frith, 1985; Perfetti, 1992; Share, 1995). Although an orthographic stage of word recognition is widely posited in developmental theories, relatively little is known about how children reach this stage or how the process of its acquisition is to be characterized. In recent work, we have attempted to explore this question by using the masked priming technique to tap the acquisition of rapid and automatic word recognition processes in developing readers (Castles, Davis, & Forster, 2003; Castles, Davis, & Letcher, 1999; Davis, Castles, & Iakovidis, 1998). This procedure involves briefly presenting participants with a letter string (the prime) and then presenting them with a second letter string (the target) and asking them to perform some task on it (e.g., naming, lexical decision). Facilitation is found to occur when the prime is orthographically the same as the target (e.g., cat fi CAT), referred to as repetition priming, or in some cases when it is very similar to it (e.g., cal fi CAT), referred to as form priming. It is generally argued that the prime improves performance in these circumstances by activating the lexical representation for the target word and therefore assisting its processing in the subsequent task (Forster, Mohan, & Hector, 2003). There are several reasons for believing this to be the case. First, participants are generally unaware of having seen the prime, so any effects of explicit conscious processing are minimized (Forster & Davis, 1984). Indeed, if the duration of a word prime is increased so that participants can report it, no form priming is obtained (Humphreys, Evett, Quinlan, & Besner, 1987). Second, because targets are displayed for at least 500 ms, the effects are unlikely to be due to visual integration (Davis & Forster, 1994). Finally, priming is typically small or nonexistent when the target is a nonword (Forster, Davis, Schoknecht, & Carter, 1987; but see Bodner & Masson, 1997, 2001; Masson & Bodner, 2003). If the effects were occurring at other than an orthographic level, equal facilitation would be expected for nonwords as for words. Masked repetition and form priming effects of various kinds have been widely observed in skilled adult readers (for a review, see Kinoshita & Lupker, 2003). More important for the current purposes, such effects have now been found in children in the process of learning to read. For example, 9-year-olds reliably show strong repetition priming effects for high-frequency words that are present in their written vocabularies (Davis et al., 1998; Pratarelli, Perry, & Galloway, 1994), and Castles and colleagues (1999) reported similar-sized effects in children as young as 7 years. With this basic finding established, it becomes possible to use the masked priming procedure to explore aspects of word recognition in developing readers in more depth by manipulating aspects of the prime and comparing the

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pattern of priming shown by children at various stages of reading acquisition with that shown by skilled adult readers. Castles and colleagues (1999) took this approach in exploring the effect of neighborhood size (N) on form priming in developing readers. Neighborhood size is a broad metric of the similarity of a word to other words and is typically operationalized as the number of other words that can be created from a particular word by changing just one letter in any position. The word clap, for example, is a high-N word because 10 other words can be created from it by changing one letter (e.g., slap, flap, clip, clam), whereas bird is a low-N word because only 1 other word can be created from it by changing only one letter (i.e., bind). Adults have consistently been shown to produce a differential pattern of masked form priming for high- and low-N words: A one letter different form prime will produce facilitation in responding to a low-N word (e.g., lird fi BIRD), but such a prime will not produce priming for a high-N word (e.g., blap fi CLAP) (Forster et al., 1987). In contrast, Castles and colleagues (1999) found that developing readers in Grades 2, 4, and 6 showed form priming from one letter different primes for all words, regardless of whether they were high or low N. Castles and colleagues (1999) interpreted this finding in terms of a modification in the tuning of the automatic word recognition system in response to growth in the size of the written vocabulary. Early in word recognition development, they argued, the system can afford to be fairly broadly tuned and to accept similar, but not identical, inputs as candidates for a target word. This is because, at this early stage, many of the similar-looking competitors of the word are not yet in the reader’s vocabulary, and so gains in efficiency can be achieved by adopting a loose criterion without compromising accuracy. As written vocabulary grows, however, the system must adapt to the presence of many more similar competing words in the lexicon in the case of high-N words and so must tighten up the input criterion so as to maintain maximum accuracy. As such, they proposed that the developing word recognition system responds to ongoing changes in the recognition requirements of particular words at an item-based level (for a similar conceptualization of word recognition development as occurring in an item-based fashion, as opposed to a stage-based fashion, see Share, 1995). We refer to this theory henceforth as the lexical tuning hypothesis. In the current study, we sought to extend the findings of Castles and colleagues (1999) in two ways with the aim of providing further support for the lexical tuning hypothesis. First, we attempted to address an anomaly in their findings regarding the magnitude of form priming for high-N words at different grade levels. Besides predicting a difference between developing readers as a group and adults in the magnitude of priming for high-N words, Castles and colleagues expected to observe a gradual decrease in priming from Grade 2 to Grade 6, reflecting the adaptation of the system to the children’s growth in written vocabulary over this time. This was not found; significant form priming for high-N words was observed even in the Grade 6 readers. However, one limitation of this study was that it was a cross-sectional design; different children were tested at each grade level, and there was considerable variation in reading ability, as well as in the amount of priming, across the different age groups. Therefore, it is possible that this variability masked the presence of any modulation in form priming effects as a function of growth in vocabulary size; indeed, when we examined a subset of the Grade 6 readers who were better readers and who showed less variability in their performance, the magnitude of priming for high-N words was found to be much smaller. In the current study, we

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attempted to address this problem by first comparing adults with developing readers and then examining the change in priming for those same developing readers at a later date. The same children were assessed on their one-letter-different form priming for high-N words at two points in time: in Grade 3 and again in Grade 5. We hoped that this design would reduce between-group variance and maximize the opportunity to observe subtle changes in the tuning of children’s word recognition mechanisms over time. The second aim of this study was to explore another index of the precision of the tuning of children’s word recognition mechanisms so as to build on and elaborate the findings for masked one letter different form priming of high-N words. Here we focused not on the coding of letter identity, as in the one letter different priming effects, but rather on the coding of letter position within words. As children learn more and more words, the position of any particular letter within a word becomes more critical to its successful identification; for example, the position of the letters in the word tap needs to be coded more precisely once the words pat and apt must also be identified. Therefore, once again, based on the lexical tuning hypothesis, we expected to see a pattern of development from a fairly loosely tuned system for the coding of the position of letters within words toward a more precisely tuned system. No research to date has explored masked priming with transposed letter primes (e.g., hopsital) in children. However, such studies have been conducted in adults. Here the results have been somewhat mixed. Several studies report significant priming from nonword transposed letter primes (e.g., anwser) to lexical decisions on target words (e.g., ANSWER) (Forster et al., 1987; Perea & Lupker, 2003; Schoonbaert & Grainger, 2004). However, Andrews (1996) reported no such facilitation on naming times for similar items. It may be that, for adults, the magnitude of priming from transposed letter primes is modulated by factors such as word length and lexical density, which varied considerably across these three studies. In the current study, we selected short, high-frequency, and high-N words with the expectation that adult tuning for letter position for these items would likely be quite precise and, therefore, that priming from transposed letter primes would be minimal. At the very least, we expected that the magnitude of priming would be smaller for these items in the adults than in our developing reader sample. In summary, in the current study, we explored one-letter-different and transposed letter form priming in a group of adults and in a group of children when they were in Grade 3 and then, 2 years later, in the same children when they were in Grade 5. For both types of prime, we predicted that the Grade 3 readers would show a greater magnitude of priming than would the adults. At follow-up, we expected that the magnitude of priming for the participants in Grade 5 would be less than that shown in Grade 3 and that the magnitude of priming might fall between that shown by the Grade 3 readers and that shown by the adults. Such a pattern, if found, would provide support for the lexical tuning hypothesis.

Method Participants The adult participants were 24 undergraduate and graduate students from the University of Melbourne (16 women and 8 men, mean age = 18 years 10 months, SD = 13 months). All spoke English as their first language.

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The child participants were 23 Grade 3 students from a Melbourne primary school1 (14 girls and 9 boys, mean age = 8 years 6 months, SD = 5 months). They comprised one class, of several in Melbourne, participating in a larger longitudinal study of word recognition development being carried out by the first two authors. We selected this age group as our youngest age cohort for two reasons. First, we wished to test children who were in the relatively early stages of reading fluency but who had a sufficiently large sight vocabulary to be able to reliably perform lexical decisions on the short high-frequency target words we had selected. Second, the results of Castles and colleagues (1999) had indicated to us that lexical decision data from Grade 2 readers tend to be highly variable and error prone. Therefore, we concluded that Grade 3 participants represented the youngest age group for which we could expect sufficiently accurate and reliable responses to target words such that any priming effects would be able to be observed. A subset of 18 of these 23 children (14 girls and 4 boys) were retested in Grade 5, when they had reached a mean age of 10 years 5 months (SD = 5 months). Of the original 23 children, 5 were unavailable for retesting due to having moved schools or being absent.2 English was the first language spoken by all participants. Materials and design There were 27 word targets of four or five letters in length and 27 nonword targets of the same length that served as foils in the lexical decision task. The word targets were relatively high in frequency on average so as to maximize the chance that they would be recognized by the child participants (mean Kucera–Francis frequency = 132, range = 5–877). They were also selected to be as high in N as possible because these are the types of items for which we expected recognition processes would be modified by growth in vocabulary size (mean N = 6.1, range = 0–17). Each word target was matched with three nonword primes: a one-letter-different substitution prime, a transposed letter transposition prime, and an all-letters-different control prime.3 Substitution primes were formed by replacing one letter in each target word with another letter (e.g., rlay fi PLAY). The position of the substitution was varied approximately equally across the primes. Transposition primes were formed by reversing the sequence of two letters in the target word (e.g., lpay fi PLAY). Again, the position of the transposition was varied systematically across the primes. Finally, control primes were nonwords that shared no letters in any position with the target (e.g., meit fi PLAY). The 1 A total of 27 children were tested initially, but 4 made more than 40% errors on the lexical decision task and so were excluded. 2 There was no evidence that any systematic bias was introduced by this attrition. The excluded participants had a mean response time of 935 ms and error rate of 24%, well within the range of the larger sample. 3 A set of comparable word primes and targets (e.g., slat fi SALT) was also included in the initial study, and some preliminary results from the combined word and nonword data are reported in Castles and colleagues (2003). However, on closer examination of the data, we observed a problem with the word prime–target pairs. As Perea and Lupker (2003) also pointed out, transposed letter word pairs are a small and select set of words; therefore, we were not able to select target words from this set that were of sufficiently high frequency for the child participants to recognize reliably. Error rates on these items for the Grade 3 readers averaged 32%, and if we had applied even our liberal criterion of rejecting participants who made more than 40% errors in lexical decision, more than a third of the Grade 3 participants would have been removed. Therefore, we made the decision not to include the word prime conditions in any further analyses.

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nonword targets were paired with primes in the same fashion as the word targets so that participants would have no strategy for guessing the lexicality of the target from the prime. Three lists, or versions of the experiment, were assembled from these items so that each of the 27 target words would appear only once in each list but that, across lists, targets would appear in each condition. Such a design ensured that participants saw each target word only once. The order of the items within the lists was randomized. A full list of primes and target words can be found in Appendix. Two items, glad and stove, were subsequently found to produce extremely high error rates in the Grade 3 readers (72% and 62%, respectively, in the control conditions) and so were removed from the analyses, leaving 25 target words. Procedure The items were presented on a high-resolution super VGA color monitor controlled by an IBM-compatible PC using the DMDX display software (Forster & Forster, 2003). Each item consisted of a sequence of three stimuli: a forward mask consisting of a row of five hash marks (#####), the prime stimulus in lowercase letters, and the target stimulus in uppercase letters (which also served as a backward mask). The forward mask and the target were presented on the screen for 800 ms, whereas the prime was displayed for a period of 57 ms. The letters on the screen were approximately 10 mm in height and 8 mm in width. Participants were seated approximately 30 cm from the viewing screen. The adult participants were seated in a sound-attenuated booth at the university and received 10 practice items (half word targets, half nonword targets) followed by the test items. They were asked to decide whether the letter sequence presented in uppercase letters was a word or not, responding as quickly and accurately as possible. They indicated their responses by pressing one of two response buttons. Each participant received a different pseudorandom ordering of items so that practice and fatigue effects would be balanced across conditions. The Grade 3 readers were assessed at their school in a quiet room. Prior to completing the experiment, the children were taken through a preliminary exercise to help them understand the requirements of the lexical decision task. They were asked to help the experimenter sort out ‘‘silly nonsense words’’ from real words that were shown to them on flashcards. On one side of each flashcard was a row of hash marks, and on the other side was a word or a nonword. The experimenter held up the cards one at a time, showing the hash marks side, and then turned the card over. The children were asked to say whether they thought the item was a real word or a nonsense word, and feedback was given. Participants were then asked to perform the same task on the computer and were given the same instructions and practice items as were the adults. All other aspects of the procedure were the same as for the adults. When retested in Grade 5, the participants were again tested in a quiet room at the school. All aspects of the procedure were the same as when they were tested in Grade 3. Data treatment Latency and error data were collected. Latency data from incorrect responses were excluded from the analysis. A lower cutoff was established at 150 ms, and responses faster

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than this were deemed to be errors. The application of this cutoff resulted in the exclusion of less than 1% of the data. Analyses of variance (ANOVAs) were carried out on the latency and error data to examine the pattern of priming effects. In each case, two analyses were conducted: one for participant data (collapsing over items, reported as F1) and another for item data (collapsing over participants, reported as F2). Factors in the analyses of the adult and Grade 3 data were participant group (adults vs. Grade 3), version of the experiment (1 vs. 2 vs. 3), and prime type (transposition vs. control, substitution vs. control). The version factor was of no theoretical interest and was included as a dummy variable only. The data for the children when retested in Grade 5 could not be included in the same analysis as those for when they were in Grade 3 because the children received different versions of the experiment across the two time periods, and this could not be controlled for within a repeated measures design. Therefore, the Grade 5 results are reported as separate ANOVAs with the factors of version (1 vs. 2 vs. 3) and prime type (transposition vs. control).

Results Table 1 presents summary statistics for the adults and the children at each grade level for the substitution, transposition, and control priming conditions. These data were analyzed in two separate sets of analyses. First, the pattern of priming for the adults versus the Grade 3 readers was compared in a set of between-group ANOVAS. Following this, the results for the follow-up testing of the participants in Grade 5 were analyzed separately. There were two reasons for dealing with the Grade 5 data separately. First, as mentioned earlier, we could not control for the different versions of the experiment that the children received across the two time periods within a repeated measures design. Second, because we had lost five participants due to attrition, combining the Grade 3 and the Grade 5 data into a single analysis would have resulted in losing valuable data at the Grade 3 level.

Table 1 Mean response times and percentage errors for the substitution, transposition, and control priming conditions for adults, Grade 3 readers, and a subset of the Grade 3 readers who were retested in Grade 5 Age group

Prime type

Example

RT (ms)

Errors (%)

Adults

Substitution Transposition Control

rlay/PLAY lpay/PLAY meit/PLAY

576 (102) 575 (93) 583 (81)

2.3 (4.6) 1.8 (5.3) 2.3 (4.6)

7 8

Grade 3

Substitution Transposition Control

rlay/PLAY lpay/PLAY meit/PLAY

875 (155) 889 (152) 953 (188)

14.0 (15.4) 14.6 (12.8) 22.3 (17.0)

78 64

Grade 5

Substitution Transposition Control

rlay/PLAY lpay/PLAY meit/PLAY

741 (176) 705 (142) 748 (152)

9.2 (13.8) 5.5 (8.7) 12.3 (14.2)

7 43

Note. Standard deviations are in parentheses. RT, response time.

Priming (ms)

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Results for adults and Grade 3 readers One-letter-different form priming As can be seen from Table 1, the skilled adult readers showed little evidence of priming from one-letter-different substitution primes for these high-frequency and high-N target words, with facilitation averaging only 7 ms. The Grade 3 readers, in contrast, showed an average facilitation effect of 78 ms. The analyses of the latency data revealed a significant main effect of participant group (adults vs. Grade 3) by both participants, F1(1, 41) = 87.12, p < .001, and items, F2(1, 22) = 198.16, p < .001, indicating that the adults made faster lexical decisions overall than did the child participants. There was also a significant main effect of prime type (substitution vs. control), indicating a priming effect overall, F1(1, 41) = 6.84, p < .05, and F2(1, 22) = 4.27, p = .05. However, this main effect was modulated by an interaction between participant group and prime type that was significant by participants, F1(1, 41) = 4.63, p < .05, but did not reach significance by items, F2(1, 22) = 2.49, p > .05. Post hoc contrasts on the participant data revealed that the priming effect was significant in the Grade 3 readers (p < .05) but not in the adults (F < 1). Analyses of the error data produced a similar pattern of results. There was a significant main effect of participant group, F1(1, 41) = 23.02, p < .001, and F2(1, 22) = 33.02, p < .001. The main effect of prime type was significant by participants, F1(1, 41) = 11.26, p < .01, and approached significance by items, F2(1, 22) = 3.51, p = .07. However, once again, these main effects need to be interpreted in the context of a significant interaction between participant group and prime type, F1(1, 41) = 11.26, p < .01, and F2(1, 22) = 4.51, p < .05. Post hoc contrasts on the participant data revealed a significant priming effect in the Grade 3 readers (p < .01) but no hint of an effect in the adults (F < 1). There were no significant priming effects in the nonword target data. Transposed letter priming As with the substitution priming, the transposed letter data presented in Table 1 suggest negligible facilitation in the skilled adult readers (M = 8 ms) but much stronger priming in the Grade 3 readers (M = 64 ms). Analyses of the latency data once again revealed a significant main effect of participant group, F1(1, 41) = 93.53, p < .001, and F2(1, 22) = 198.05, p < .001. There was also a significant transposed letter priming effect overall, as indicated by a main effect of prime type (transposition vs. control), F1(1, 41) = 7.09, p < .05, and F2(1, 22) = 7.08, p < .05. These main effects were again supplemented by an interaction between participant group and prime type that was significant by both participants, F1(1, 41) = 4.04, p = .05, and items, F2(1, 22) = 4.60, p < .05. Post hoc contrasts on the participant data revealed significant transposed letter priming in the Grade 3 readers (p < .05) but not in the adults. This pattern of priming results across the two age groups was again largely mirrored in the error data. Overall, adults made fewer errors than did Grade 3 readers, F1(1, 41) = 39.89, p < .01, and F2(1, 22) = 34.68, p < .001. There were also fewer errors made overall in the transposition condition than in the control condition, a priming main effect that approached significance by participants, F1(1, 41) = 3.89, p = .055, and was significant by items, F2(1, 22) = 7.92, p < .05. There was a strong trend toward a significant interaction between participant group and prime type that approached significance by

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Table 2 Mean transposed letter priming effect in response times and percentage errors for adults and Grade 3 readers as a function of position of transposition Age group

Priming Internal letters

Adults Grade 3

External letters

RT (ms)

Errors (%)

RT (ms)

Errors (%)

8 (53) 155 (233)

0.0 (10.7) 10.8 (13.9)

5 (101) 20 (179)

1.0 (6.2) 8.7 (20.1)

Note. Standard deviations are in parentheses. RT, response time.

participants, F1(1, 41) = 3.00, p = .09, and was significant by items, F2(1, 22) = 5.18, p < .05. Again, there were no significant priming effects in the nonword target data.

Effect of position of transposition Perea and Lupker (2003) reported stronger transposed letter priming in their item set for items where the transposition occurred in the internal letters of the word than for items where it occurred in the external letters of the word. This finding is also consistent with the developmental work of Ehri (1991), who has amassed considerable evidence that the consonant frame is particularly important for early word recognition. Therefore, we examined our item data to see whether we could find any evidence for priming in participants that was limited to transpositions in particular positions in the prime and whether this varied across the adult and Grade 3 participants. We categorized the items in Appendix according to whether the transposition prime involved an alternation of external letters (e.g., noes fi NOSE) or of internal letters (e.g., drak fi DARK), and we compared the magnitude of priming for the two categories. There were 12 items with internal transpositions and 13 with external transpositions. The results are presented in Table 2. As can be seen, there was little apparent evidence of any variation in priming as a function of position of transposition in the adults, but there was some indication of an effect in the latencies for the Grade 3 readers; the internal letter transpositions produced a very large mean priming effect of 155 ms, whereas the mean priming effect from the external letter transpositions averaged only 20 ms. However, as is evident from the standard deviations, the data were highly variable. To explore the pattern of results further, the priming data were submitted to an ANOVA with version (1 vs. 2 vs. 3), participant group (adults vs. Grade 3), and position of transposition (internal vs. external) as factors. Because the classification of internal versus external transposition had taken place post hoc, frequency was not perfectly controlled across the two conditions,4 so we also included Kucera–Francis frequency scores as a covariate in the analysis. There was no overall main effect on magnitude of priming of position of transposition in either latencies, F2(1, 18) = 2.71, p < .05, or errors, F2 < 1, and there was no significant interaction between participant group and position of transposition in either latencies, F2(1, 18) = 2.61, p < .05, or errors, F2 < 1. However, given that this analysis was post hoc, and that a relatively small number of items contributed to the 4

Mean Kucera–Francis frequency of the internal transposition items was 186 (SD = 239) and of the external items was 92 (SD = 133), a difference that did not reach significance, t(23) = 1.22, p > 05.

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mean for each condition (12 for internal, 13 for external), these suggestive results probably should be treated with caution. Results for follow-up testing of participants in Grade 5 Descriptive statistics for the performance of the children when followed up in Grade 5 are presented in Table 1. As can be seen, the magnitude of the one letter different form priming effect appeared to have decreased substantially; in the latency data, the substitution primes produced an average of only 7 ms facilitation compared with the control condition. This was not close to significance by either participants or items (all Fs < 1). There were also no significant priming effects in the error data (all Fs < 1). There remained, however, significant transposed letter priming effects. In the latency data, the transposition primes produced an average facilitation effect of 43 ms compared with the control condition that was significant by both participants and items, F1(1, 15) = 5.96, p < .05, and F2(1, 22) = 4.33, p < .05. There was a similar trend in the error data, with fewer errors being made in the transposition condition than in the control condition, a difference that trended toward significance by participants, F1(1, 15) = 2.79, p = .11, and items, F2(1, 22) = 4.48, p = .058. Once again, there was no interaction between prime type (transposition vs. control) and position of transposition (internal vs. external) in either the latency or error data (all F2s < 1). There were again no priming effects in the nonword data.

Discussion We begin this section by summarizing the results first for the adults and Grade 3 readers and second for the follow-up testing of the participants in Grade 5. We then consider how the findings might be interpreted in terms of theoretical models of the developing word recognition system. Results summary Adults and Grade 3 readers The Grade 3 readers in this experiment showed a distinctly different pattern of priming effects from those displayed by the adults, with the children displaying strong facilitation on lexical decision responses for words preceded by both transposed letter and one letter different primes and the adults displaying no facilitation from either type of prime. Let us consider the results in terms of our initial hypotheses. For the adults, in relation to the one-letter-different substitution primes, no priming was the expected result. This finding is consistent with numerous previous studies reporting negligible form priming effects for words from high-density orthographic neighborhoods such as those used here (Forster & Davis, 1991; Forster et al., 1987; Segui & Grainger, 1990). As such, against the background of findings of significant one letter different form priming effects for words with few or no neighbors, the results support our general conceptualization of the word recognition system as having adapted to the differing discrimination demands of words. For the relatively difficult-to-discriminate words used in the current study, it would appear that there is a high level of precision in adult recognition mechanisms.

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For the transposed letter priming effects, the predicted pattern of results for adults was more difficult to formulate. Whereas some studies have reported robust facilitation from transposed letter primes in adults (Forster et al., 1987; Perea & Lupker, 2003), others have found no priming (Andrews, 1996). We found no evidence for priming in the current study. This was the case regardless of the position of the transposition within the prime, in contrast to the findings of Perea and Lupker (2003). We suspect that the magnitude of facilitation observed from transposed letter primes in skilled adult readers is most likely modulated by factors such as the length, frequency, and overall neighborhood density of the target words. Indeed, when we looked post hoc at the size of the transposed letter priming effect for the adults in our study as a function of N (P5 vs. <5), we found an average effect of 3 ms for the high-N items and an average effect of 26 ms for the low-N items, seemingly suggesting that N is a critical factor here (although the interaction was not significant, F1(1, 21) = 2.05, p > .05). It would seem, therefore, that the high frequency and density of the words overall in the current study minimized the opportunity to observe priming effects in adults because of the precise recognition mechanisms required to quickly and accurately recognize these kinds of words. Most important for the current purposes, our finding of negligible one letter different or transposed letter form priming for the current set of items in skilled adult readers provides a baseline against which to examine analogous priming effects in developing readers. For adults, recognition mechanisms for these items appear to have become very finely tuned. According to the lexical tuning hypothesis, however, less skilled readers should have much less precise mechanisms for these words, and this should be reflected in generally stronger priming effects from nonidentical primes. Overall, the results for the Grade 3 readers supported this hypothesis. As was expected from the neighborhood size work reported in Castles and colleagues (1999), the Grade 3 readers showed significant priming from one letter different form primes even for high-N targets. They also showed strong and significant priming from the transposition primes, with the magnitude of the effect averaging more than 60 ms. Effects of this size are typical of what we have seen in identity priming for children of this age (see, e.g., Castles et al., 1999; Davis et al., 1998), suggesting that, for these participants, one letter different and transposed letter primes may have been as effective in activating the representations for the target words as were the words themselves. In the case of the one letter different primes, this suggests that the immature word recognition system tolerates a degree of mismatch in letter identity even for words that are highly similar to other words. In the case of the transposed letter primes, because these primes shared all of their letters with the target words but differed in the position of those letters, it would appear that the developing word recognition system tolerates a degree of error in letter position if letter identity requirements are fulfilled. The Grade 3 readers, of course, had slower response times overall than did the adults. One possible alternative explanation of our results is that they might simply be a function of the time course of orthographic activation. That is, Grade 3 readers took longer to respond than did adults, and this extra processing time provided more opportunity for the children to show prime-based activation compared with the adults. If this were the case, lexical density per se would not be contributing to the difference in priming across the two groups. However, we do not believe that a time course explanation can fully account for our current set of results for the following reason: This hypothesis would predict that there would be a strong correlation between overall response time and magnitude

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of priming; faster children should show less priming than should slower children. We calculated overall response times for the Grade 3 readers by averaging across all conditions and then correlated this with magnitude of priming. The correlations tended to be in the positive direction, in support of this hypothesis, but they were weak and nonsignificant for both the correlation between one letter different priming and response time, r(23) = .22, p > .05, and the correlation between transposed letter priming and response time, r(23) = .23, p > .05. Therefore, there does not seem to be a strong relation between priming and overall response time. Interestingly, the priming effects observed were apparent not just in the latencies but also in the error data. For these very early readers, it may be that, although a representation exists for a particular word, the presented target does not always produce sufficient activation for that word to be successfully recognized and responded to in the lexical decision task. However, when the same word is preceded by a prime that preactivates the representation of the target word (in this case the substitution and transposed letter primes), the boost provided makes a failure of recognition less likely to occur. Presumably, the same would also be true of adults in those cases where a prime has successfully activated the representation of a target word, but adult performance on these kinds of lexical decision tasks is typically so close to ceiling that there is little room for such effects to be observed. In summary, consistent with the lexical tuning hypothesis, it would seem that there is less precision in the recognition processes of Grade 3 readers compared with adults both in terms of the requirements for letter identity matching and in terms of the requirements for letter position. Even stronger support for this hypothesis would be provided if the Grade 3 readers in this experiment began to show evidence for more precise tuning as they grew older and acquired larger written vocabularies. This question was explored by following up the children two years later, when they were in Grade 5. Follow-up testing of participants in Grade 5 The findings for one letter different substitution priming in the participants in Grade 5 were consistent with the lexical tuning hypothesis. Whereas these children had shown significant facilitation from these primes in Grade 3, they no longer showed any priming effects in Grade 5 and indeed their pattern of priming closely resembled the adult pattern. It would appear that the children’s recognition mechanisms for these words had been modified over the time period such that a nonidentical prime that was previously sufficient to activate the lexical representation for a target word no longer was able to do so. This finding is particularly compelling given that the modulation in priming was shown for the same children over time. This would seem to provide strong evidence that recognition mechanisms for individual words change as a function of the increasing size and density of the overall lexical system.5

5 Even stronger evidence for the lexical tuning hypothesis would be provided if these same Grade 5 participants continued to show significant one letter different priming for a set of low-N words. We did not manipulate N in the current study. However, as mentioned, these children were part of a larger longitudinal study of word recognition development and had in fact been examined on their priming for a set of low-N words in Grade 5 as part of some separate, but related, research. The items were, of course, different from those in this study, but as a general indication they showed an average priming effect of 65 ms (SD = 59), suggesting that substantial priming was still evident for low-N words.

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The one-letter-different priming result contrasts with that reported by Castles and colleagues (1999), who found significant form priming for high-N target words even in a group of Grade 6 readers. However, there was a large amount of variation in Castles and colleagues’ data, and the better readers in the group tended to show the smallest priming effects. In addition, the Grade 4 readers in that study did show a substantial reduction in priming. This would suggest that the magnitude of priming for the Grade 6 readers in that study may have been misestimated. Sources of variation in the magnitude of priming shown by individual children is an issue that deserves further exploration, ideally using a more extensive and detailed examination of individual priming effects than was carried out in the current group-based study. The transposed letter priming effects in the participants in Grade 5 were a notable contrast to the one letter different effects. In this case, robust priming effects remained. One interpretation of this result is that the word recognition system is on average slower to develop precision in letter position coding than to develop precision in letter identity coding (at least for high-N words). So, by Grade 5, a change in letter identity will disrupt the ability of a prime to activate the lexical representation of a high-frequency and high-density target word, but a change in letter position, with letter identity preserved, will be insufficient for the system to reject the prime as a candidate. This hypothesis is in some ways consistent with the adult transposed letter priming data; findings of significant transposed letter priming effects for some words, even in adults, suggests that letter position coding presents a particular challenge to even the most skilled word recognition system (Forster et al., 1987; Perea & Lupker, 2003). The current experiment aimed to extend the findings of Castles and colleagues (1999) in two ways. The first was to explore the effects of small changes in letter identity on the magnitude of priming observed for the same items in the same children at two different ages, and the second was to explore the effects of changes in letter position with identity preserved. In both cases, we found support for the hypothesis that children’s word recognition mechanisms are less precisely tuned than those of adults, consistent with the lexical tuning hypothesis. However, the two different manipulations of lexical similarity produced different patterns developmentally across the two child age groups. For changes in letter identity, tuning appeared to have reached adult levels by Grade 5 for the items used, whereas for changes in letter position, the Grade 5 readers continued to show broader tuning than did the adult readers. We now turn to considering in detail precisely how these developmental changes in lexical tuning might be accounted for within models of word recognition. What is tuned in a developing word recognition system? Letter identity effects We turn first to the results for changes in letter identity, where participants were presented with neighbor nonword primes that were one letter different from the subsequently presented target words (e.g., rlay fi PLAY). In the adult word recognition literature, priming effects of this kind have been interpreted in terms of two classes of models: the search model (Forster, 1976, 1998) and the interactive activation model (McClelland & Rumelhart, 1981). In the most recent instantiation of the search model, lexical access involves a two-stage filtering process. In the first stage, lexical entries that closely match

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a presented letter string are selected as candidates, and these entries are then opened. This opening process takes time and is a necessary first step to retrieve information from the entry. In the second stage, each candidate is subjected to a more detailed verification process in which the orthographic properties retrieved from the lexical entry are compared with the stimulus input and the closest matching candidate is selected. The other entries are then closed and the word is identified. Masked substitution priming effects such as those described earlier are explained by assuming that a sufficiently close matching prime opens the target entry at the first stage, allowing faster verification of the subsequently presented target at the second stage. The reason why substitution priming is not observed in high-density environments is that the orthographic overlap between the prime and the target is not sufficient for the target entry to become a candidate. This could be because words with many neighbors require a higher ‘‘match coefficient’’ to become a candidate, or it could be because the words in a dense cluster are recoded to minimize their overlap (a case of acquired distinctiveness), as argued by Forster and Taft (1994). This reduction in overlap is achieved by recoding the words in terms of higher order letter units such as bodies. Thus, the overlap between the words fact and face would be minimal if these words were coded in terms of bodies (e.g., f + [act] and f + [ace]) because [act] and [ace] are completely separate units. Under these conditions, there would be no reason to expect priming for pairs such as fane– FACE. Therefore, the gradual reduction in this kind of priming that we see from the youngest to oldest participants here could reflect developmental changes in the way in which the match coefficient is computed. For early readers, to assist rapid recognition of a small number of known words, the required match coefficient may be preset to something less than 100% overlap for all words. However, for older readers, increased exposure to other similar-looking words forces an adjustment to the matching process either by requiring 100% letter overlap for words in high-density neighborhoods or by changing the level at which the overlap is computed, that is, switching from the letter level to the body level. In the interactive activation model, masked substitution priming effects are modeled somewhat differently. On this account, when a word is presented, the target word itself and all of its neighbors are activated, with the target receiving the most activation due to its complete letter overlap with the presented string. At the word level, the target word and all of its neighbors have inhibitory links with each other so that recognition is achieved when the target word successfully inhibits the other neighbor competitors due to its higher overall activation. Masked substitution priming effects are modeled by assuming that a neighbor prime (a word or a nonword) preactivates the target word such that it is already boosted significantly before the target is presented. Therefore, activation levels on presentation of the target are able to reach criterion more quickly than for unprimed words. However, this will not be successful if the prime also preactivates too many other neighbor words because lateral inhibition between them will cancel out any boost that is provided. Therefore, on this model, the gradual reduction in priming seen developmentally would be represented in terms of an increasing number of inhibitory links being formed with a target word as a child learns new similar-looking written words. We have represented this schematically in Fig. 1. Early in reading acquisition, for example, a child may have a representation for the word cat but not for any of its neighbors (e.g., cap, hat, sat). Therefore, a substitution prime (dat) will preactivate cat, but no other words that can inhibit this will be preactivated, so priming will occur. However, as written vocabulary grows, the prime dat will also activate many other words (e.g., hat, sat), and

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Fig. 1. Schematic representation of the modeling of developmental changes in masked substitution priming within an interactive activation framework. (a) Word recognition system of an early reader, where only a small number of representations have been formed. Preactivation of the target word by the prime produces facilitation in responding when the target is presented subsequently. (b) Word recognition system of a more advanced reader, where the number of word representations has grown. Preactivation of the target word by the prime does not produce facilitation because the target’s activation is reduced by lateral inhibition from neighboring words.

lateral inhibition between these words and the target word will reduce priming. (Note, on this account, that it is the shared neighbors between the prime and target that will be particularly important, as proposed by Van Heuven, Dijkstra, Grainger, & Schriefers, 2001.) Letter position effects We now turn to the results for changes in letter position, where participants were presented with masked primes that retained all of their letters in common with the target but with a position transposition (e.g., lpay fi PLAY). Even in Grade 5, readers continued to show significant priming for these items, although their priming by this age from one letter different neighbor primes had dropped to nonsignificant levels. The adult readers in this study did not show priming for these items. These results are difficult to account for within models of word recognition as currently formulated, and indeed the demonstration of masked transposed letter priming effects even in adults in some circumstances has been seen as a significant challenge to the letter coding schemes used in current models (Andrews, 1996; Perea & Lupker, 2003; Schoonbaert & Grainger, 2004; Van Assche & Grainger, 2006). Let us consider the interactive activation model. On this model, as discussed, letter identity is coded within a particular position (i.e., it is slot coded), and words that differ from a target by one letter in any slot position are classified as neighbors. Thus, on this model, sale is a neighbor of salt, but slat is not. Based on the mechanism for masked form priming proposed above, therefore, transposed letter primes should produce less priming than should one letter different neighbor primes as a rule because the former will activate fewer competing words to inhibit the target word than will the latter. Thus, the presence of strong and persistent transposed letter priming effects for the children in the current study up to Grade 5, and their

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presence even in adults in some circumstances, would seem to suggest the need for some modification of the architecture of existing dominant models of word recognition. The solution may lie in a move toward spatial letter coding schemes, such as those employed in the SOLAR model of Davis (1999), rather than slot coding schemes. In this model, all letter codes are position independent and the order of the letters in a presented word is coded by a monotonically decreasing sequence of activations: The first letter of a word receives the highest activation, the second letter receives slightly less, and so on through to the end of the word. On this coding, therefore, slat is a close neighbor of salt because all four letters receive some activation in common. If a scheme such as this were incorporated into the interactive activation model, the current results could be accounted for by proposing that spatial coding in early readers contains little differentiation in activation levels as a function of letter order but that this differentiation increases with age and reading experience. So, in very early readers, the precise order of letters within words may be slow to resolve, and a prime such as lpay may activate the representation for the target play nearly as strongly as does the input of play itself. With increasing experience, a steeper decrease in the activation function across letters will decrease the lexical similarity between lpay and play and, therefore, will reduce priming. Of course, we have no direct evidence from the current study that these hypothesized changes in activation function occur as a result of increases in lexical density as opposed to being the result of greater reading experience per se. However, as noted earlier, there appeared to be a tendency for the skilled readers in our sample to show strong transposed letter priming for the lower N items in the set but much weaker transposed letter priming for the higher N items. This would appear to suggest that there is an association between lexical density and the adaptation of letter coding processes. Further work is needed to explore this link more closely; it may be that what is required is an explanation for transposed letter effects that involves both item-specific effects as a function of density and a general developmental trajectory toward greater position differentiation at the letter level. Two phenomena remain to be explained on such an account. First, why do adults show masked priming from transposed letters in some circumstances (Forster et al., 1987; Guerrera, 2004; Perea & Lupker, 2003; Schoonbaert & Grainger, 2004)? As with one letter different priming, and as discussed, one would expect an interaction with neighborhood size. For words with few neighbors and little lateral inhibition at the word level, the level of activation from a transposed letter prime may still be sufficient to produce priming, whereas for words with many neighbors such as those presented here, the inhibition at the word level would act to cancel out any activation from the letter level. Second, why should the adaptation of the system in the precision of its coding of letter position take longer than the adaptation for letter identity? One must assume here that developmental modifications at the word level of the interactive activation model occur first, with increasing numbers of representations of newly learned words being formed and exerting lateral inhibition on other words. Subsequent to, and perhaps in part because of, this growing lexical density, modifications at the letter coding level occur to maximize differentiation between letter positions and so increase the precision of information fed through to the lexical level. In conclusion, in the current study we have attempted to use masked priming to examine in detail the nature of word recognition processes used by early readers and to determine the ways in which these processes change with age and reading expertise. The results are consistent with a conceptualization of orthographic development as proceeding from a

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broadly tuned mechanism to a very precisely tuned mechanism. Such findings are not unique to the orthographic domain. They can be placed in the broader context of recent work suggesting that spoken word recognition (Metsala, 1997) and speech production (German & Newman, 2004) are also modulated by lexical density factors. Together, this body of work points to a fruitful way forward for understanding the ways in which children adapt to the enormous growth in their spoken and written vocabularies over time.

Appendix. Primes and target words ITEMS and transposition, substitution, and control priming conditions, respectively: ANGRY, angyr, angrf, atler; BAKER, abker, bcker, mivet; BAND, badn, banc, lese; CAPE, acpe, dape, bati; DARK, drak, derk, eban; EACH, eahc, eafh, ibnd; FAST, afst, gast, eben; GLAD, gald, ghad, blet; HATE, haet, hati, obok; HEART, herat, heaet, derad; HORSE, ohrse, gorse, prdea; ITCH, tich, ikch, drae; JUNK, jnuk, julk, crat; KICK, kikc, kicm, bais; LAMB, almb, namb, cohp; LEMON, lmeon, ledon, mivet; NIGHT, ngiht, nilht, blaes; NORTH, nroth, nosth, teerg; NOSE, noes, nosp, beda; OCEAN, coean, opean, erget; PLAY, lpay, rlay, meit; SALE, slae, sase, obth; SHAPE, shaep, shaie, diert; SLIDE, sldie, slire, reast; STOVE, sotve, smove, celat; THING, thnig, thiog, srane; WHITE, whiet, whitn, esgde.

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