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Location invariance in masked repetition priming of letters and words
Acta Psychologica 142 (2013) 23–29
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Location invariance in masked repetition priming of letters and words Yousri Marzouki a,⁎, Martijn Meeter b, Jonathan Grainger a, c a b c
Laboratoire de Psychologie Cognitive, Aix-Marseille University, Marseille, France Department of Cognitive Psychology, Vrije Universiteit Amsterdam Centre National de le Recherche Scientifique, France
a r t i c l e
i n f o
Article history: Received 12 June 2012 Received in revised form 27 October 2012 Accepted 30 October 2012 Available online 21 November 2012 PsycINFO classification: 2346 2340 2323
a b s t r a c t Earlier studies have suggested that information from a prime stimulus can be integrated with target information even when the two stimuli appear at different spatial locations. Here, we examined such location invariance in a masked repetition priming paradigm with single letter and word stimuli. In order to neutralize effects of acuity and spatial attention on prime processing, subliminal prime stimuli always appeared on fixation. Target location varied randomly from trial to trial along the horizontal meridian at one of seven possible locations for letter stimuli (−7° to +7°) and three positions for word stimuli (−4°, 0°, +4°). Speed of responding to letter and word targets was affected by target location, and by priming, but the size of repetition priming effects did not vary as a function of target location. These results suggest that masked repetition priming is mediated by representations that integrate information about object identity independently of object location. © 2012 Elsevier B.V. All rights reserved.
Keywords: Letter and word perception Masked repetition priming Eccentricity Location-invariance
1. Introduction Masked repetition priming has been extensively used to investigate early processes involved in letter and word perception (e.g., Forster & Davis, 1984; Holcomb & Grainger, 2006; Jacobs & Grainger, 1991; Seguì & Grainger, 1990). The standard way of using this technique consists in presenting the prime and the target at the same spatial location, and comparing performance to target stimuli that are preceded by the same or a different prime stimulus. However, some recent studies have departed from this tradition by manipulating the location of prime and target stimuli in masked repetition priming (e.g., Marzouki & Grainger, 2008; Marzouki, Meeter, & Grainger, 2008). In one critical condition in these studies, targets were always presented on fixation and prime location varied randomly from trial to trial at various eccentricities along the horizontal meridian. Priming effects were found to diminish as prime eccentricity increased, with both word targets (Marzouki & Grainger, 2008) and single letter targets (Marzouki, Meeter, et al., 2008; Marzouki, Midgley, Holcomb, & Grainger, 2008). Visual acuity was excluded as the causal factor since priming effects did not diminish in conditions where prime and target position co-varied, with ⁎ Corresponding author at: Laboratoire de Psychologie Cognitive, Université d'AixMarseille, 3 place Victor Hugo, 13331 Marseille cedex 1. Tel.: + 33 488 57 69 08; fax: + 33 488 57 68 95. E-mail address: [email protected] (Y. Marzouki). 0001-6918/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.actpsy.2012.10.006
primes and targets occupying the same randomly varying location. If visual acuity were the critical factor, one would also expect priming effects to diminish as a function of prime eccentricity in these conditions. Although there was some evidence that acuity was having an influence, it was clearly not entirely responsible for the pattern of priming effects found with centrally located targets. Two accounts of this pattern of priming effects were put forward by Marzouki, Meeter, et al. (2008), Marzouki, Midgley, et al. (2008). According to one account, called the integration account, it is the spatial separation of prime and target stimuli that is the critical factor. Here it is hypothesized that limits in translation invariance determine how well information extracted from the prime will influence processing of the target. According to an alternative account, called the attentional account, it is the amount of spatial attention directed to the prime location that is the critical factor. When targets always appear on fixation, then attention is hypothesized to be maximal on fixation and diminishes as a function of eccentricity, therefore causing priming effects to diminish. Marzouki, Meeter, et al. (2008), Marzouki, Midgley, et al. (2008) put these two accounts to test in an experiment that dissociated the confounding factors of prime eccentricity and prime-target spatial separation by orthogonally manipulating prime and target locations, with three different positions (−2.3°, 0°, +2.3°). The results showed that priming effect sizes were not affected by either prime or target location. Thus, prime-target spatial separation was not influencing the size of repetition priming effects. This led Marzouki et al. to propose that it is
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changes in the deployment of spatial attention that best accounts for the pattern of priming effects reported by Marzouki and Grainger (2008). Furthermore, this account is in line with the results of studies showing that an exogenous spatial cue can modulate the size of masked repetition priming effects (Besner, Risko, & Sklair, 2005; Lachter, Forster, & Ruthruff, 2004; Marzouki, Grainger, & Theeuwes, 2007; Marzouki, Midgley, et al., 2008). Marzouki, Meeter, et al. (2008), Marzouki, Midgley, et al. (2008) proposed an adaptation of Grainger and van Heuven's (2003) model of orthographic processing, shown in Fig. 1, as a specific implementation of their attentional account of variations in repetition priming effects as a function of prime eccentricity. This adaptation draws a distinction between location-specific and location-independent letter detectors. Location-specific detectors code for the presence of a given letter identity at a given location along the horizontal meridian and send activation on to the corresponding location-independent letter detector that codes for the presence of a given letter independently of its location. Another important aspect of this proposal is that spatial attention can modulate the activity of location-specific letter detectors in the alphabetic array. The key ingredient of Marzouki, Meeter, et al.'s (2008), Marzouki, Midgley, et al.'s (2008) account of effects of prime and target location on masked repetition priming is the role played by high-level location-invariant letter representations in integrating information across prime and target stimuli independently of their location (see Fig. 1). Visual acuity and spatial attention are two factors that can modulate this integration process by affecting the level of activity in location-specific letter representations. One straightforward prediction of this account is that when possible influences of acuity and attention are controlled for, then the size of repetition priming effects should not depend on the distance separating prime and target, even for quite large separations (larger than that used in Experiment 2 of the Marzouki, Meeter, et al., 2008; Marzouki, Midgley, et al., 2008, study). In order to rule out any role of acuity and attention in the present study, prime stimuli were always located on fixation and target location varied randomly from trial to trial at different eccentricities along the horizontal meridian. According to Marzouki et al., masked repetition priming effects should not vary as a function of target eccentricity. Experiment 1 tests this prediction with single letter stimuli, and Experiment 2 tests the same prediction using words. 2. Experiment 1 2.1. Method 2.1.1. Participants Five individuals (mean age = 24 years, 3 females) participated in this experiment for monetary compensation. All were right-handed and reported having normal or corrected-to-normal vision. 2.1.2. Design and stimuli Sixteen letters (all consonants) served as targets along with sixteen pseudo-letters designed using Font Creator 4.0 software and tested in prior research (Marzouki, Meeter, et al., 2008; Marzouki, Midgley, et al., 2008; Marzouki et al., 2007; New & Grainger, 2011). Each pseudo-letter was derived from a target letter by re-arranging the component features while avoiding rotated or mirror versions by sometimes removing or adding small segments, or by dividing larger segments into smaller ones (see Fig. 2). Each target letter/ pseudo-letter was primed either by the same letter/pseudo-letter (repetition prime) or a different letter/pseudo-letter (unrelated prime), defining the two levels of the factor Relatedness. Letter targets were always primed by a letter, and pseudo-letter targets always primed by a pseudo-letter. Target stimuli could appear at 7 different positions along the horizontal meridian: − 7° (extreme left), − 4.7°, − 2.3°, 0° (on fixation), + 2.3°, + 4.7°, and + 7° (extreme right).
Only target position was manipulated and primes always appeared centrally. Target Eccentricity was crossed with Relatedness in a 7 × 2 factorial design. In each experimental block, each letter/pseudo-letter target was seen twice by each participant, once in the repetition prime condition and once with an unrelated prime, for a total of 448 trials per block and 4480 trials per participant. 2.1.3. Procedure The experiment was run inside a dimly lit room and was controlled using DMDX software (Forster & Forster, 2003). Participants were seated in front of a computer screen on which stimuli were displayed in white on a black background in VGA mode (75 Hz refresh). The background luminance of the screen was 0.01 cd/m 2 and the luminance of all stimuli was 5.1 cd/m 2. The procedure is described in Fig. 3. Each trial began with a central fixation cross for 2000 ms. The fixation cross was then replaced by a forward mask for 10 ms consisting of a string of 7 white squares with black crossed stripes, each square occupying one of the 7 possible target positions. Prime stimuli appeared immediately after this for a duration of 16 ms, 1 and were followed by a backward mask (identical to the forward mask) lasting 10 ms. The backward mask was replaced by the target stimulus, which remained on the screen until participants' response. Participants performed an alphabetical decision task (letter/ pseudo-letter classification) and were asked to respond as rapidly and as accurately as possible by pressing one of two triggers on a game-pad with their index fingers. In order to avoid any influence of stimulus–response compatibility (i.e., a Simon effect — Simon, 1969), hand of response was counterbalanced across the two blocks within each session. The reported visual angles for each level of eccentricity define the distance between the central fixation location and the center of the stimulus. The viewing distance was 80 cm. Each participant was tested in 5 separate sessions each composed of two blocks, with each block containing the complete set of primetarget pairings (448 trials). On each session, participants were first presented with a set of practice trials consisting of 16 letters and pseudo-letters, followed by the 448 trials of the first block in random order, and the same number of trials in the second block with a change in response hand. Each session lasted about 1 h. After finishing all sessions, each participant performed a visibility test using exactly the same stimuli and procedure as the main experiment. In the visibility test, participants were informed of the presence of prime stimuli appearing before the target and were instructed to report on every trial whether the prime was a letter or a pseudo-letter (two-alternative forced choice). 3. Results Participants responded with a high level of accuracy in all sessions of the experiment (overall average error rate M = 2.15%, SD = 1.52). Correct RTs to letter targets were analyzed following a repeated measures 7 (Eccentricity) × 2 (Repetition) design. As RT distributions tend to deviate from normality, we transformed our data before running any analyses. We investigated the optimal transformation to normality in our RT distribution using the Box-Cox transformation (Box & Cox, 1964). The log-likelihoods profile of the lambda parameter (λ) of the Box-Cox power transform was obtained by plotting RTs against a linear model including the additive effects of Eccentricity and Relatedness plus their interaction. We found that the log-likelihood of λ peaked around −1, strongly suggesting a reciprocal transformation of data [Rec (RT) = 1 − (1/RT)] as being optimal. 1 The combination of centrally located primes and off-center targets caused an increase in prime visibility relative to the conditions tested by Marzouki, Meeter, et al. (2008), Marzouki, Midgley, et al. (2008). For this reason a pilot study with two volunteers was used to determine below threshold prime durations using a standard staircase method.
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Fig. 1. Adaptation of Grainger and van Heuven's (2003) model of orthographic processing to the case of single letters. Spatial attention is hypothesized to operate at the level of location-specific letter detectors that feedforward information to abstract location-invariant letter representations.
3.1. Letter analysis Random effects of participants and experimental sessions were taken into consideration in a multilevel regression analysis of the reciprocal-transformed RTs using the linear mixed-effect (LME) model technique (e.g., Baayen, 2008; Baayen, Davidson, & Bates, 2008; Bates, 2005). This enables us to simultaneously consider fixed and random effects in more detail than with traditional averaging by participant and by item.2 Fig. 4 shows means RTs as a function of target eccentricity for letter targets. The pattern of results strongly suggests an absence of interaction between Eccentricity and Relatedness. Two models were tested based on a dataset of 12,985 observations from 6 participants making alphabetic decisions on 2240 trials for letter targets over 10 experimental sessions. The first model, referred to as M1, includes the interaction between the two fixed effects Eccentricity and Relatedness, whereas the second model M2 does not consider this interaction. Likelihood tests using both AIC (19244.6 for M2 versus 19254.11 for M1) or BIC (19326.7 for M2 versus 19381.2 for M1) indicated that M2 should be preferred over M1. 3 Henceforth, in the following analyses we consider only M2. We observed a significant main effect of Relatedness, t(12,982) = 4.96, p b .0001, Cohen's d = 1.95, JZS Bayes Factor 4 b 0.033, and a marginally significant main effect of Eccentricity, t(12,982) = − 1.95, p = .0515, d = 0.76, 5 JZS Bayes Factor = 21.3. We investigated the possible presence of mixed effects (i.e. random slopes) between Eccentricity and Participants and Relatedness and 2 We also performed standard ANOVAs with participants and items as random variables and obtained the same absence of interaction between Relatedness and Eccentricity (all Fs b 1). In this analysis, the main effect of Relatedness was only significant by participants (F1(1, 59) = 13.4, MSE = 1.4, p b .001). 3 An estimate of the model that best minimizes the information loss can be obtained by calculating the relative likelihood value of model 1: exp((19244.6 − 19254.11)/ 2) = 0.0086. This implies that model 1 is 0.0086 times as probable as the second model in minimizing the information loss. 4 The JZS Bayes Factor estimates the evidence for the null hypothesis or the alternative (Rouder, Speckman, Sun, Morey, & Iverson, 2009). Values greater than 10 indicate strong evidence for the null hypothesis, and values greater than 30 are very strong evidence, while values less than 0.1 indicate strong evidence for the alternative hypothesis, and values less than 0.033 very strong evidence (Jeffreys, 1961). 5 As the estimate of the degrees of freedom for multi-level models can be problematic, we confirmed these results using a Markov Chain Monte Carlo sampling approach (cf., Baayen, 2008).
Participants. We did not find any evidence for differences across participants with respect to the magnitude of the effects of Eccentricity, χ 2(25) = 19.4, p > .1, or Relatedness, χ 2(2) = 4.4, p > .1. Hence, there is no reason to consider the individual results separately instead of the whole group of participants as illustrated by Fig. 4. 3.2. Pseudo-letter analysis The same type of analysis was performed on the RT data for pseudo-letter targets. There were no main effects of Relatedness, t(12,982) = 1.44, p = .15, d = .04, JZS Bayes Factor = 50.1 or Eccentricity, t(12,982) = − 1.43, p = .23, d = .03, JZS Bayes Factor = 51.2 in this analysis and the interaction between these two factors was also not significant (p = .86). 3.3. Visibility test analysis We correlated individual d′ measures (M = 2.1, SD = 1.4) for each participant in the visibility test with the net priming effect (difference between all repeated and unrelated conditions) obtained across all experimental sessions (e.g., Draine & Greenwald, 1998). This correlation was not significant (r = .25, p = .62), and a linear regression between d′ and net priming only reduced the total variation in the
Fig. 2. The set of letters and their corresponding pseudo-letters used in Experiment 1.
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4. Discussion
Fig. 3. Structure of the experimental procedure used in the present study. Primes were always centrally located whereas target stimuli occupied 7 possible positions (from −7° to +7°) defining the 7 levels of prime eccentricity. The eccentricity values (degrees of visual angle) represent the distance from fixation to the center of the target stimulus.
amount of priming by 6.3%. These results clearly suggest that priming effects in the indirect measure (experimental sessions) are not likely to be affected by the level of visibility of prime stimuli in the direct measure (the visibility test).
Experiment 1 was designed to provide a further test of Marzouki, Meeter, et al.'s (2008), Marzouki, Midgley, et al.'s (2008) account of masked repetition priming with letter stimuli when prime and target stimuli occupy distinct spatial locations. According to this account, priming is principally driven by the integration of information across prime and target stimuli via abstract location-invariant letter detectors (see Fig. 1). The size of repetition priming effects can be modulated, however, by visual acuity and spatial attention, both of which are hypothesized to affect the amount of information that can be extracted from prime stimuli. Here, the role of both of these factors was neutralized by having prime stimuli always appear centered on fixation. We therefore predicted that, in these conditions, priming effects should not be affected by the distance separating prime and target stimuli (i.e., target eccentricity). The results of Experiment 1 are in line with this prediction. Strong effects of repetition priming were found for letter targets, and the size of these priming effects did not depend on target eccentricity. Experiment 2 provides a further test of location invariance in masked repetition priming, this time with word and pseudo-word stimuli. Marzouki and Grainger (2008) tested 5-letter words and pseudo-words with centrally located targets and primes presented at a wide range of eccentricities (their Experiment 1B). They found that priming effects had already disappeared with primes located at 4° of eccentricity. However, when primes and targets were presented at the same location (their Experiment 1A), they found significant priming at 4° of eccentricity. As argued in the Introduction, this pattern can be explained by spatial attention being focused on the central target location when this does not vary, hence resulting in less efficient processing of peripherally located primes. Therefore, following the logic of Experiment 1 of the present study, effects of spatial attention were neutralized by having prime stimuli always centered on fixation while varying target location. In Experiment 2, primes and targets were 3-letter words, and target location was either at the same location as the primes, or shifted to the right or the left of fixation by 4° of visual angle. 5. Experiment 2 5.1. Method 5.1.1. Participants 27 Dutch native speakers individuals (mean age = 23, 16 female) were paid for their participation in the study. All participants were right-handed and reported having normal or corrected-to-normal vision. 23 participants were retained for the final analysis, after excluding 4 participants with excessive error rates. 5.1.2. Design and stimuli 180 three-letter Dutch words and 180 three-letter pseudo-words served as targets. Each target word/pseudo-word was primed either by the same word/pseudo-word (repetition prime) or a different word pseudo-word (unrelated prime), defining the two levels of the factor Relatedness. Word targets were always primed by a word, and pseudo-word targets always primed by a pseudo-word. Target stimuli could appear at 3 different positions along the horizontal meridian: − 4° (extreme left); 0° (foveal position); + 4° (extreme right). Prime stimuli were always centrally located (see Fig. 5). Two factors were manipulated in this experiment: Eccentricity (− 4°; 0°; and + 4°), and Relatedness (repeated vs. unrelated prime) in a 3 × 2 factorial design.
Fig. 4. Mean reaction time for letter targets as a function of prime condition (related/ unrelated to target) and target eccentricity with centrally located primes in Experiment 1 (a), and for purposes of comparison, the results of Marzouki et al. (2008, Experiment 1B) with varying prime eccentricity and centrally located targets (b). Error bars show SEM.
5.1.3. Procedure The experimental set-up was similar to Experiment 1, with E-prime software® used to present stimuli on a PC computer monitor
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Fig. 5. Experimental procedure used in Experiment 2 with words and pseudo-words. Target stimuli occupied three possible positions (−4°, 0° and +4°) as indicated by the three white arrows. The eccentricity values (degrees of visual angle) represent the distance from fixation to the central letter of the target stimulus.
in VGA mode (120-Hz refresh) in white on a black background. Background luminance of the screen and stimulus luminance were approximately the same as for Experiment 1 (0.01 cd/m 2 and 5.1 cd/m 2, respectively). The procedure is described in Fig. 5. Each trial began with a forward mask for 500 ms, formed of a string of hash marks with two vertical bars (I) placed above and below the center. Prime stimuli appeared immediately after this for a duration of 60 ms and were followed by a backward mask (a string of hash marks the same length as the forward mask) lasting 12 ms, which, in turn, was replaced by the target stimulus, which remained on the screen until the participant's response. The prime and target stimuli were presented in 14-point bold Courier New font with each letter subtending 0.9° of visual angle. The width of each letter was 0.4 cm, and the height was 0.5 cm. The reported visual angles for each level of eccentricity defined the distance between the central fixation zone and the center of the target stimulus at a viewing distance of 80 cm. The participants were asked to respond as rapidly and as accurately as possible by pressing one of two keyboard buttons with their index fingers: right button for words and left button for pseudowords. Each word/pseudo-word target was seen twice by each participant, once in the repetition prime condition and once with an unrelated prime. Standard counterbalancing with a Latin square allowed each target to be tested at each level of the combination of Eccentricity and Relatedness across participants. Manual response was also counterbalanced across participants in order to avoid any influence of stimulus response compatibility (i.e., — a Simon effect; Simon, 1969). The participants first performed a practice session with a set of 18 words and pseudowords that did not appear in the main experiment, followed by the 360 trials of the main experiment in random order. The experiment lasted approximately 45 min. 6. Results 6.1. Visibility test analysis The Area Under the Roc Curve (AUC) was calculated per participant. The AUC provides a powerful measure to discriminate between participants based on their visibility performance under a non-parametric assumption about the distribution of hits and false alarms (see Marzouki et al., 2007). Each individual AUC value was tested against the theoretical area 0.5 with a 95% Confidence Interval. If the confidence interval does not include the 0.5 value, then this can be taken as evidence that
discrimination performance is significantly greater than chance (Zhou, Obuchowski, & McClish, 2002). Results showed that only 7 participants had an AUC that deviated significantly from chance. Mean RTs for the remaining 16 participants are shown in Fig. 6. 6.2. Word analysis An LME analysis was performed on RTs in the same way as Experiment 1 except that no transformation of the data was required prior to analysis (the log-likelihood of λ peaked around 1). Eccentricity (4°, 0°, +4°) and Relatedness (repeat vs. unrelated) were entered as fixed factors and Participants as random factor. There was only a significant main effect of Relatedness, t(838)=5.52, pb .05, d=1.95, JZS Bayes Factorb 0.033. There was no main effect of Eccentricity, t(838)=−0.98, p=.32, d= .02, JZS Bayes Factor=22.5. and no interaction between Relatedness and Eccentricity (p>.1). The same pattern of results was observed on the accuracy data with a significant effect of Relatedness, pb .01, but no effect of Eccentricity and no interaction (p>.1).6 6.3. Pseudo-word analysis There were no main effects of Eccentricity, t(838) = − 0.54, p = .29, d = − 0.1, JZS Bayes Factor = 31.4 or Priming, t(838) = 1.48, p = .93, d = .25, JZS Bayes Factor = 12.2 and no interactions in the analysis of the RT data or the accuracy data (all ps > .05). 7. Discussion The results of Experiment 2 demonstrate that the location-invariant repetition priming effects seen in Experiment 1 with single letter stimuli can also be obtained with words. This provides evidence for a general mechanism for location-invariant processing of visual objects that applies to both single letters and single words. The comparison of the results of Experiment 1B of Marzouki and Grainger (2008) with those of Experiment 2 of the present study is quite striking (see Fig. 6). This clearly demonstrates that the drop in priming effects with increasing prime eccentricity (for centrally located targets) reported by Marzouki 6 As with Experiment 1 we performed standard ANOVAs with participants and items as random variables and obtained the same absence of interaction between Relatedness and Eccentricity (all Fs b 1). In this analysis, the main effect of Relatedness was highly significant, by participants (F1(1, 15)= 22.5, MSE = 7570.6, p b .0005) and by items (F2(1, 269) = 236.3, MSE = 13250.5, p b .0001).
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Fig. 6. Results of Experiment 2 with word targets (a). Results only included participants (N=16) performing at chance level performance in the visibility task. For comparison, results were taken from Marzouki and Grainger (2008) Experiment 1B, with central targets and varying prime location (b). Error bars show SEM.
and Grainger (2008) cannot be due to the increased spatial separation of primes and targets, since this tended to have the opposite effect in Experiment 2 of the present study.
8. General discussion In the present study we tested two accounts of the sensitivity of masked repetition priming to shifts in location of prime and target stimuli, as found in our prior work using single letter and word stimuli (Marzouki & Grainger, 2008; Marzouki, Meeter, et al., 2008; Marzouki, Midgley, et al., 2008). Our prior work had shown that the size of repetition priming effects obtained with the masked priming paradigm and centrally located targets diminished as a function of prime eccentricity (Marzouki & Grainger, 2008; Marzouki, Meeter, et al., 2008; Marzouki, Midgley, et al., 2008). We put forward two accounts of this pattern: the integration account, according to which it is the increased spatial separation of prime and target stimuli that causes the reduction in priming effects with greater prime eccentricity; and the attentional account, according to which it is the drop in spatial attention as a function of distance from the location of visible targets that causes the reduction in priming effects (note that since strong priming was found when primes and targets occupied the same peripheral location, the drop in priming effects with central targets and peripheral primes cannot be due to the drop in visual acuity with eccentricity). These two accounts were put to test in the present study by having primes always centered on fixation and varying target location (the exact opposite to what was done in Marzouki & Grainger, 2008, Experiment 1B and Marzouki, Meeter, et al., 2008; Marzouki, Midgley, et al., 2008, Experiment 1B, the results of which are shown in Figs. 4 and 6 for comparison).
The results of both experiments are in favor of the attentional account, and therefore with the hypothesis that information extracted from prime and target stimuli can be integrated across quite large distances in masked repetition priming with single letters and words. According to the model presented in Fig. 1, when the stimuli are single letters this integration process is performed by location-independent letter representations. Visual features extracted from briefly presented prime stimuli cause a rise in activation in location-specific letter detectors that send activation forward to detectors that code for the presence of a given letter independently of its location. This pre-activation of location-independent letter detectors then facilitates processing of upcoming letter targets that are the same letter as the prime, independently of prime-target spatial separation. Peressotti and Grainger (1995) drew a similar distinction between location-specific and location-independent letter representations on the basis of masked priming with letter triples. In that study, the time to decide that all three targets were letters (e.g., PGS vs. TM%) was decreased by primes that contained the same letters as targets in different positions (e.g., SPG-PGS). A further advantage was obtained for primes that contained the same letters in the same positions, and this was hypothesized to reflect the combined advantage of pre-activation in location-specific and location-independent letter detectors. Nevertheless, it should be noted that in more recent research, we have failed to observe significant masked repetition priming in a letter-in-string identification task (Massol, Grainger, Midgley, & Holcomb, 2012). In this work, random strings of 7 consonants were presented briefly and participants had to identify the letter that had occurred at a post-cued location that varied randomly from trial to trial. Identification of uppercase target letters was not significantly affected by prior presentation of primes formed of the same lowercase letters compared with different letter primes. One explanation for this observed absence of repetition priming would be that it is difficult to integrate information across prime and target stimuli at the level of location-specific letter detectors, possibly because of increased sensitivity to masking at this level of processing. Thus, priming effects obtained with the masked priming paradigm would be mostly subtended by location-invariant representations, which could be single letters, letter combinations, or whole-words. However, it should be noted that the hypothesized locationindependent letter detectors shown in Fig. 1 are not thought to be part of the normal process of skilled word reading. In Grainger and van Heuven's (2003) model of orthographic processing, from which Fig. 1 is derived, location-specific letter identities are mapped onto location-invariant word identities via word-centered, prelexical orthographic representations that take the form of ordered letter combinations (see Dehaene, Cohen, Sigman, & Vinckier, 2005, for a similar proposal). There is no role specified for location-independent representations of single letters. Nevertheless, such location-invariant letter representations are likely developed through initial contact with single letters (when children learn the alphabet), and may well be used in the initial stages of learning to read, prior to the development of parallel independent letter processing that is the hallmark of skilled reading. In the model shown in Fig. 1, such parallel independent letter processing involves location-specific letter detectors. Location-independent letter detectors then serve to connect the various location-specific detectors for a given letter identity at different positions, indicating that they all represent the same letter identity. Grainger, Rey, and Dufau (2008) noted the evidence for the existence for such abstract location-invariant letter detectors in a brain region situated anterior to the standard region for orthographic processing (the visual word form area, VWFA, Cohen et al., 2002). This fits nicely with the idea that these letter detectors would receive input from location-specific letter detectors involved in letter-string processing, as proposed in our model. The results of Experiment 2 of the present study are easily accommodated by models of visual word recognition that postulate the existence of location-invariant representations for printed words. Such
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representations are required in order to integrate information across multiple within-word fixations, where upon each fixation the same word occupies a different retinal location. The fact that masked repetition priming effects were unaffected by prime-target spatial overlap in Experiment 2 of the present study, suggests that much of these effects are carried by location-invariant orthographic representations. However, these location-invariant orthographic representations could be whole-word or sub-word representations. Evidence for a role for both types of representation has been provided by research using a combination of masked priming and even-related potentials (ERPs) while manipulating prime-target spatial overlap (Dufau, Grainger, & Holcomb, 2008; Ktori, Grainger, Dufau, & Holcomb, 2012). Locationindependent priming effects begin to emerge in the ERP signal at around 200 ms post-target onset, which roughly corresponds to the point in time when the transition from location-specific letter representations to location-independent, word-centered sublexical orthographic representations is thought to be complete (Grainger & Holcomb, 2009). Finally, the absence of priming effects with pseudo-word targets in Experiment 2 is a typical finding in experiments using masked priming and the lexical decision task. This fits with accounts of the lexical decision task according to which responses to pseudo-word stimuli are determined by how much lexical activation the pseudo-words generate (e.g., Dufau, Grainger, & Ziegler, 2012; Grainger & Jacobs, 1996). Since repetition priming can act to enhance the lexical activation generated by a pseudo-word (e.g., the prime “toble” activates “table”, which continues to receive support during processing of the target “toble”), this inhibitory influence of priming will counter any benefits that the repetition primes offer at the level of sublexical processing. The fact that the same pattern was seen with pseudo-letter targets in Experiment 1 suggests that the mechanisms involved in performing the alphabetic decision task are essentially the same as those used to perform lexical decision. That is, responses to pseudo-letter stimuli will be determined by how well these pseudo-letters activate real letter representations, with greater letter-level activity generating slower responses and more errors. Hence repetition priming with pseudo-letters will also suffer from the same trade-off as with word stimuli, with facilitation driven by shared features in the repetition condition trading-off with the inhibitory influence of stronger letter-level activation favoring the incorrect response. Summing up, we have shown that when prime stimuli are presented on fixation and only target location varied, then the size of masked repetition priming effects does not depend on prime-target spatial separation, at least for the distances tested in the present study. This was true for a wide range of target locations (−7°, −4.7°, −2.3°, 0°, +2.3°, +4.7°, +7°) for the single letter targets tested in Experiment 1, and also for word targets in Experiment 2 with targets appearing at one of three possible locations (−4°, 0°, +4°). These results suggest that location-invariant letter and word representations can integrate information over a large spatial range, and that prior evidence for limits in such spatial integration during masked repetition priming most likely reflects the influence of spatial attention on the processing of prime stimuli. Acknowledgment The authors would like to thank Fermin Moscoso del Prado Martín for his help with the LME technique for data analysis. This research was supported by ERC grant 230313 awarded to Jonathan Grainger and NWO grant 452-09-007 to Martijn Meeter.
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Contents lists available at SciVerse ScienceDirect
Acta Psychologica journal homepage: www.elsevier.com/ locate/actpsy
Location invariance in masked repetition priming of letters and words Yousri Marzouki a,⁎, Martijn Meeter b, Jonathan Grainger a, c a b c
Laboratoire de Psychologie Cognitive, Aix-Marseille University, Marseille, France Department of Cognitive Psychology, Vrije Universiteit Amsterdam Centre National de le Recherche Scientifique, France
a r t i c l e
i n f o
Article history: Received 12 June 2012 Received in revised form 27 October 2012 Accepted 30 October 2012 Available online 21 November 2012 PsycINFO classification: 2346 2340 2323
a b s t r a c t Earlier studies have suggested that information from a prime stimulus can be integrated with target information even when the two stimuli appear at different spatial locations. Here, we examined such location invariance in a masked repetition priming paradigm with single letter and word stimuli. In order to neutralize effects of acuity and spatial attention on prime processing, subliminal prime stimuli always appeared on fixation. Target location varied randomly from trial to trial along the horizontal meridian at one of seven possible locations for letter stimuli (−7° to +7°) and three positions for word stimuli (−4°, 0°, +4°). Speed of responding to letter and word targets was affected by target location, and by priming, but the size of repetition priming effects did not vary as a function of target location. These results suggest that masked repetition priming is mediated by representations that integrate information about object identity independently of object location. © 2012 Elsevier B.V. All rights reserved.
Keywords: Letter and word perception Masked repetition priming Eccentricity Location-invariance
1. Introduction Masked repetition priming has been extensively used to investigate early processes involved in letter and word perception (e.g., Forster & Davis, 1984; Holcomb & Grainger, 2006; Jacobs & Grainger, 1991; Seguì & Grainger, 1990). The standard way of using this technique consists in presenting the prime and the target at the same spatial location, and comparing performance to target stimuli that are preceded by the same or a different prime stimulus. However, some recent studies have departed from this tradition by manipulating the location of prime and target stimuli in masked repetition priming (e.g., Marzouki & Grainger, 2008; Marzouki, Meeter, & Grainger, 2008). In one critical condition in these studies, targets were always presented on fixation and prime location varied randomly from trial to trial at various eccentricities along the horizontal meridian. Priming effects were found to diminish as prime eccentricity increased, with both word targets (Marzouki & Grainger, 2008) and single letter targets (Marzouki, Meeter, et al., 2008; Marzouki, Midgley, Holcomb, & Grainger, 2008). Visual acuity was excluded as the causal factor since priming effects did not diminish in conditions where prime and target position co-varied, with ⁎ Corresponding author at: Laboratoire de Psychologie Cognitive, Université d'AixMarseille, 3 place Victor Hugo, 13331 Marseille cedex 1. Tel.: + 33 488 57 69 08; fax: + 33 488 57 68 95. E-mail address: [email protected] (Y. Marzouki). 0001-6918/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.actpsy.2012.10.006
primes and targets occupying the same randomly varying location. If visual acuity were the critical factor, one would also expect priming effects to diminish as a function of prime eccentricity in these conditions. Although there was some evidence that acuity was having an influence, it was clearly not entirely responsible for the pattern of priming effects found with centrally located targets. Two accounts of this pattern of priming effects were put forward by Marzouki, Meeter, et al. (2008), Marzouki, Midgley, et al. (2008). According to one account, called the integration account, it is the spatial separation of prime and target stimuli that is the critical factor. Here it is hypothesized that limits in translation invariance determine how well information extracted from the prime will influence processing of the target. According to an alternative account, called the attentional account, it is the amount of spatial attention directed to the prime location that is the critical factor. When targets always appear on fixation, then attention is hypothesized to be maximal on fixation and diminishes as a function of eccentricity, therefore causing priming effects to diminish. Marzouki, Meeter, et al. (2008), Marzouki, Midgley, et al. (2008) put these two accounts to test in an experiment that dissociated the confounding factors of prime eccentricity and prime-target spatial separation by orthogonally manipulating prime and target locations, with three different positions (−2.3°, 0°, +2.3°). The results showed that priming effect sizes were not affected by either prime or target location. Thus, prime-target spatial separation was not influencing the size of repetition priming effects. This led Marzouki et al. to propose that it is
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changes in the deployment of spatial attention that best accounts for the pattern of priming effects reported by Marzouki and Grainger (2008). Furthermore, this account is in line with the results of studies showing that an exogenous spatial cue can modulate the size of masked repetition priming effects (Besner, Risko, & Sklair, 2005; Lachter, Forster, & Ruthruff, 2004; Marzouki, Grainger, & Theeuwes, 2007; Marzouki, Midgley, et al., 2008). Marzouki, Meeter, et al. (2008), Marzouki, Midgley, et al. (2008) proposed an adaptation of Grainger and van Heuven's (2003) model of orthographic processing, shown in Fig. 1, as a specific implementation of their attentional account of variations in repetition priming effects as a function of prime eccentricity. This adaptation draws a distinction between location-specific and location-independent letter detectors. Location-specific detectors code for the presence of a given letter identity at a given location along the horizontal meridian and send activation on to the corresponding location-independent letter detector that codes for the presence of a given letter independently of its location. Another important aspect of this proposal is that spatial attention can modulate the activity of location-specific letter detectors in the alphabetic array. The key ingredient of Marzouki, Meeter, et al.'s (2008), Marzouki, Midgley, et al.'s (2008) account of effects of prime and target location on masked repetition priming is the role played by high-level location-invariant letter representations in integrating information across prime and target stimuli independently of their location (see Fig. 1). Visual acuity and spatial attention are two factors that can modulate this integration process by affecting the level of activity in location-specific letter representations. One straightforward prediction of this account is that when possible influences of acuity and attention are controlled for, then the size of repetition priming effects should not depend on the distance separating prime and target, even for quite large separations (larger than that used in Experiment 2 of the Marzouki, Meeter, et al., 2008; Marzouki, Midgley, et al., 2008, study). In order to rule out any role of acuity and attention in the present study, prime stimuli were always located on fixation and target location varied randomly from trial to trial at different eccentricities along the horizontal meridian. According to Marzouki et al., masked repetition priming effects should not vary as a function of target eccentricity. Experiment 1 tests this prediction with single letter stimuli, and Experiment 2 tests the same prediction using words. 2. Experiment 1 2.1. Method 2.1.1. Participants Five individuals (mean age = 24 years, 3 females) participated in this experiment for monetary compensation. All were right-handed and reported having normal or corrected-to-normal vision. 2.1.2. Design and stimuli Sixteen letters (all consonants) served as targets along with sixteen pseudo-letters designed using Font Creator 4.0 software and tested in prior research (Marzouki, Meeter, et al., 2008; Marzouki, Midgley, et al., 2008; Marzouki et al., 2007; New & Grainger, 2011). Each pseudo-letter was derived from a target letter by re-arranging the component features while avoiding rotated or mirror versions by sometimes removing or adding small segments, or by dividing larger segments into smaller ones (see Fig. 2). Each target letter/ pseudo-letter was primed either by the same letter/pseudo-letter (repetition prime) or a different letter/pseudo-letter (unrelated prime), defining the two levels of the factor Relatedness. Letter targets were always primed by a letter, and pseudo-letter targets always primed by a pseudo-letter. Target stimuli could appear at 7 different positions along the horizontal meridian: − 7° (extreme left), − 4.7°, − 2.3°, 0° (on fixation), + 2.3°, + 4.7°, and + 7° (extreme right).
Only target position was manipulated and primes always appeared centrally. Target Eccentricity was crossed with Relatedness in a 7 × 2 factorial design. In each experimental block, each letter/pseudo-letter target was seen twice by each participant, once in the repetition prime condition and once with an unrelated prime, for a total of 448 trials per block and 4480 trials per participant. 2.1.3. Procedure The experiment was run inside a dimly lit room and was controlled using DMDX software (Forster & Forster, 2003). Participants were seated in front of a computer screen on which stimuli were displayed in white on a black background in VGA mode (75 Hz refresh). The background luminance of the screen was 0.01 cd/m 2 and the luminance of all stimuli was 5.1 cd/m 2. The procedure is described in Fig. 3. Each trial began with a central fixation cross for 2000 ms. The fixation cross was then replaced by a forward mask for 10 ms consisting of a string of 7 white squares with black crossed stripes, each square occupying one of the 7 possible target positions. Prime stimuli appeared immediately after this for a duration of 16 ms, 1 and were followed by a backward mask (identical to the forward mask) lasting 10 ms. The backward mask was replaced by the target stimulus, which remained on the screen until participants' response. Participants performed an alphabetical decision task (letter/ pseudo-letter classification) and were asked to respond as rapidly and as accurately as possible by pressing one of two triggers on a game-pad with their index fingers. In order to avoid any influence of stimulus–response compatibility (i.e., a Simon effect — Simon, 1969), hand of response was counterbalanced across the two blocks within each session. The reported visual angles for each level of eccentricity define the distance between the central fixation location and the center of the stimulus. The viewing distance was 80 cm. Each participant was tested in 5 separate sessions each composed of two blocks, with each block containing the complete set of primetarget pairings (448 trials). On each session, participants were first presented with a set of practice trials consisting of 16 letters and pseudo-letters, followed by the 448 trials of the first block in random order, and the same number of trials in the second block with a change in response hand. Each session lasted about 1 h. After finishing all sessions, each participant performed a visibility test using exactly the same stimuli and procedure as the main experiment. In the visibility test, participants were informed of the presence of prime stimuli appearing before the target and were instructed to report on every trial whether the prime was a letter or a pseudo-letter (two-alternative forced choice). 3. Results Participants responded with a high level of accuracy in all sessions of the experiment (overall average error rate M = 2.15%, SD = 1.52). Correct RTs to letter targets were analyzed following a repeated measures 7 (Eccentricity) × 2 (Repetition) design. As RT distributions tend to deviate from normality, we transformed our data before running any analyses. We investigated the optimal transformation to normality in our RT distribution using the Box-Cox transformation (Box & Cox, 1964). The log-likelihoods profile of the lambda parameter (λ) of the Box-Cox power transform was obtained by plotting RTs against a linear model including the additive effects of Eccentricity and Relatedness plus their interaction. We found that the log-likelihood of λ peaked around −1, strongly suggesting a reciprocal transformation of data [Rec (RT) = 1 − (1/RT)] as being optimal. 1 The combination of centrally located primes and off-center targets caused an increase in prime visibility relative to the conditions tested by Marzouki, Meeter, et al. (2008), Marzouki, Midgley, et al. (2008). For this reason a pilot study with two volunteers was used to determine below threshold prime durations using a standard staircase method.
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Fig. 1. Adaptation of Grainger and van Heuven's (2003) model of orthographic processing to the case of single letters. Spatial attention is hypothesized to operate at the level of location-specific letter detectors that feedforward information to abstract location-invariant letter representations.
3.1. Letter analysis Random effects of participants and experimental sessions were taken into consideration in a multilevel regression analysis of the reciprocal-transformed RTs using the linear mixed-effect (LME) model technique (e.g., Baayen, 2008; Baayen, Davidson, & Bates, 2008; Bates, 2005). This enables us to simultaneously consider fixed and random effects in more detail than with traditional averaging by participant and by item.2 Fig. 4 shows means RTs as a function of target eccentricity for letter targets. The pattern of results strongly suggests an absence of interaction between Eccentricity and Relatedness. Two models were tested based on a dataset of 12,985 observations from 6 participants making alphabetic decisions on 2240 trials for letter targets over 10 experimental sessions. The first model, referred to as M1, includes the interaction between the two fixed effects Eccentricity and Relatedness, whereas the second model M2 does not consider this interaction. Likelihood tests using both AIC (19244.6 for M2 versus 19254.11 for M1) or BIC (19326.7 for M2 versus 19381.2 for M1) indicated that M2 should be preferred over M1. 3 Henceforth, in the following analyses we consider only M2. We observed a significant main effect of Relatedness, t(12,982) = 4.96, p b .0001, Cohen's d = 1.95, JZS Bayes Factor 4 b 0.033, and a marginally significant main effect of Eccentricity, t(12,982) = − 1.95, p = .0515, d = 0.76, 5 JZS Bayes Factor = 21.3. We investigated the possible presence of mixed effects (i.e. random slopes) between Eccentricity and Participants and Relatedness and 2 We also performed standard ANOVAs with participants and items as random variables and obtained the same absence of interaction between Relatedness and Eccentricity (all Fs b 1). In this analysis, the main effect of Relatedness was only significant by participants (F1(1, 59) = 13.4, MSE = 1.4, p b .001). 3 An estimate of the model that best minimizes the information loss can be obtained by calculating the relative likelihood value of model 1: exp((19244.6 − 19254.11)/ 2) = 0.0086. This implies that model 1 is 0.0086 times as probable as the second model in minimizing the information loss. 4 The JZS Bayes Factor estimates the evidence for the null hypothesis or the alternative (Rouder, Speckman, Sun, Morey, & Iverson, 2009). Values greater than 10 indicate strong evidence for the null hypothesis, and values greater than 30 are very strong evidence, while values less than 0.1 indicate strong evidence for the alternative hypothesis, and values less than 0.033 very strong evidence (Jeffreys, 1961). 5 As the estimate of the degrees of freedom for multi-level models can be problematic, we confirmed these results using a Markov Chain Monte Carlo sampling approach (cf., Baayen, 2008).
Participants. We did not find any evidence for differences across participants with respect to the magnitude of the effects of Eccentricity, χ 2(25) = 19.4, p > .1, or Relatedness, χ 2(2) = 4.4, p > .1. Hence, there is no reason to consider the individual results separately instead of the whole group of participants as illustrated by Fig. 4. 3.2. Pseudo-letter analysis The same type of analysis was performed on the RT data for pseudo-letter targets. There were no main effects of Relatedness, t(12,982) = 1.44, p = .15, d = .04, JZS Bayes Factor = 50.1 or Eccentricity, t(12,982) = − 1.43, p = .23, d = .03, JZS Bayes Factor = 51.2 in this analysis and the interaction between these two factors was also not significant (p = .86). 3.3. Visibility test analysis We correlated individual d′ measures (M = 2.1, SD = 1.4) for each participant in the visibility test with the net priming effect (difference between all repeated and unrelated conditions) obtained across all experimental sessions (e.g., Draine & Greenwald, 1998). This correlation was not significant (r = .25, p = .62), and a linear regression between d′ and net priming only reduced the total variation in the
Fig. 2. The set of letters and their corresponding pseudo-letters used in Experiment 1.
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4. Discussion
Fig. 3. Structure of the experimental procedure used in the present study. Primes were always centrally located whereas target stimuli occupied 7 possible positions (from −7° to +7°) defining the 7 levels of prime eccentricity. The eccentricity values (degrees of visual angle) represent the distance from fixation to the center of the target stimulus.
amount of priming by 6.3%. These results clearly suggest that priming effects in the indirect measure (experimental sessions) are not likely to be affected by the level of visibility of prime stimuli in the direct measure (the visibility test).
Experiment 1 was designed to provide a further test of Marzouki, Meeter, et al.'s (2008), Marzouki, Midgley, et al.'s (2008) account of masked repetition priming with letter stimuli when prime and target stimuli occupy distinct spatial locations. According to this account, priming is principally driven by the integration of information across prime and target stimuli via abstract location-invariant letter detectors (see Fig. 1). The size of repetition priming effects can be modulated, however, by visual acuity and spatial attention, both of which are hypothesized to affect the amount of information that can be extracted from prime stimuli. Here, the role of both of these factors was neutralized by having prime stimuli always appear centered on fixation. We therefore predicted that, in these conditions, priming effects should not be affected by the distance separating prime and target stimuli (i.e., target eccentricity). The results of Experiment 1 are in line with this prediction. Strong effects of repetition priming were found for letter targets, and the size of these priming effects did not depend on target eccentricity. Experiment 2 provides a further test of location invariance in masked repetition priming, this time with word and pseudo-word stimuli. Marzouki and Grainger (2008) tested 5-letter words and pseudo-words with centrally located targets and primes presented at a wide range of eccentricities (their Experiment 1B). They found that priming effects had already disappeared with primes located at 4° of eccentricity. However, when primes and targets were presented at the same location (their Experiment 1A), they found significant priming at 4° of eccentricity. As argued in the Introduction, this pattern can be explained by spatial attention being focused on the central target location when this does not vary, hence resulting in less efficient processing of peripherally located primes. Therefore, following the logic of Experiment 1 of the present study, effects of spatial attention were neutralized by having prime stimuli always centered on fixation while varying target location. In Experiment 2, primes and targets were 3-letter words, and target location was either at the same location as the primes, or shifted to the right or the left of fixation by 4° of visual angle. 5. Experiment 2 5.1. Method 5.1.1. Participants 27 Dutch native speakers individuals (mean age = 23, 16 female) were paid for their participation in the study. All participants were right-handed and reported having normal or corrected-to-normal vision. 23 participants were retained for the final analysis, after excluding 4 participants with excessive error rates. 5.1.2. Design and stimuli 180 three-letter Dutch words and 180 three-letter pseudo-words served as targets. Each target word/pseudo-word was primed either by the same word/pseudo-word (repetition prime) or a different word pseudo-word (unrelated prime), defining the two levels of the factor Relatedness. Word targets were always primed by a word, and pseudo-word targets always primed by a pseudo-word. Target stimuli could appear at 3 different positions along the horizontal meridian: − 4° (extreme left); 0° (foveal position); + 4° (extreme right). Prime stimuli were always centrally located (see Fig. 5). Two factors were manipulated in this experiment: Eccentricity (− 4°; 0°; and + 4°), and Relatedness (repeated vs. unrelated prime) in a 3 × 2 factorial design.
Fig. 4. Mean reaction time for letter targets as a function of prime condition (related/ unrelated to target) and target eccentricity with centrally located primes in Experiment 1 (a), and for purposes of comparison, the results of Marzouki et al. (2008, Experiment 1B) with varying prime eccentricity and centrally located targets (b). Error bars show SEM.
5.1.3. Procedure The experimental set-up was similar to Experiment 1, with E-prime software® used to present stimuli on a PC computer monitor
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Fig. 5. Experimental procedure used in Experiment 2 with words and pseudo-words. Target stimuli occupied three possible positions (−4°, 0° and +4°) as indicated by the three white arrows. The eccentricity values (degrees of visual angle) represent the distance from fixation to the central letter of the target stimulus.
in VGA mode (120-Hz refresh) in white on a black background. Background luminance of the screen and stimulus luminance were approximately the same as for Experiment 1 (0.01 cd/m 2 and 5.1 cd/m 2, respectively). The procedure is described in Fig. 5. Each trial began with a forward mask for 500 ms, formed of a string of hash marks with two vertical bars (I) placed above and below the center. Prime stimuli appeared immediately after this for a duration of 60 ms and were followed by a backward mask (a string of hash marks the same length as the forward mask) lasting 12 ms, which, in turn, was replaced by the target stimulus, which remained on the screen until the participant's response. The prime and target stimuli were presented in 14-point bold Courier New font with each letter subtending 0.9° of visual angle. The width of each letter was 0.4 cm, and the height was 0.5 cm. The reported visual angles for each level of eccentricity defined the distance between the central fixation zone and the center of the target stimulus at a viewing distance of 80 cm. The participants were asked to respond as rapidly and as accurately as possible by pressing one of two keyboard buttons with their index fingers: right button for words and left button for pseudowords. Each word/pseudo-word target was seen twice by each participant, once in the repetition prime condition and once with an unrelated prime. Standard counterbalancing with a Latin square allowed each target to be tested at each level of the combination of Eccentricity and Relatedness across participants. Manual response was also counterbalanced across participants in order to avoid any influence of stimulus response compatibility (i.e., — a Simon effect; Simon, 1969). The participants first performed a practice session with a set of 18 words and pseudowords that did not appear in the main experiment, followed by the 360 trials of the main experiment in random order. The experiment lasted approximately 45 min. 6. Results 6.1. Visibility test analysis The Area Under the Roc Curve (AUC) was calculated per participant. The AUC provides a powerful measure to discriminate between participants based on their visibility performance under a non-parametric assumption about the distribution of hits and false alarms (see Marzouki et al., 2007). Each individual AUC value was tested against the theoretical area 0.5 with a 95% Confidence Interval. If the confidence interval does not include the 0.5 value, then this can be taken as evidence that
discrimination performance is significantly greater than chance (Zhou, Obuchowski, & McClish, 2002). Results showed that only 7 participants had an AUC that deviated significantly from chance. Mean RTs for the remaining 16 participants are shown in Fig. 6. 6.2. Word analysis An LME analysis was performed on RTs in the same way as Experiment 1 except that no transformation of the data was required prior to analysis (the log-likelihood of λ peaked around 1). Eccentricity (4°, 0°, +4°) and Relatedness (repeat vs. unrelated) were entered as fixed factors and Participants as random factor. There was only a significant main effect of Relatedness, t(838)=5.52, pb .05, d=1.95, JZS Bayes Factorb 0.033. There was no main effect of Eccentricity, t(838)=−0.98, p=.32, d= .02, JZS Bayes Factor=22.5. and no interaction between Relatedness and Eccentricity (p>.1). The same pattern of results was observed on the accuracy data with a significant effect of Relatedness, pb .01, but no effect of Eccentricity and no interaction (p>.1).6 6.3. Pseudo-word analysis There were no main effects of Eccentricity, t(838) = − 0.54, p = .29, d = − 0.1, JZS Bayes Factor = 31.4 or Priming, t(838) = 1.48, p = .93, d = .25, JZS Bayes Factor = 12.2 and no interactions in the analysis of the RT data or the accuracy data (all ps > .05). 7. Discussion The results of Experiment 2 demonstrate that the location-invariant repetition priming effects seen in Experiment 1 with single letter stimuli can also be obtained with words. This provides evidence for a general mechanism for location-invariant processing of visual objects that applies to both single letters and single words. The comparison of the results of Experiment 1B of Marzouki and Grainger (2008) with those of Experiment 2 of the present study is quite striking (see Fig. 6). This clearly demonstrates that the drop in priming effects with increasing prime eccentricity (for centrally located targets) reported by Marzouki 6 As with Experiment 1 we performed standard ANOVAs with participants and items as random variables and obtained the same absence of interaction between Relatedness and Eccentricity (all Fs b 1). In this analysis, the main effect of Relatedness was highly significant, by participants (F1(1, 15)= 22.5, MSE = 7570.6, p b .0005) and by items (F2(1, 269) = 236.3, MSE = 13250.5, p b .0001).
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Fig. 6. Results of Experiment 2 with word targets (a). Results only included participants (N=16) performing at chance level performance in the visibility task. For comparison, results were taken from Marzouki and Grainger (2008) Experiment 1B, with central targets and varying prime location (b). Error bars show SEM.
and Grainger (2008) cannot be due to the increased spatial separation of primes and targets, since this tended to have the opposite effect in Experiment 2 of the present study.
8. General discussion In the present study we tested two accounts of the sensitivity of masked repetition priming to shifts in location of prime and target stimuli, as found in our prior work using single letter and word stimuli (Marzouki & Grainger, 2008; Marzouki, Meeter, et al., 2008; Marzouki, Midgley, et al., 2008). Our prior work had shown that the size of repetition priming effects obtained with the masked priming paradigm and centrally located targets diminished as a function of prime eccentricity (Marzouki & Grainger, 2008; Marzouki, Meeter, et al., 2008; Marzouki, Midgley, et al., 2008). We put forward two accounts of this pattern: the integration account, according to which it is the increased spatial separation of prime and target stimuli that causes the reduction in priming effects with greater prime eccentricity; and the attentional account, according to which it is the drop in spatial attention as a function of distance from the location of visible targets that causes the reduction in priming effects (note that since strong priming was found when primes and targets occupied the same peripheral location, the drop in priming effects with central targets and peripheral primes cannot be due to the drop in visual acuity with eccentricity). These two accounts were put to test in the present study by having primes always centered on fixation and varying target location (the exact opposite to what was done in Marzouki & Grainger, 2008, Experiment 1B and Marzouki, Meeter, et al., 2008; Marzouki, Midgley, et al., 2008, Experiment 1B, the results of which are shown in Figs. 4 and 6 for comparison).
The results of both experiments are in favor of the attentional account, and therefore with the hypothesis that information extracted from prime and target stimuli can be integrated across quite large distances in masked repetition priming with single letters and words. According to the model presented in Fig. 1, when the stimuli are single letters this integration process is performed by location-independent letter representations. Visual features extracted from briefly presented prime stimuli cause a rise in activation in location-specific letter detectors that send activation forward to detectors that code for the presence of a given letter independently of its location. This pre-activation of location-independent letter detectors then facilitates processing of upcoming letter targets that are the same letter as the prime, independently of prime-target spatial separation. Peressotti and Grainger (1995) drew a similar distinction between location-specific and location-independent letter representations on the basis of masked priming with letter triples. In that study, the time to decide that all three targets were letters (e.g., PGS vs. TM%) was decreased by primes that contained the same letters as targets in different positions (e.g., SPG-PGS). A further advantage was obtained for primes that contained the same letters in the same positions, and this was hypothesized to reflect the combined advantage of pre-activation in location-specific and location-independent letter detectors. Nevertheless, it should be noted that in more recent research, we have failed to observe significant masked repetition priming in a letter-in-string identification task (Massol, Grainger, Midgley, & Holcomb, 2012). In this work, random strings of 7 consonants were presented briefly and participants had to identify the letter that had occurred at a post-cued location that varied randomly from trial to trial. Identification of uppercase target letters was not significantly affected by prior presentation of primes formed of the same lowercase letters compared with different letter primes. One explanation for this observed absence of repetition priming would be that it is difficult to integrate information across prime and target stimuli at the level of location-specific letter detectors, possibly because of increased sensitivity to masking at this level of processing. Thus, priming effects obtained with the masked priming paradigm would be mostly subtended by location-invariant representations, which could be single letters, letter combinations, or whole-words. However, it should be noted that the hypothesized locationindependent letter detectors shown in Fig. 1 are not thought to be part of the normal process of skilled word reading. In Grainger and van Heuven's (2003) model of orthographic processing, from which Fig. 1 is derived, location-specific letter identities are mapped onto location-invariant word identities via word-centered, prelexical orthographic representations that take the form of ordered letter combinations (see Dehaene, Cohen, Sigman, & Vinckier, 2005, for a similar proposal). There is no role specified for location-independent representations of single letters. Nevertheless, such location-invariant letter representations are likely developed through initial contact with single letters (when children learn the alphabet), and may well be used in the initial stages of learning to read, prior to the development of parallel independent letter processing that is the hallmark of skilled reading. In the model shown in Fig. 1, such parallel independent letter processing involves location-specific letter detectors. Location-independent letter detectors then serve to connect the various location-specific detectors for a given letter identity at different positions, indicating that they all represent the same letter identity. Grainger, Rey, and Dufau (2008) noted the evidence for the existence for such abstract location-invariant letter detectors in a brain region situated anterior to the standard region for orthographic processing (the visual word form area, VWFA, Cohen et al., 2002). This fits nicely with the idea that these letter detectors would receive input from location-specific letter detectors involved in letter-string processing, as proposed in our model. The results of Experiment 2 of the present study are easily accommodated by models of visual word recognition that postulate the existence of location-invariant representations for printed words. Such
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representations are required in order to integrate information across multiple within-word fixations, where upon each fixation the same word occupies a different retinal location. The fact that masked repetition priming effects were unaffected by prime-target spatial overlap in Experiment 2 of the present study, suggests that much of these effects are carried by location-invariant orthographic representations. However, these location-invariant orthographic representations could be whole-word or sub-word representations. Evidence for a role for both types of representation has been provided by research using a combination of masked priming and even-related potentials (ERPs) while manipulating prime-target spatial overlap (Dufau, Grainger, & Holcomb, 2008; Ktori, Grainger, Dufau, & Holcomb, 2012). Locationindependent priming effects begin to emerge in the ERP signal at around 200 ms post-target onset, which roughly corresponds to the point in time when the transition from location-specific letter representations to location-independent, word-centered sublexical orthographic representations is thought to be complete (Grainger & Holcomb, 2009). Finally, the absence of priming effects with pseudo-word targets in Experiment 2 is a typical finding in experiments using masked priming and the lexical decision task. This fits with accounts of the lexical decision task according to which responses to pseudo-word stimuli are determined by how much lexical activation the pseudo-words generate (e.g., Dufau, Grainger, & Ziegler, 2012; Grainger & Jacobs, 1996). Since repetition priming can act to enhance the lexical activation generated by a pseudo-word (e.g., the prime “toble” activates “table”, which continues to receive support during processing of the target “toble”), this inhibitory influence of priming will counter any benefits that the repetition primes offer at the level of sublexical processing. The fact that the same pattern was seen with pseudo-letter targets in Experiment 1 suggests that the mechanisms involved in performing the alphabetic decision task are essentially the same as those used to perform lexical decision. That is, responses to pseudo-letter stimuli will be determined by how well these pseudo-letters activate real letter representations, with greater letter-level activity generating slower responses and more errors. Hence repetition priming with pseudo-letters will also suffer from the same trade-off as with word stimuli, with facilitation driven by shared features in the repetition condition trading-off with the inhibitory influence of stronger letter-level activation favoring the incorrect response. Summing up, we have shown that when prime stimuli are presented on fixation and only target location varied, then the size of masked repetition priming effects does not depend on prime-target spatial separation, at least for the distances tested in the present study. This was true for a wide range of target locations (−7°, −4.7°, −2.3°, 0°, +2.3°, +4.7°, +7°) for the single letter targets tested in Experiment 1, and also for word targets in Experiment 2 with targets appearing at one of three possible locations (−4°, 0°, +4°). These results suggest that location-invariant letter and word representations can integrate information over a large spatial range, and that prior evidence for limits in such spatial integration during masked repetition priming most likely reflects the influence of spatial attention on the processing of prime stimuli. Acknowledgment The authors would like to thank Fermin Moscoso del Prado Martín for his help with the LME technique for data analysis. This research was supported by ERC grant 230313 awarded to Jonathan Grainger and NWO grant 452-09-007 to Martijn Meeter.
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