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Background: Novel word acquisition is generally believed to be a rapid process, essential for ensuring a flexible and efficient communication system; at least in spoken language, learners are able to construct memory traces for new linguistic stimuli after just a few exposures. However, such rapid word learning has not been systematically found in visual domain, with different confounding factors obscuring the orthographic learning of novel words. This study explored the changes in human brain activity occurring online, during a brief training with novel written word‑forms using a silent reading task Results: Single‑trial, cluster ‑based random permutation analysis revealed that training caused an extremely fast (after just one repetition) and stable facilitation in novel word processing, reflected in the modulation of P200 and N400 components, possibly indicating rapid dynamics at early and late stages of the lexical processing. Furthermore, neural source estimation of these effects revealed the recruitment of brain areas involved in orthographic and lexico ‑seman‑ tic processing, respectively. Conclusions: These results suggest the formation of neural memory traces for novel written word‑forms after a mini‑ mal exposure to them even in the absence of a semantic reference, resembling the rapid learning processes known to occur in spoken language. Keywords: Word learning, ERP methodology, Cluster‑based random permutation analysis, N400, P200 Background language domain, extensive behavioral research has con- Human brain possesses an impressive ability to learn sistently proven the acquisition of new vocabulary as a novel vocabulary, not only during the first years of life very fast process, with learning outcomes obtained after when language development is taking off but also in relatively short training periods, in some cases involv- adulthood, when learning a foreign language or acquir- ing just a few exposures [22, 26, 27, 33, 42, 44, 45, 51, 66, ing new terms in the native one. Moreover, this capabil- 105]. Indeed, this process was referred to as fast mapping ity of learning new vocabulary is highly efficient, as the in early developmental studies, in which children showed acquisition and representation of novel words unfolds in rapid and incidental learning for the association between a particularly fast and accurate fashion. u Th s, in spoken new auditory forms and their referents see [19, 20]. There is accumulating body of evidence from studies using methodologies such as fMRI [16, 92], PET  or EEG, By novel words we understand linguistic stimuli never experienced before [5, 53, 54, 102, 104, 115], suggesting the existence of a and hence unknown, with no information stored about either their auditory/ neural mechanism supporting the rapid learning of novel visual form or meaning yet. spoken words, whose activity can be traced by measuring *Correspondence: email@example.com brain signals before and after a learning session, or even Centre for Cognition and Decision Making, Institute for Cognitive online, during the process of learning. In particular, a Neuroscience, National Research University Higher School of Economics, Moscow, Russian Federation number of recent ERP studies have reported an increase Full list of author information is available at the end of the article © The Author(s) 2020. 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The Creative Commons Public Domain Dedication waiver (http://creat iveco mmons .org/publi cdoma in/ zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Bermúdez‑Margaretto et al. Behav Brain Funct (2020) 16:11 Page 2 of 17 of brain activity in core language circuits (most crucially u Th s, the rapid N400 decrease obtained in these studies, left temporal lobe) elicited by novel spoken word-forms after only a handful of exposures to novel word-forms after only a few minutes of auditory exposure to them, presented within meaningful contexts [8, 76, 82] or even and even in the absence of meaning referent or active after just one single visual presentation within a highly- rehearsal [53, 54, 102, 104, 115]. Most typically, this constrained semantic context , is considered to reflect modulation of brain activity is manifested rather early in the facilitation in the processing of these stimuli and their the electrophysiological response, at ~50–150 ms after integration in the lexico-semantic system through mean- the novel spoken word-form’s disambiguation point (the ingful associations. point in time when the phonotactical configuration of Therefore, the build-up process of new linguistic repre - specific stimulus diverges from other spoken word-forms sentations can be hypothesized to be a very fast process, with identical initial phonemes, enabling its identifica - both in spoken and visual domains. Some other studies tion as a unique item during speech perception). Cru- suggest, however, that it is only after an intensive and cially, this response enhancement, which is explained as meaningful training with novel words, involving a higher an activation of a newly built lexical representation, has number of exposures (including even weekly training been found under both attentive [53, 54] and passive lis- sessions) and consolidation periods (at least overnight, tening conditions [102, 104, 115], showing the formation but often including days of practice), that it may be pos- of neural memory traces to be not only a very fast but to sible to ensure the build-up of new representations a large degree also an automatic process (see , for a fully-integrated into the mental lexicon, both in spoken review). [27, 33, 42, 72] and in visual modalities [6, 17, 73, 107]. However, language is not confined to the auditory Although apparently contradictory at first glance, both modality; visually presented language and the neu- sets of findings—those supporting rapid fast mapping ral mechanisms underpinning the acquisition of novel and those favoring slower learning—likely reflect two dif - vocabulary through reading are no less important. In this ferent stages involved in the acquisition of novel vocabu- sense, the acquisition of new orthographic forms is con- lary, achieved at different points during word learning. sidered crucial for reading fluency, as it allows a reader Indeed, these two stages have been described as lexical to transfer from a serial grapheme-to-phoneme decoding configuration and lexical engagement [ 65]. Thus, dur - of the novel word to a holistic whole-word recognition ing the early stage of lexical configuration, the specific strategy [100, 101]. A number of behavioral studies have features of the surface word-form are acquired, such as reported effects suggestive of fast orthographic learning, its orthography, phonology or meaning, with a relatively which can be achieved by training novel word-forms in short exposure allowing the fast acquisition of memory reading-aloud tasks involving very few (from four to six) traces for novel word-forms. Later on, during the lexical repetitions [, 15, 61, 62, 68, 96, 109]. For instance, it has engagement stage, the intensive exposure to these stim- been found that such training significantly improves the uli allows its integration into the lexicon, and hence its speed and accuracy of novel word-form recognition, dynamic interaction—in terms of facilitation and compe- leading to the elimination of the so-called lexicality effect, tition—with other word units at similar processing level. i.e. the differences between novel and previously known Different behavioral studies have reported data support - words . Such a short exposure to novel written words ing this two-stage process [35, 42, 47, 111], showing the has been reported to reduce the naming latency differ - early phase of configuration as a necessary condition ence between short and long novel words caused by the for novel word learning. Indeed, the foundations of new serial, letter-by-letter decoding of unfamiliar stimuli (the word acquisition are likely established during this early so-called length effect, [2, 61, 62, 68]). These findings stage, through the formation of episodic memory repre- clearly indicate the formation of directly-accessed ortho- sentations. Later on, the word’s connections become dis- graphic representations in the mental lexicon, causing tributed over the entire language neural network due to a change in the reading strategy for the trained words, extensive experience, going beyond the initial encoding evolving from serial decoding to a parallel, whole-form in isolated episodic use. In the present study, we aimed to recognition strategy. Furthermore, similar to the spoken further investigate the neurophysiological underpinnings domain, several ERP studies have also provided evidence of the early lexical configuration in the written domain, suggesting the existence of a neurophysiological mecha- that is, the rapid acquisition of novel word´s orthography. nism which enables rapid formation of mental represen- In contrast to a substantial amount of ERP research tations for novel written word-forms perceived visually in spoken language, focused on rapid learning of pure [8, 14, 76, 82]. Most typically, these studies report the phonological word-forms, the evidence regarding the modulation of the N400, an ERP component considered putative neurophysiological mechanisms underlying the to reflect the lexico-semantic processing of stimuli . acquisition of orthographic word-forms is rather limited. B ermúdez‑Margaretto et al. Behav Brain Funct (2020) 16:11 Page 3 of 17 The vast majority of studies in this strand of research 91, 97]. This, in turn, implies that the build-up of a sur - (including the ones listed above) combined training in face word-form representation in the presence of seman- both the orthography and the meaning of new words [8, tics may obscure the brain dynamics responsible for the 14, 76, 82], whereas only few of them evaluated the brain acquisition of orthographic traces for novel words as dynamics underlying purely orthographic learning as such. Therefore, to fully understand this process, it seems such (e.g., [11, 13, 80]). However, the underlying neural crucial to determine the activation patterns which occur mechanisms for the acquisition of a novel surface form during the visual encounter with novel written word- per se and the meaning attached to it are likely dissocia- forms per se and enable the formation of orthographic ble, with one related to the analysis of visual features and traces, without confounding them by semantic effects the orthographic recognition of the surface form and taking place in parallel. the other related to the access of its associated concept However, the majority of studies addressing fast learn- [23, 86]. Indeed, ERP studies on visual word recognition ing of novel written word-forms have used a meaning- have provided evidence of dissociation of orthographic based training approach—and consequently reported and semantic processes at neurophysiological level. The the modulation of the N400—thus preventing us from orthographic processes related to the extraction of visual disentangling putative orthographic surface-level from features and word-form analysis appear to be reflected semantic effects. Some of the very few ERP studies using in early brain responses, elicited within the first 250 ms semantics-free paradigms have shown that the brief of word processing [3, 4, 10, 25, 59]. In particular, the exposure to novel written word-forms involves the acti- amplitude of the P200, a fronto-centrally distributed ERP vation of episodic memory processes [11, 13]. In particu- component, is known to be modulated by the access of lar, a short and meaningless training with novel written the orthographic word-form, with larger responses for word-forms (only six exposures) was found to produce an high-frequency words in comparison to low-frequency enhancement of the so-called Late Positive Component words or to pseudowords , therefore, this ERP is con- (LPC), to the point that differences between responses sidered to be an index of holistic word-form recogni- to these stimuli and those to known words disappeared tion [7, 21, 67, 89, 114]. On the other hand, the access of by the end of the task . This ERP component is a late word meaning has in turn been found to elicit later brain (~500–700 ms) central positivity, typically observed in responses, ~250–500 ms after stimulus onset, most nota- repetition and old-new paradigms, and its enhancement bly reflected in the amplitude of the N400 component, has been related to the encoding and strengthening of already mentioned above. This parietally-distributed episodic memory traces that enable recognition of previ- negativity is largely known as a robust brain correlate of ously presented stimuli (see Rugg and Curran , for a lexico-semantic processing, sensitive to both lexical sta- review). tus and semantic context of the stimuli [9, 56], Feder- The visual training carried out in these studies did not meier and Kutas 2011). Thus, smaller N400 responses produce, however, any effect linked to earlier lexical pro - are considered to reflect the ease of processing and inte - cesses, indicative of orthographic learning (reflected, grating the word into the preceding context, as well as for instance, in the modulation of P200). Even the well- its context-driven expectancy. Besides N400 semantic stablished N400 repetition effect, mentioned above, was priming effects (in which prior presentation of seman - either missing or very weak in these studies, despite the tically related word reduces the N400 amplitude), ERP repeated exposure to novel stimuli over the training. One research has shown the sensitivity of this component to plausible explanation could be the use of a non-natural the physical repetition of the stimuli, with more positive- reading context—a lexical decision task—for the train- going (i.e. less negative) responses for repeated than for ing of novel word-forms [11, 13]. Such manipulation unrepeated stimuli, which is interpreted as a sign of facil- could enhance the attention—and hence linked episodic itation in the semantic access because of the repetition- processes—on novel word-forms in order to actively induced pre-activation of lexico-semantic entries [30, 63, categorize them during the task, masking or blurring 85, 94]. Importantly, although, based on the above, the the activation at the earlier stages of processing. Indeed, orthographic and semantic analyses might be considered similar LPC enhancements have been also found in other as consecutive processes, there is also evidence of earlier studies in which an explicit categorization (i.e. semantic lexico-semantic activation during visual word recogni- judgement) was required for stimuli previously trained tion (between 100 and 200 ms), suggesting a cascaded- in both orthography and meaning (e.g.: [6, 8, 82]), which interactive nature of the linguistic processing [31, 32, 46, suggests the link between this late modulation and non-lexical processes driven by particular task require- A pseudoword is a word-like sequence of phonemes/letters, observing the ments (explicit attention-demanding lexical or semantic phonotactic and orthotactic rules of participant’s language, but devoid of categorization). meaning. Bermúdez‑Margaretto et al. Behav Brain Funct (2020) 16:11 Page 4 of 17 In more detail, previous ERP studies have shown the sentence reading, in which few exposures of the novel influence that attentional processes, driven by such cat - word—usually less than 10—are provided). egorization demands, commence at earlier stages during In sum, the putative brain mechanisms for the for- visual word recognition [38, 57, 60, 93]. In these studies, mation of purely visual word-form representations the N400 effect found under lexical or semantic categori - require further investigation. In particular, more stud- zation tasks is actually overlapped by the modulation of ies are needed that could avoid the confound between the P300, a component related to attentional mechanisms orthographic learning and semantic or categorization activated to accomplish the task [87, 88], thus confound- processes, and would employ more natural paradigms ing the interpretation of ER effects at the lexico-semantic similar to those used in behavioral research, involving stage of word processing. Furthermore, the modulation brief and attentive exposure to new words and using of the P300 has been found to differ across tasks varying reading, rather than lexical categorization or other unre- in the amount of explicit demands over the stimuli [12, lated visual tasks. Here, we asked whether a brief train- 93], with the subsequent P300-N400 overlap observed at ing—up to six exposures—with novel word-forms in an high-level (i.e. lexical decision task) but not at low-level attentive reading task (resembling the training conditions demand tasks (i.e. reading task). Therefore, enhanced in behavioral studies), could produce neural changes attention driven by specific categorization of the novel indicative of a build-up of lexical memory traces. More words may prevent the observation of changes at early specifically, we hypothesized that this training would (most crucially, orthographic) stages of their processing. allow us to detect changes particularly related with the In this sense, the use of training contexts which do not orthographic learning of the novel written word-forms, involve overt categorization or other behaviorally spe- in the absence of other confounding factors. There - cific responses to the trained stimuli seems essential to fore, it was expected this learning would be reflected in study the effects of visual training at early stages of their the modulation of the P200 component, a known neu- processing. ral marker of orthographic word-form access. Besides Indeed, earlier effects relative to lexical processing this, we might also expect modulation of N400 and LPC of novel word-forms have been found in visual domain components, since changes in these ERPs have been when the training involved a more automatic, “task- often found in previous studies addressing novel word free” learning , in a similar way as previously found learning. However, since our training paradigm avoids in spoken domain [53, 54, 102, 104, 115]. In particular, precisely the conditions that are believed to affect these in their MEG study, Partanen and colleagues found that late responses (such as inclusion of semantic context or unattended exposure to novel meaningless written word- requirement of stimulus categorization along the task), forms during a non-linguistic distraction task caused the predictions for these components are somewhat less a modulation of the brain response at earlier stages of straightforward. Nonetheless, since the repeated expo- stimulus processing (around ~100 and 200 ms). No mod- sure to novel word-forms was expected to cause the for- ulation was found at later time windows (around ~300 mation of new orthographic traces, their activation could, or 500 ms) in this attention- and task-free non-semantic in turn, facilitate the lexical processing of these stimuli in paradigm. Remarkably, the visual exposure implemented upcoming encounters, which might be reflected in the in this study was outside the focus of reader’s attention, progressive reduction of the N400. Moreover, the re- using parafoveal tachistoscopic presentation of stimuli, activation of word memory traces through their repeated and thus advocating automaticity of the memory trace exposure could potentially trigger the episodic process- build-up memory even in visual domain. However, read- ing for these stimuli, which might be reflected in the LPC ing—especially involving encounters with novel visual enhancement (although, notably, this effect has been information—is usually an attentive process. Moreover, particularly linked to categorization demands, which are the training implemented in the Partanen et al. visual absent in the present training task). Accordingly, EEG study, as well as in similar studies in spoken domain, methodology was used to explore changes in both early involved a massive exposure to novel word-forms, with (150–250 ms) and late (250–800 ms) brain’s electrical sig- many repetitions over the experimental session (over nals, generated on-line during the repeated exposure to 100). This approach contrasts with the short exposure novel written word-forms in a reading task. The impact carried out in the ERP studies using attentive-categori- of each individual encounter with the novel word-form zation tasks for training, and particularly with behavioral was tested by means of a single-trial, cluster-based ran- studies in this strand of research, wherein training para- dom permutation analysis of EEG data. Thus, rather than digms are usually more similar to the learning conditions just comparing pre and post training effects, by using this in visual domain than in the above M/EEG studies (i.e. fine-grain method we also estimated the contribution of attentive low-level demand tasks, such as single-word or each repetition along the training into the changes in the B ermúdez‑Margaretto et al. Behav Brain Funct (2020) 16:11 Page 5 of 17 brain electrical response elicited by novel word-forms. both P200 and N400 time windows was very fast (taking In addition, an exploratory, data-driven analysis of neu- place after the first exposure) and was maintained across ral source estimation was carried out in order to identify the training session. the brain generators responsible for the ERP modulations Additional comparisons with control stimuli—known found at surface level. We hypothesized that, if an early words—over the averaged time windows and significant ERP modulation (i.e. P200) actually encodes the putative channels identified in previous cluster analysis resulted orthographic learning of the novel word-forms, then the in a significant lexicality effect in the P200 time window, differences in the processing of these stimuli before and with a stronger P200 response exhibited by known words st after the training would be observed in the brain regions in comparison to novel word-forms presented at the 1 related to orthographic processing (such as left lingual trial (t(25) = 2.84, p = 0.017; difference between known and fusiform gyrus) [78, 83, 84, 87, 90]. words vs. novel word-forms at 1st trial: 1.30 µV). How- ever, with training, these differences vanished, with both novel and known words showing similar brain activity Results already at the 2nd exposure and until the end of the task Cluster analysis carried out for the effect of training (con - (all ps > 0.05; see Fig. 2). Therefore, the modulation of the trasting novel word-forms at the beginning and at the end brain’s electrophysiological response produced by the of the training) resulted in two significant clusters of dif - orthographic training of novel word-forms reduced the ferences, obtained in the tests carried out over the early P200 lexicality effect such that it was eliminated after just (150–250 ms) and late (250–800 ms) temporal segments. one visual repetition. A somewhat different pattern of The first cluster extended from 191–210 ms (t(25)= effects was found for the N400 time window. No signifi - −3.17, p= 0.041), with maximal activity at 201 ms, show- cant difference was detected between known and novel ing a fronto-central distribution and revealing more posi- word-forms presented at the 1st trial (known words tive amplitude for novel words presented in the last than vs. novel word-forms difference at 1st trial: −0.05 µV, in the first block of repetitions (diff. 1st vs. 6th trial= p > 0.05). However, lexical differences emerged at the −1.48 µV). The second cluster of differences extended second exposure to novel word-forms (t(25) = −3.03; from 373–550 ms (t(25)= −3.06, p=0.005), maximal at p = 0.004; diff. = −1.90 µV), which were maintained 460 ms, with a centro-posterior distribution showing less across the training for all remaining trials (all ps < 0.01). negative amplitude at the last than at the first trial (diff. Neural source reconstruction of the orthographic 1st vs. 6th trial = 2.00 µV). Both the latency and the scalp training ERP effect (novel word-forms presented at first distribution of these two effects likely suggest the modu - vs. at last trial) was carried our using LAURA distributed lation of P200 and N400 components, respectively, as can source estimation method. Two ROIs were identified also be observed in the averaged waveforms of ERPs and as the most likely neural contributors to the early P200 topographic maps plotted in Fig. 1. increase observed at surface level, namely the left lin- The activity at each resulting time window (191– gual gyrus (left LG, maximal in x = −16.72, y = −55.47, 210 ms and 373–550 ms) was averaged across significant z = 5.88, Talairach Coordinates, corresponding to BA 18, channels and complementary analyses were carried out Talairach and Tournoux ) and the bilateral superior in order to further explore the effect of each single rep - frontal gyrus (right SFG: x = 3.34, y = 62.33, z = −0.008; etition along the entire orthographic training. Results left SFG: x = −3.34, y = 62.33, z = −0.008, corresponding for P200 time window (191–210 ms) showed significant to BA 10). See Fig. 3 (left panel). Further analyses car- increase of the positivity elicited by novel word-forms ried out in both ROIs revealed the increase of activation across training trials (see Fig. 1 for mean amplitudes val- from the first to the last exposure with the novel written ues elicited across exposures). Crucially, the strongest word-forms (left LG: t(25) = 2.84, p = 0.009, diff. 1st vs. change was found from the 1st to the 2nd training trial trial: −3.27 A/mm ; Right SFG: t(25) = 2.86, p = 0.008, (t(25) = −2.47, p = 0.036; diff. = −1.12 µV) whereas no diff. 1st vs. 6th trial: −2.13 A/mm ; Left SFG: t(25) = 2.84, significant differences were found from the 2rd to the p = 0.009, diff. 1st vs. 6th trial: −1.84 A/mm ). Conse- remaining trials of exposure (all ps > 0.05). Similarly, the quently, differences exhibited between novel and known repeated exposure to novel word-forms was found to words at the beginning of the training (left LG: t(25) = modulate the N400 amplitude especially in the begin- −2.075, p = 0.048, diff. novel vs. known: −3.06 A/mm ; ning of the training; thus, the strongest reduction in the Right SFG: t(25) = −2.19, p = 0.038, diff. novel vs. known: N400 amplitude was observed from the 1st to the 2nd −1.93 A/mm ; Left SFG: t(25) = −3.24, p = 0.003, diff. trial (t(25) = −2.99, p = 0.001; diff. = −1.85 µV), whereas novel vs. known: −2.56 A/mm ) were found as eliminated no significant modulations were found between subse - at the last exposure with novel word-forms (all ts(25) < 1, quent blocks (all ps > 0.05, see Fig. 1 for details). There - all ps > 0.4). In addition, the left postcentral gyrus was fore, this pattern of results shows that the modulation in Bermúdez‑Margaretto et al. Behav Brain Funct (2020) 16:11 Page 6 of 17 Fig. 1 Averaged ERP waveforms at midline scalp sites for novel word‑form exposures across the six different training trials. Panels on the left and on the right show the training effects found at P200 and N400 intervals, respectively. Topographic maps above each set of ERP waveforms depict scalp distribution and electrodes in which the general effect of novel word training (first vs. last trial of exposure) was significant in the cluster ‑based random permutation analysis (time windows are highlighted in grey shaded areas). Topographic maps below each set of waveforms show the scalp distribution of the differences between novel word‑forms across each new exposure. Bar graphs below each panel show the mean amplitude of each ERP obtained for novel words across the training blocks. Cluster analysis for each pair‑ wise comparison carried out across training trials revealed that changes at both P200 and N400 time windows were very fast (already at the second exposure) and stable over the rest of the training also identified as another likely neural source of the train - to BA 22), right inferior parietal lobule (including angu- ing effect (left PostCG, x = −56.85, y = −20.88, z = 41.50), lar gyrus, rAG, x = 30.09, y = − 60.09, z = 31.04, corre- showing the decrease of activity from the first to the last sponding to BA39/40) and the left middle frontal gyrus training trial (t(25) = −2.62, p = 0.015, diff. 1st vs. 6th (lMFG, x = −43.47, y = 12.14, z = 46.07, correspond- trial: 3.3 A/mm ) and thus causing the increase of differ - ing to BA 6). At these locations, a reduction of activity ences with control known words (first trial: t(25) = −0.43, was found from the first to the last exposure to novel p = 0.66, diff. novel vs. known: −0.46 A/mm ; last trial: word-forms (rMTG, posterior section: t(25) = −5.008, t(25) = −2.85, p = 0.009, diff. novel vs. known: −3.76 A/ p = 0.000, diff. 1st vs. 6th trial: 3.84 A/mm ; rMTG, ante- mm ). rior section: t(25) = −3.22, p = 0.004, diff. 1st vs. 6th trial: Figure 3 (right panel) shows the most likely neural 3.03 A/mm ; rSTG: t(25) = −3.73, p = 0.001, diff. 1st vs. sources responsible for the reduction of N400 activ- 6th trial: 2.97 A/mm ; rAG: t(25) = −4.11, p = 0.000, ity, identified in the right middle and superior temporal diff. 1st vs. 6th trial: 4.01 A/mm ; lMFG: t(25) = −4.63, gyrus (rMTG, posterior section, x = 36.78, y = −61.09, p = 0.000, diff. 1st vs. 6th trial: 6.81 A/mm ) thus increas- z = 24.84, corresponding to BA 39; rMTG, anterior sec- ing differences between novel and known words from the tion, x = 36.78, y = −3.98, z = −14, corresponding to BA beginning (all ts < 1.6, ps > 0.1) to the end of the training 21; rSTG: x = 36.78, y = −54.85, z = 18.30, corresponding (rMTG, posterior section: t(25) = −3.57, p = 0.001, diff. B ermúdez‑Margaretto et al. Behav Brain Funct (2020) 16:11 Page 7 of 17 Fig. 2 Averaged ERP waveforms for control known words and for novel words across the six different training trials. Panels on the left and on the right show the training effect found in the P200 and N400 time windows, respectively (highlighted in grey shaded areas). Topographical maps below each set of waveforms show differences in scalp distribution between known and novel words across the training trials. Bar graphs below each panel show the mean amplitude of each ERP obtained for known words and for novel words across the training blocks. For the P200 time window, pair‑ wise comparisons revealed that mean activity elicited by control and novel words differed at the first trial but became similar already at the second exposure of novel words and was maintained throughout the rest of the training. However, for the N400 time window, pair‑ wise comparisons revealed that lexical differences emerged after the second exposure with novel word‑forms and were maintained across the rest of training trials The brief orthographic training with novel word-forms novel vs. known: −4.78 A/mm ; rMTG, anterior section: produced a strikingly fast and stable enhancement of an t(25) = −4.36, p = 0.000, diff. novel vs. known: −4.54 A/ early positivity, as observed in the amplitude of the P200 mm ; rSTG: t(25) = −2.79, p = 0.01, diff. novel vs. known: component. This ERP component has been related to −3.58 A/mm ; rAG: t(25) = −2.54, p = 0.018, diff. novel the extraction of orthographic and phonological word vs. known: −4.27 A/mm ; lMFG: t(25) = −4.11, p = 0.000, features at early stages of word processing , Carre- diff. novel vs. known: −9.53 A/mm ). riras et al. 2005; [67, 89, 114]. More specifically, smaller P200 amplitudes have been associated with more sub- Discussion lexical orthographic activation. In this sense, the P200 In this study we report ultra-rapid changes in the brain’s enhancement could be related to a modification of sub- electrophysiological signal elicited by novel meaning- lexical orthographic process, switching from the letter- less written word-forms, showing the influence of a very by-letter decoding to a more holistic lexical-type access short training (6 exposures only) at both early and late of newly formed representations. This interpretation is lexical stages of the processing of these stimuli. In par- also supported by the elimination of P200 differences ticular, the single-trial analysis carried out in this study between trained and already known words, possibly revealed that the strongest change in the brain electrical reflecting the process of establishing the whole-word response to novel word-forms took place between their recognition strategy for these new items, similar to that first two exposures, reflected in the modulation of both used for the reading of well-known lexical stimuli. Note P200 and N400 components. Bermúdez‑Margaretto et al. Behav Brain Funct (2020) 16:11 Page 8 of 17 Fig. 3 LAURA neural source reconstruction of the ERP training effects (last vs. first exposure with novel word‑forms) obtained for P200 and N400 time windows. T‑maps represent the brain location of differences in current source density between the last and first exposure to novel words, with the loci of maximal differences framed in red. For the early, P200 time window (left panel), the left lingual gyrus (lLG) and bilateral superior frontal gyrus (SFG) were found as the most probable neural sources for the P200 increase obtained at scalp level, whose activity was found stronger along the exposures with novel word‑forms. For the late time window (right panel), the neural generators of the N400 reduction were most expressed in the right middle and superior temporal gyrus, right angular gyrus and the left middle frontal gyrus, whose activity was found reduced from the first to the last exposure with novel written word‑forms. Graphs show the mean current source magnitudes at significant ROIs. Labels refer to neural sources: lLG (left Lingual Gyrus), rSFG (right Superior Frontal Gyrus), lSFG (left Superior Frontal Gyrus), lPostCG (left Postcentral Gyrus), rMTGant (right Middle Temporal Gyrus, anterior section), rMTGpos (right Middle Temporal Gyrus, posterior section), rSTG (right Superior Temporal Gyrus), rAG (right Angular Gyrus), lMFG (left Middle Frontal Gyrus) that our known and novel stimuli were matched in vari- letter- or bigram-related effect. Furthermore, the find - ous low-level psycholinguistic features (incl. syllabic ings at source level are also in agreement with this argu- and bigram frequency), which implies that this dynamic ment, with the left lingual gyrus as one of the most likely likely reflects whole-form acquisition rather than a neural sources responsible for the P200 enhancement B ermúdez‑Margaretto et al. Behav Brain Funct (2020) 16:11 Page 9 of 17 found at surface level. Indeed, this visual region has been written word-forms became similar to those elicited by proposed, together with the fusiform gyrus, as part of known, already lexicalized words as a consequence of the word-form processing system involved in the ortho- their repeated exposure, which also speaks to the linguis- graphic analysis of real versus false fonts or non-letter tic nature of this activity pattern. Nonetheless, to fully strings, carried out during early stages of reading (Nobre validate this explanation and rule out the habituation vs. et al. 1994, Petersen et al. 1988, 1990; Puce et al. 1996). language-related nature of the effects, future experiments Whereas the left fusiform has been related to the pro- should use additional control conditions including famil- cessing of local features, the left lingual gyrus is engaged iar words and non-orthographic visual patterns as stimuli in global shape processing, activated when attention is (i.e. symbol strings),indeed, the repetitive presentation of directed to the processing of global parts, such as the non-orthographic stimuli together with the set of main whole word-form [39, 40, 75]. Thus, the increase of acti - experimental word-forms could help disentangle ortho- vation found in this region likely indicates the stronger graphic from perceptual learning effects while avoid - whole-shape discrimination for the novel written word- ing potential confounds introduced by the repetition of forms through their repetitions. Indeed, whereas novel meaningful stimuli (such as formation of new semantic words initially exhibited lower activation than known associations, similar enhancement of orthographic mem- words at this region, as was similarly reported in previ- ory traces for both sets, etc.). ous studies [43, 74], novel words reached a similar level A similarly fast effect of visual repetition was reflected of activation than known words after the training. Taken in the amplitude of an N400 response, showing a remark- together, these results likely indicate the enhancement able decrease from the first to the second visual presen - of a whole-form based reading strategy for novel written tation of the novel written words, which also remained words as a consequence of this short visual exposure. stable until the end of the training. As reported in previ- These findings are in line with cognitive models devel - ous ERP studies with novel written words trained under oped in psycholinguistics to account for reading pro- meaningful contexts [8, 14, 76, 82], such a reduction in cesses, and particularly for the visual recognition of the N400 time interval could reflect the facilitation in already known and newly-experienced words [23, 86]. the lexico-semantic access of novel stimuli, due to preac- According to these models, the more often a particu- tivation of the respective concept, previously associated lar form is encountered, the lower is the threshold for through repetition. However, taking into account that its activation in the orthographic lexicon and therefore, in the present training context we only deal with visual the faster its visual recognition is. Thus, repeated visual word-forms devoid of semantic content, such an N400 exposure with novel word-forms allows the reader to pass effect cannot be generated by semantic activations per se. from a sub-lexical reading, which operates by means of In fact, given the novel word-forms trained in the present a serial phonological decoding of each grapheme into its study were unique stimuli, not derived from real words, corresponding phoneme, to a holistic reading, character- such an N400 modulation could not be triggered by ized by parallel letter decoding. Thus, the P200 modula - accessing the meaning of any related word either. Other tion found in the present study along the visual exposure explanations, therefore, must be considered for the N400 to novel written word-forms, as well as the estimated modulation. neural generators of this effect, likely reflect neurophysi - Importantly, the repetition of linguistic stimuli is con- ological changes that underlie the evolution from a sub- sidered to produce the formation of memory represen- lexical to a more lexical, whole-word reading strategy. tations, which contain recently processed information On a more cautious side, it may in principle be possible whose pre-activation facilitates the processing of the that early ERP modulations found across repetitive stimu- repeated stimuli at each new encounter . Such facili- lus presentation are not language or learning-specific and tation, understood as an easier or more fluent processing, simply reflect unspecific sensory-level effects of stimulus is reflected in the present study already at the early stage repetition. However, this non-linguistic explanation does of the linguistic processing. Thus, the activation of such not seem likely, given that repetition effects are typically new mental representations, containing orthographic, expressed as suppression/habituation of ERP responses, surface-related information, contributes to the enhance- which is not what we observe here [48, 77], although ment of a whole-form processing strategy for these stim- also see  for increased neural responses during rep- uli, as indexed in the P200 modulation. However, even if etition). The present changes in P200 amplitude do not these mental representations lack meaning, the knowl- show a suppression through the training, but instead edge gained through the exposures likely contributes to manifest a clear facilitation, as predicted by the theoreti- the ease or their processing at a later stage, as reflected cal account of new memory trace build-up and activation. in more positive-going N400 responses, typically asso- Moreover, as a result of training, P200 responses to novel ciated with less effortful lexico-semantic processing [9, Bermúdez‑Margaretto et al. Behav Brain Funct (2020) 16:11 Page 10 of 17 55, 56]. Indeed, previous studies have reported similar reduction obtained at scalp level across word repetitions, N400 reduction effects caused by repetition of mean - this effect is considered to reflect the decrease of unnec - ingless stimuli, including novel stimuli not derived from essary or redundant activation, due to the pre-activation real words [30, 63], they interpreted this effect as driven of the word representation after its previous presenta- by the intrinsic nature of textual stimuli, which suggests tion. Importantly, the posterior section of the middle all orthographic stimuli as potentially meaningful and temporal gyrus and surrounding areas including superior thus activates language-based processes during their temporal sulcus and inferior parietal gyrus, found here silent reading, including orthography (P200) and lexi- as the main generators of the N400 modulation, have cal semantics (N400). That could in principle be true for been proposed as the best candidates for the storage and the novel word-forms trained in the present study, since access of lexical, rather than semantic representations these stimuli were in fact real—but extremely infrequent [34, 49, 64]. This supports the argument that the present and hence unknown—word-forms with a specific mean - N400 modulation likely reflects lexical facilitation caused ing attached to them, thus, the semantic nature of these by pre-activation of whole-form surface representa- stimuli could theoretically boost the orthographic learn- tions for previously presented stimuli. Nonetheless, the ing and contribute to the rapid ERP changes observed, right-hemispheric activation found in the present study although this effect is improbable taking into account does not follow the left lateralization typically observed these stimuli were completely unknown to our partici- for language, however, it must be noted that the right- pants. Future comparisons across different stimuli sets hemisphere distribution of the N400 responses and its could answer the question whether the ecological value associated neural sources has been also reported in the of real linguistic stimuli (in comparison to artificially cre - literature, proving the right hemisphere as lesser but ated ones) underlies the boost of rapid word learning robust generator of this ERP [24, 52, 58, 113]. Indeed, a observed in ERP effects. Nonetheless, from that obliga - recent N400 study has reported a very similar pattern tory-semantics view discussed in aforementioned stud- of source activation as observed here, with the activity ies, and in the context of a learning task in which these decrease in the right superior and middle temporal gyrus stimuli were intended to be attended and learnt as much along with stimulus repetition , nonetheless, the as possible, the N400 reduction observed here likely comparison between both studies must be careful, since reflects the increased ease of their lexical processing, the N400 effect reported in Ströberg et al. was found caused by pre-activation of previously repeated informa- under a semantic priming paradigm, and particularly for tion containing surface whole-form rather than concep- familiar words preceded by primes presented repeatedly, tual features. Indeed, the increase in N400 differences hence not directly measured for repeated stimuli as in the between control stimuli and novel words also suggests present study. the activation of such facilitatory memories for repeated In general, it seems feasible to conclude that the pat- stimuli, in comparison to non-repeated control words. tern of ERP results found in this study reflects the fast Data from neural source estimation is also in agreement built-up of memory traces during the early stage of word with the view of the N400 reduction as a language-related learning. However, despite this fast and sustained mem- effect, connected to the potential lexico-semantic status ory formation process, it may be difficult to claim that of the stimuli; the localization of this effect revealed a visual word-form representations built for these stimuli set of areas typically associated with the lexico-semantic have been fully integrated into the mental lexicon at this processing as the most likely neural generators of this initial stage. Results found here, particularly the P200 ERP modulation, namely the right middle and superior modulation, reflect the fast acquisition of orthographic temporal gyri as well as the right inferior parietal cor- features for novel trained words, enabling the construc- tex (including the angular gyrus). These findings are in tion of surface-only word-forms and contributing to their agreement with previous literature, which has reported lexical configuration. Nonetheless, the lexicalization pro - these language-related areas among those responsible cess for these stimuli is likely still in progress, and a more for the N400 response [36, 52], see , for a review). intensive training and/or consolidation are most proba- Moreover, the specific pattern of N400 source activation bly required for their integration and further engagement obtained in the present study, showing the decrease of into the mental lexicon, as suggested in previous studies brain activity at temporal regions with novel word repeti- [6, 17, 27, 33, 42, 72, 73, 107]. tion, corroborates a number of previous studies [69, 98, In general, the present results are in agreement with 99]. Using similar stimulus repetition paradigms to the previous ERP data, confirming the high speed with one employed here, with no addition of semantic infor- which neurophysiological traces for novel written words mation, these studies found the decrease in the left tem- are built up [8, 14, 76, 82]. Importantly, these previ- poral and frontal gyri as neural generators of the N400 ous studies were not able to disentangle orthographic B ermúdez‑Margaretto et al. Behav Brain Funct (2020) 16:11 Page 11 of 17 and semantic processes during the acquisition of novel and N400 components. Therefore, the present data show word-forms, as they employed novel stimuli with mean- the remarkable speed of the human brain to evolve from ings attached to them. In contrast, the brain dynamics a serial to a whole-form reading strategy—after just a reported here more likely reflect the neural mechanisms couple of exposures to the novel orthographic stimulus, underlying purely orthographic processes during the an ability likely fundamental for learning to read as well initial acquisition of novel written word-forms, in the as for acquiring new vocabulary when reading. Moreover, absence of semantics. Similar suggestion of fast acqui- these results suggest the impact of the automaticity of the sion of purely orthography-based word-forms have been training in obtaining a clear neurophysiological modu- made previously [11, 13, 80]. However, in those studies lation at the early stages of the processing, thus indicat- the attention was not specifically focused on the learn - ing the importance of using low-level demand tasks to ing of novel word-forms but was instead diverted to study novel word learning. Nevertheless, further research accomplishing other visual tasks, such as the categoriza- is needed that could overcome the limitations of the tion of non-related stimuli during the parafoveal repeti- present study, providing behavioral measures of learn- tion of the novel words  or the lexical categorization ing as well as including the repetition of different types of stimulus items [11, 13]. Importantly, when the context of stimuli as additional control conditions. That would of training prioritizes the categorization of the trained confirm the pattern of rapid word learning obtained stimulus instead of their simple visual recognition as at neural level and strengthen the interpretation of the in the above mentioned studies, the effect of training is effects found as language-related, indicative of the fast only reflected in the LPC component, a late modulation orthographic learning of novel written words. Besides typically related to episodic memory process. Such pro- this, future research could extend the present findings cesses, probably recruited to carry out the required overt by addressing the neural underpinnings of the two stages reaction, may distort effects at earlier lexical process - of the lexicalization process, exploring the conditions ing stages and thus confound the effects of learning, as that could enable the fast engagement of the novel writ- already suggested in previous research comparing effects ten words into the reader’s lexicon. In this sense, future of novel word learning in high and low demanding tasks ERP investigation might consider the use of post-learn- . ing, low-level demand tasks to study the putative interac- In contrast, here we use a more natural context of tion of the novel word-forms with other existing lexical training, characterized by the attentive encounter with entrances after short training periods. novel word-forms in a silent reading task, and involving a small number of exposures. This approach allowed us to Methods detect fast orthographic learning effects at both early and Twenty-six students (18 females and 8 males; age range late stages in the lexical processing of novel word-forms. 18–29 years; SD = 2.84) took part in the experiment for u Th s, when word learning is carried out under a rela - course credits. All of them were right-handed, native tively automatic and free-demand task, the effect of train - Spanish speakers with no psychiatric or neurological dis- ing is only shaping their lexical processing at early and orders. Their brain activity was recorded by means of 64 late stages—as reflected in both P200 and N400 modu - Ag/AgCl active electrodes connected to an EEG ampli- lations, with no effects on later components such as the fier (ActiChAmp, Brain Products GmbH, Gilching, Ger - LPC, linked to episodic, categorization-related processes. many) during a silent reading task. Ocular activity was Remarkably, the early effect found in the present study recorded using horizontal and vertical EOG recordings. is consistent with previous findings both in the auditory During recordings, all electrodes were referenced to the [53, 54, 80, 102, 104, 115] and, more recently, in the visual vertex (Cz); two additional electrodes were placed on domain , during the exposure to novel words under the mastoid bones for off-line re-referencing of the sig - relatively non-attentive conditions of exposure to novel nal using the mean activity in these two electrodes. EEG spoken and written word-forms. signal was amplified and digitized at a 1000 Hz sampling rate and high and low pass filters at 0.1 and 100 Hz, Conclusions respectively, as well as a 50 Hz notch filter, were applied. Overall, the present study provides new evidence for Figure 4 shows the experimental procedure. The read - rapid word learning in visual domain, even in the absence ing task included 24 known words (medium frequency of a semantic reference. The online neural changes Spanish words, extracted from Alameda and Cuetos ), obtained through a very short naturalistic encounter used as control items and 24 previously unknown word- with meaningless word-forms show for the first time the forms (obscure words, with mean lexical frequency of 0 activation of early and late lexical stages of the process- occurrences per million, Martinez and Garcia ) act- ing for these stimuli, reflected in the modulation of P200 ing as novel words to be trained. Obscure (or rare) words Bermúdez‑Margaretto et al. Behav Brain Funct (2020) 16:11 Page 12 of 17 Fig. 4 Experimental procedure and sequence of stimuli presentation. During EEG recordings, participants were asked to pay attention to the center of the screen and read silently the stimuli presented. A set of novel written word‑forms (e.g.: nabla) was presented repeatedly six times across six successive training trials. Additionally, another set of real Spanish known words (e.g.: nieve) was presented to participants in order to establish a control comparison between known and novel written words. For both sets of stimuli, equal sequence of presentation was followed with the same elements, with presentation durations indicated to the right of each rectangle. Red triangles on the schematic ERP epoch (lower right) indicate, for each ERP effect, the latencies when maximal ERP changes were found as consequence of the repeated exposure to novel written word‑forms are real words existing in the dictionary, but due to their (independent-samples t-tests confirmed no statistical dif - very low lexical frequency these stimuli are unknown for ferences, with all contrasts at p >.05) by means of the Bus- participants, thus acting as novel words to be learned. capalabras database . The set of known words (e.g., The selection of such stimuli as novel words, instead nieve, Eng. snow, balcón, Eng. balcony) was presented of building them by changing letters of real words, has first, followed by the set of unknown written word-forms been often carried out for the study of novel word learn- (e.g., nabla, ancient musical instrument,jínjol, a type of ing [2, 41, 82]. This procedure ensures ecological learn - buckthorn), which was repeatedly presented in six dif- ing effects by means of fully naturalistic materials—new ferent blocks, each containing one trial of each stimu- entrances in participant’s native orthography, as well as lus in different randomized order across blocks (hence, prevents the activation of real words led by excessive each stimulus was exposed across 6 different trials). The orthographic similarity. Participants were asked at the presentation of known words was included in order to end of the training, to ensure they were naïve and had establish an additional control comparison between no previous knowledge about the novel stimuli. Both already known and novel written words. As a note, the known and novel words were disyllabic stimuli, 5–6 let- explicit repetition of these stimuli was avoided in order ter long (see Table 1 for characteristics of the stimuli). to prevent semantic association between these and novel The novel and known words were matched for the num - words, thus ensuring the assessment of purely ortho- ber of letters and syllables, mean syllable frequency, graphic mechanisms during novel word learning. Such bigram frequency and number of orthographic neighbors procedure was also aimed to limit a possible re-activation B ermúdez‑Margaretto et al. Behav Brain Funct (2020) 16:11 Page 13 of 17 Table 1. Main psycholinguistic properties of the stimuli used Novel Words Known Words t (46) value p value Lexical frequency 0 57.78 (103.99) – – Number of syllables 2 (0) 2 (0) 0 1 Number of letters 5.50 (0.51) 5.50 (0.51) 0 1 Number of orthographic Neighbors 1.42 (1.31) 1.46 (1.21) − 0.11 0.91 Bigram frequency (token type) 518.92 (285.91) 601.7 (350.51) − 0.89 0.37 Mean (1st and 2nd) Syllable Frequency 2046.83 (3150.97) 2108.54 (2997.74) − 0.07 0.94 Stimuli were maximally matched between the experimental conditions. Standard deviation is shown in brackets. Independent‑samples t‑tests confirmed no differences between novel and known words across the variables Novel words used in the study: cofín, dorna, fudre, bruño, gelfe, nabla, notro, pajel, paila, sisón, cuatí, facón, dolmán, puntel, reitre, roblón, runcho, seisén, holmio, trujal, jínjol, pambil, timple, carmes; Known words: color, toldo, valle, traje, golfo, bicho, litro, papel, nieve, mujer, baile, gafas, balcón, doctor, huella, millón, rastro, violín, garfio, crimen, cactus, césped, templo, pintor. and strengthening of orthographic traces for known whole ERP segment, and finds clusters (data points in words, a mechanism that could not easily be disentangled close temporal and spatial proximity) of significant dif - from—as well as confounded with—the acquisition of ferences between conditions, while effectively controlling novel orthographic information. The task was introduced for type 1 error . Two steps were followed for cluster- to participants as an experiment for learning new words, based analyses: they were instructed to read the stimuli presented on the First, the general effect of the orthographic training screen silently, by means of covert articulation, paying was studied by analyzing the differences between novel as much attention as possible and trying to learn them. written word-forms presented at the beginning (first Familiarization trials (using other stimuli) were provided trial) and at the end of the task (sixth trial). In particu- before the start of the task; breaks were taken after each lar, two temporal windows were defined, from 150–250 block in order to avoid fatigue. Stimuli were displayed in ms and from 250–800 ms, in order to test the training the center of a computer screen in white, 18-point bold effect at early and late stages of the processing; these Courier New font over a black background by means of two time windows were selected based on previous E-Prime 2.0 software (Psychology Software Tools Inc., ERP literature in which early (before 250 ms) and late Pittsburgh, USA). First, a fixation mark was displayed responses have been distinguished during visual word during 1000 ms, followed by the presentation of the stim- recognition (see for instance, [7, 21, 50, 55]. Then, the ulus for another 1000 ms. A blank screen was then pre- two conditions (novel words in blocks 1 and 6) were sented for 500 ms and finally the instruction ‘‘blink now’’ contrasted by t-tests computed for every sample point for 1000 ms. across each temporal window (across 1500 sample Preprocessing of the EEG data was carried out using points for the early temporal window of 150 to 250 ms Fieldtrip Toolbox . Raw data were low pass-filtered segment, i.e. 25 time samples × 60 channels, and across at 30 Hz and downsampled to 256 Hz. Recordings were 8460 sample points for the late temporal window of 250 epoched between −200 to 1000 ms post stimulus onset to 800 ms segment, i.e. 141 time samples × 60 chan- and the baseline was corrected using the 200-ms interval nels). Those samples below or equal to a predetermined preceding the stimulus onset. Independent component alpha level (0.05) were grouped together based on spa- analysis (ICA) was used to remove ocular artifacts and tial and temporal adjacency (a minimum of 2 adjacent a triangular interpolation of bad channels was applied. sample points was required). The cluster effect size Additional artifact rejection (using exclusion criteria at ± was then calculated by taking the sum of all individ- 100 µV) was applied to remove any remaining contami- ual t-test values of every temporo-spatial grouping (or nated epochs. Data were re-referenced offline to average cluster). In order to correct for multiple comparisons mastoid reference. Finally, EEG epochs were averaged per carried out, a cluster-based test statistic method was subject and per condition and ERPs were computed for then implemented. In particular, the null distribution novel word-forms at each task block, as well as for known of cluster-level statistic was calculated by randomly words (with a mean of 20 epochs included per condition). assigning ERP segments to the experimental condition The resulting ERPs were submitted to a cluster-based (here, 1000 times). A new cluster effect size was calcu - random permutation analysis in order to test the effect of lated after each randomization and the cluster with the the orthographic training. This is a method which deals largest effect size entered in the distribution. Finally, with multiple comparisons in space and time, over the the cluster was considered significant if the probability Bermúdez‑Margaretto et al. Behav Brain Funct (2020) 16:11 Page 14 of 17 Author’s contributions of the null hypothesis was below or equal 5%, i.e., if the BBM conducted the experimental task. BBM and DB analyzed the data. BBM proportion of cases, in which the values of this distri- and YS wrote the manuscript. AD and FC designed the experimental task. All bution were larger than the observed cluster-level sta- authors read and approved the final manuscript. tistic. Once a cluster was detected in either of the ERP Funding segments, a new contrast was carried out to further The article was prepared within the framework of the HSE University Basic explore the scalp localization of the resulting cluster, Research Program and funded by the Russian Academic Excellence Project ‘5–100.’ averaging its time interval. Next, in a second step we aimed to study the effect Availability of data and materials of each single repetition along the whole orthographic The datasets used and analyzed during the current study are available from the corresponding author on reasonable request. training, by using a more detailed, trial-by-trial approach. Thus, for each resulting cluster, a new analy - Ethics approval and consent to participate sis taking the mean amplitude over the time windows This research was approved by the Ethics Committee of the Psychology Department of the University of Oviedo. Before starting the experimental task, and electrodes of the resulting significant clusters was participants received information about the purpose of the study, the task, carried out to contrast novel words presented at spe- and its duration and gave their written informed consent. cific training trials (i.e., first vs. second, second vs. Consent for publication third, etc.); additional comparisons were also carried Not applicable. out between novel and control known words in each trial. The same cluster-based test statistic method as Competing interests The authors declare that they have no competing interests. implemented in the first step was used to account for multiple comparisons during this second analysis. Author details Finally, brain sources underlying the ERP effect of Centre for Cognition and Decision Making, Institute for Cognitive Neurosci‑ ence, National Research University Higher School of Economics, Moscow, orthographic training were estimated using the LAURA Russian Federation. Center of Functionally Integrative Neuroscience, Aarhus distributed source estimation method , implemented University, Aarhus, Denmark. Instituto Universitario de Neurociencia (IUNE) in Cartool Software . The solution space was calcu - and Facultad de Psicología, Universidad de La Laguna, Tenerife, Spain. Facul‑ tad de Psicología, Universidad de Oviedo, Oviedo, Spain. lated using a realistic head model, including 4011 nodes defined at regular distances within the grey matter of a Received: 31 January 2020 Accepted: 21 November 2020 standard magnetic resonance image (MRI) template, which is based on the average of 305 healthy adult brain MRIs (created by the Montreal Neurological Institute, MNI, see Evans et al. ). Current source magnitudes References 1. Alameda JR, Cuetos Vega F. Diccionario de frecuencias de las unidades (ampere per squared millimeter) at each node were cal- lingüísticas del castellano. Servicio de Publicaciones: Universidad de culated for each participant and condition (novel written Oviedo; 1995. words at first and last training trial, and control words) 2. Álvarez‑ Cañizo M, Suárez‑ Coalla P, Cuetos F. Orthographic learning in Spanish children: influence of previous semantic and phonological over averaged time windows showing significant repeti - knowledge. 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