1.1. Stems and Theme Vowel
Diachronically, Romance languages, such as Catalan, Italian, Portuguese, Romanian, and Spanish, inherited their verbal system from Latin, where we observe on the right side a tense node containing the tense (T) and agreement (Agr) inflectional suffixes, and on the left side a stem (v) formed by the root (√) with semantic content and a theme vowel (Th) with a functional class distribution, yielding the general verbal hierarchical structure [[[parl]
√[e]
Th]
v[[r]
T[ont]
Agr]
T]
TP parleront ‘they will speak’ [
3,
4]. Although the suffixal system is extremely regular and paradigmatic, stem formation presents specific rules and allomorphy, which are consistently affected by the Th and inflectional suffixes [
5]. Synchronically, Romance languages reduced the four Latin Th to three (i.e., ‘a’, ‘e’, and ‘i’), as shown in
Table 1. The Th is an empty morph without semantic content but with an exclusive functional role in verbal class distribution [
6]. However, the French verbal system presents many singularities in stem formation and is not traditionally described in terms of Th [
7], but see [
3].
French is traditionally described as having three verbal classes: the first class with the [-er] ending is fully regular, is productive for new verbs (e.g.,
googler ‘to google’), and includes more than 8500 verbs [
8]. The second class with the [-ir] ending with allomorphy in [-iss-] is fully regular, is not productive, and comprises approximately 250 verbs [
9]. The third class presents many micro-classes with specific stem formation processes, is not productive, and has approximately 350 verbs [
7]. Compared to other Romance languages, French changed the ‘a’ Th to ‘e’ in the first class (but kept ‘a’ in obsolete tenses, such as in the indicative simple past and subjunctive imperfect past); in the second class, French defined the regular [-ir] ending, which was irregular verbs from the fourth class in Latin and third class in other Romance languages; and in the third class, French suppressed the short ‘ĕ’ Latin Th, establishing verbs without a Th, changed the long ‘ē’ Th to ‘oi’; and developed many endings as a function of the ‘i’ Th [
3].
In the present study, we tested different verbal micro-classes in French to examine whether different types of processing, as postulated by dual-mechanism models, are needed to handle their identification. Indeed, even if the third class presents many different micro-classes (more than 25, see [
7]), from a cognitive perspective, it could also be seen as a regular domain driven by combinatorial and allomorphic morphological rules for lexical access and word recognition [
10].
1.2. Inflectional Processing
Three families of morphological processing models have emerged since the 1970s: symbolic manipulation models, associative activation models, and dual-mechanism models. The first type argues that the mental lexicon contains morphological representations that are activated in word (de)composition, such as the Obligatory Decomposition model [
11] and the Single Route model [
12]. The second type advocates that the mental lexicon is formed by associative whole word representations [
13]. In contrast, Parallel Distributed Processing models compute different weights in the overlap of orthography, phonology, and semantics in hidden units [
14]. Alternatively, the amorphous Naïve Discriminative Leaning model predicts no activation of word structure in a direct form-to-meaning computation [
15].
Dual-mechanism models propose two routes for word recognition, a direct whole word route and another route operating through morphological decomposition; for example, the Augmented Addressed Morphology (AAM) model dissociates known and unknown words [
16], the Parallel Dual-Route model asserts a race between both routes [
17], and the Words and Rules (W&R) model differentiates regular and irregular words [
18]. Alternatively, the Minimalist Morphology (MM) model proposes a route where irregular words are represented as sub-lexical entries that contain variables [
19].
According to Marantz [
20] (p. 906), “the separationist understanding of morphemes allows for a complete integration of morphology with the syntax (the ‘grammar’ in everyday language) such that the internal structure of words finds analysis within the same syntactic architecture and subject to the same syntactic principles as the internal structure of phrases and sentences.” However, the morphological hierarchical structure of words has rarely been considered in psycho- and neurolinguistic models of word processing and recognition, although it might have a role in word recognition. This implies that while a simple insertion rule such as
lire/lisons ‘to read/we read’ may demand few resources, a complex substitution rule such as
joindre/joignons ‘to join/we join’ may demand greater resources, impacting the participant’s behavior.
Recent findings in English verbal inflection have shown that the morphological structure of words contributes to the form-based decomposition. It seems that the transitional probability from stem to (pseudo)affix modulates early behavioral and neurological components [
12,
21,
22]. Recent work in the English irregular past tense demonstrated that form-based subregularities can be quantified and offer an optimal predictor for morphological processing [
23,
24]. Fruchter et al. [
25] showed that the phonological rules relating irregular forms are a reliable predictor for the morphological decomposition and word recognition. Also, Yang [
26] showed that more than the transitional probabilities, the acquisition of the English past tense can be realistically explained when constrained by the phonological and morphological structures. These studies provide evidence for a single-mechanism account in morphological processing exploring form-based and transitional probabilities [
20].
Existing Romance language studies on the processing of different verbal classes have yielded contradictory results. Domínguez et al. [
27] propose that Spanish verbs from the first class are fully combinatorial whereas verbs from the second and third classes are lexically represented, according to the AAM model, but see [
4,
28]. A similar pattern of results was observed in Catalan [
29], but see [
5]. Veríssimo and Clahsen [
30] argue that in Portuguese, although first class verbs are fully decomposed using a root-based mechanism, morphophonological first class verbs (e.g.,
afogar/afɔgo ‘to drown/I drown’) and third class verbs are partially decomposed, having different stem representations in the mental lexicon, according to the MM model, but see [
31].
Italian presents more controversial results: Caramazza et al. [
16] argue in favor of the AAM model, whereas Say and Clahsen [
32] propose that regular first class verbs are accessed through the combinatorial route and irregular second and third class verbs are accessed through the whole word route, according to the W&R model. In contrast, Orsolini and Marslen-Wilson [
33] show that priming effects in regular and irregular verbs from the three classes do not differ, suggesting a single-mechanism model with decomposition.
In French, Meunier and Marslen-Wilson [
34] observe that different verb types, i.e., fully regular (e.g.,
aimons/aimer ‘we love/to love’), morphophonological (e.g.,
jɛttes/jeter ‘you throw/to throw’), sub-regular (e.g.,
peignent/peindre ‘they paint/to paint’), and idiosyncratic verbs (e.g.,
vont/aller ‘they go/to go’), may be decomposed for lexical access, suggesting that word recognition in French is morpheme based. Thus, even if stems present morphophonological or allomorphic processes, the paradigmatic suffixal system seems to trigger the decomposition between the stem and inflectional suffixes in all French verbs [
35]. Bonami et al. [
9] show the systematic regularity of the second class French verbs, which might indeed be considered fully regular because (a) there are no morphophonological verbs in the second class; (b) there are no phonological/orthographic rules in the second class, such as the <c/ç> and <g/ge> rules found in the first class (e.g.,
placer/plaçons ‘to place/we place’,
manger/mangeons ‘to eat/we eat’); and (c) in the second class, ‘i’ Th was completely incorporated into all second class roots where all forms have been stabilized in either [√i] before the boundary and consonant or [√iss-] before vocalic (V) suffixes (i.e., [√i] → [√iss-]/_V).
Nevertheless, even if Romance languages such as Catalan, Italian, Portuguese, and Spanish have a more transparent morphological structure where the root, Th, and inflectional suffixes of T and Agr can be easily identified in verbal forms, and thus, more full priming would be expected, psycholinguistic studies have found full priming in the first class and partial priming in the second and third classes. The main argument for these results is the default fully regular behavior of the first class stem formation with high productivity when compared to the second and third classes [
2].
Therefore, differently from these Romance languages, (a) French verbal classes are not traditionally described in terms of Th [
7]; (b) the French first class presents morphophonological irregularities (e.g.,
jeter/jɛttes ‘to throw/you throw’,
adorer/adɔres ‘to adore/you adore’) [
10] and phonological/orthographic irregularities (e.g.,
placer/plaçons ‘to place/we place’,
manger/mangeons ‘to eat/we eat’) [
8]; the second class is fully regular [
9]; and the third class presents many micro-classes with regular and productive allomorphic processes [
34]. Also, (c) French presents an iambic prosodic system with the stress always falling on the last pronounced syllable, while the other Romance languages present a trochaic prosodic system with the stress falling on the ultimate, penultimate, or antepenultimate syllable [
3], and (d) French shows a large inconsistency between the orthographic and phonetic forms, especially regarding the pronunciation of schwa and inflectional suffixes in verbal forms.
The general picture drawn is that Romance languages, even if structurally close, seem different at the morphological processing level; therefore, cross-linguistic comparisons can provide descriptive information for a better understanding of the morphological processing in word recognition [
2].
1.3. Stem Formation
We conducted a cross-modal experiment and a masked priming experiment with a visual lexical decision task to investigate how stem formation impacts processing of French verbs. Cross-modal priming elicits conscious processing, resulting in lexical core activation through different modalities; however, it captures semantic and orthographic effects, which are undesired in our investigation [
34,
36]. In contrast, visual masked priming relies on unconscious and automatic effects, providing evidence for rapid and early processes in word recognition [
37]. We used a 52 ms masked prime presentation in our experiment to track morphological processing. At this duration, orthographic overlap produces inhibition and semantic activation is not measurable; thus, priming facilitation can be interpreted as morphological rather than in terms of orthographic overlap or semantic relatedness [
21,
38].
We tested four different verb types that present specific stem formation processes (one from the first class and three from the third class) and that are considered higher subdivisions in the verbal classes that assemble individual micro-classes. They represent 92.19% of the total verbs in French, and the three third class verb types tested represent 84.24% of the total third class verbs, they are also representative of different stem formation processes. The first verb type is fully regular verbs from the first class in [-er] (e.g.,
parler/parlons ‘to speak/we speak’), the second is regular verbs from the third class in [-ir] (e.g.,
sentir/sentons ‘to feel/we feel’), the third is verbs from the third class in [-ire] with allomorphic insertion (e.g.,
lire/lisons ‘to read/we read’), and the fourth is verbs from the third class in [-indre] with allomorphic substitution (e.g.,
joindre/joignons ‘to join/we join’) [
3,
7,
8].
We tested three different priming conditions (i.e., control, identity, and test). The identity priming condition reflects full priming and the unrelated control priming condition reflects no priming; thus, the test priming condition was compared to these two conditions. On the one hand, full priming in the test condition indicates that words are fully decomposed into atomic units and that even the stem is parsed into the root and Th, indicating that stem formation is root-based; on the other hand, no priming in the test condition implies that words are not decomposed, favoring whole word representation. Partial priming may indicate that different, albeit linked, allomorphic stem representations are activated in the prime and target pair [
1,
22,
30,
32,
38,
39].
Our hypothesis is that all French verbs from the different micro-classes are morphologically decomposed for lexical access and word recognition; therefore, the morphemic representations are processed in the mental lexicon through hierarchical structures, where the stem is processed by means of morphological and phonological rules [
24,
25,
40]. The alternative hypothesis is that only the first class is default root-based and fully decomposed for lexical access, whereas the other classes are word based or stem based, accessed by whole word or sub-lexical entries [
16,
17,
18,
19].