Davide Crepaldi

A place for nouns and a place for verbs? A critical review of neurocognitive data on grammatical-class effects

It is generally held that noun processing is specifically sub-served by temporal areas, while the neural underpinnings of verb processing are located in the frontal lobe. However, this view is now challenged by a significant body of evidence accumulated over the years. Moreover, the results obtained so far on the neural implementation of noun and verb processing appear to be quite inconsistent. The present review briefly describes and critically re-considers the anatomo-correlative, neuroimaging, MEG, TMS and cortical stimulation studies on nouns and verbs with the aim of assessing the consistency of their results, particularly within techniques. The paper also addresses the question as to whether the inconsistency of the data could be due to the variety of the tasks used. However, it emerged that neither the different investigation techniques used nor the different cognitive tasks employed fully explain the variability of the data. In the final section we thus suggest that the main reason for the emergence of inconsistent data in this field is that the cerebral circuits underlying noun and verb processing are not spatially segregated, at least for the spatial resolution currently used in most neuroimaging studies.

Nouns and verbs in the brain: grammatical class and task specific effects as revealed by fMRI.

The wide variety of techniques and tasks used to study the neural correlates of noun and verb processing has resulted in a body of inconsistent evidence. We performed a functional magnetic resonance imaging (fMRI) experiment to detect grammatical class effects that generalize across tasks. A total of 12 participants undertook a grammatical-class switching task (GCST), in which they were presented with a noun (or a verb) and were asked to retrieve the corresponding verb (or noun), and a classical picture naming task (PNT) widely used in the previous aphasiological and imaging literature. The GCST was explicitly designed to ensure control over confounding variables, such as stimulus complexity or imageability. Conjunction analyses of the haemodynamic responses measured in the two tasks indicated a shared verb-related activation of a dorsal premotor and posterior parietal network, pointing to a strong relationship between verb representation and action-oriented (visuo-)spatial knowledge. On the other hand, no brain area was consistently associated with nouns in both tasks. Moreover, there were task-dependent differences between noun and verb retrieval both at behavioural and at physiological level; the grammatical class that elicited the longest reaction times in both tasks (i.e., verbs in the PNT and nouns in the GCST) triggered a greater activation of the left inferior frontal gyrus. Therefore, we suggest that this area reflects a general increase in task demand rather than verb processing per se.

Verb-noun double dissociation in aphasia: theoretical and neuroanatomical foundations.

This paper reports the results of several studies on the mechanisms underlying Verb-Noun (V-N) dissociation. The objectives of the studies were to ascertain the location of the lesions causing predominant V or N impairment and to shed light on the different mental representations of these word classes through analyses of the data from neuropsychological patients. With regard to lesion sites, results obtained through an anatomo-correlative study on 15 V-impaired and 5 N-impaired aphasic patients indicate that lesions causing predominant N impairment were mostly located in the middle and inferior left temporal area. Three alternative lesion sites were associated with a V deficit (left posterior temporo-parietal lesions; large left fronto-temporal perisylvian lesions; deep lesions of the insula and/or the basal ganglia). In contrast to the results obtained from several neuropsychological and neuroimaging studies, none of the V-impaired patients had an isolated frontal lesion. The second aim is to discuss grammatical class interaction with semantic factors such as actionality or imageability (said to be the real cause of V-N dissociation). The retrieval of Ns and Vs in a sentence context was tested on 16 V-impaired aphasic patients and the resulting data indicate that imageability interacts with the retrieval of Ns and Vs, but cannot completely account for their dissociation.

Morphemes in their place: Evidence for position-specific identification of suffixes.

Previous research strongly suggests that morphologically complex words are recognized in terms of their constituent morphemes. A question thus arises as to how the recognition system codes for morpheme position within words, given that it needs to distinguish morphological anagrams like overhang and hangover. The present study focused specifically on whether the recognition of suffixes occurs in a position-specific fashion. Experiments 1 and 2 revealed that morphologically complex nonwords (gasful ) are rejected more slowly than orthographic controls (gasfil ) but that the same interference effect is not present when the morphemic constituents are reversed ( fulgas vs. filgas). Experiment 3 went further in demonstrating that reversing the morphemes within words (e.g., nesskind) does not yield morpheme interference effects against orthographic controls (e.g., nusskind). These results strongly suggest that suffix identification is position specific, which imposes important constraints on the further development of models of morphological processing.

Clustering the lexicon in the brain: a meta-analysis of the neurofunctional evidence on noun and verb processing

Although it is widely accepted that nouns and verbs are functionally independent linguistic entities, it is less clear whether their processing recruits different brain areas. This issue is particularly relevant for those theories of lexical semantics (and, more in general, of cognition) that suggest the embodiment of abstract concepts, i.e., based strongly on perceptual and motoric representations. This paper presents a formal meta-analysis of the neuroimaging evidence on noun and verb processing in order to address this dichotomy more effectively at the anatomical level. We used a hierarchical clustering algorithm that grouped fMRI/PET activation peaks solely on the basis of spatial proximity. Cluster specificity for grammatical class was then tested on the basis of the noun-verb distribution of the activation peaks included in each cluster. Thirty-two clusters were identified: three were associated with nouns across different tasks (in the right inferior temporal gyrus, the left angular gyrus, and the left inferior parietal gyrus); one with verbs across different tasks (in the posterior part of the right middle temporal gyrus); and three showed verb specificity in some tasks and noun specificity in others (in the left and right inferior frontal gyrus and the left insula). These results do not support the popular tenets that verb processing is predominantly based in the left frontal cortex and noun processing relies specifically on temporal regions; nor do they support the idea that verb lexical-semantic representations are heavily based on embodied motoric information. Our findings suggest instead that the cerebral circuits deputed to noun and verb processing lie in close spatial proximity in a wide network including frontal, parietal, and temporal regions. The data also indicate a predominant—but not exclusive—left lateralization of the network.

Meaning is in the beholder’s eye: Morpho-semantic effects in masked priming

A substantial body of literature indicates that, at least at some level of processing, complex words are broken down into their morphemes solely on the basis of their orthographic form (e.g., Rastle, Davis, & New, Psychonomic Bulletin and Review 11:1090–1098, 2004). Recent evidence has shown that this process might not be obligatory, as indicated by the fact that morpho-orthographic effects were not found in a cross-case same–different task—that is, when lexical access was not necessarily required (Duñabeitia, Kinoshita, Carreiras, & Norris, Language and Cognitive Processes 26:509–529, 2011). In this study, we employed a task that required understanding a series of words and, thus, implied lexical access. Masked primes were shown very briefly right before the appearance of the target word; prime–target pairs entertained a morpho-semantic (dealer–DEAL), a morpho-orthographic (corner–CORN), or a purely orthographic (brothel–BROTH) relationship. Eye fixation times clearly indicated facilitation for transparent pairs, but not for opaque pairs (or for orthographic pairs, which were used as a baseline). Conversely, the usual morpho-orthographic pattern was found in a control experiment, employing a lexical decision task. These results indicate that the access to a morpho-orthographic level of representation is not always necessary for lexical identification, which challenges models of visual word identification that cannot account for task-induced effects.

Semantic transparency in free stems: The effect of Orthography-Semantics Consistency on word recognition.

A largely overlooked side effect in most studies of morphological priming is a consistent main effect of semantic transparency across priming conditions. That is, participants are faster at recognizing stems from transparent sets (e.g., farm) in comparison to stems from opaque sets (e.g., fruit), regardless of the preceding primes. This suggests that semantic transparency may also be consistently associated with some property of the stem word. We propose that this property might be traced back to the consistency, throughout the lexicon, between the orthographic form of a word and its meaning, here named Orthography-Semantics Consistency (OSC), and that an imbalance in OSC scores might explain the “stem transparency” effect. We exploited distributional semantic models to quantitatively characterize OSC, and tested its effect on visual word identification relying on large-scale data taken from the British Lexicon Project (BLP). Results indicated that (a) the “stem transparency” effect is solid and reliable, insofar as it holds in BLP lexical decision times (Experiment 1); (b) an imbalance in terms of OSC can account for it (Experiment 2); and (c) more generally, OSC explains variance in a large item sample from the BLP, proving to be an effective predictor in visual word access (Experiment 3).

Lexical-Semantic Variables Affecting Picture and Word Naming in Chinese: A Mixed Logit Model Study in Aphasia

Lexical-semantic variables (such as word frequency, imageability and age of acquisition) have been studied extensively in neuropsychology to address the structure of the word production system. The evidence available on this issue is still rather controversial, mainly because of the very complex interrelations between lexical-semantic variables. Moreover, it is not clear whether the results obtained in Indo-European languages also hold in languages with a completely different structure and script, such as Chinese. The objective of the present study is to investigate this specific issue by studying the effect of word frequency, imageability, age of acquisition, visual complexity of the stimuli to be named, grammatical class and morphological structure in word and picture naming in Chinese. The effect of these variables on naming and reading accuracy of healthy and brain-damaged individuals is evaluated using mixed-effect models, a statistical technique that allows to model both fixed and random effects; this feature substantially enhances the statistical power of the technique, so that several variables–and their complex interrelations–can be handled effectively in a unique analysis. We found that grammatical class interacts consistently across tasks with morphological structure: all participants, both healthy and brain-damaged, found simple nouns significantly easier to read and name than complex nouns, whereas simple and complex verbs were of comparable difficulty. We also found that imageability was a strong predictor in picture naming, but not in word naming, whereas the contrary held true for age of acquisition. These results are taken to indicate the existence of a morphological level of processing in the Chinese word production system, and that reading aloud may occur along a non-semantic route (either lexical or sub-lexical) in this language.

Masked suffix priming and morpheme positional constraints

Although masked stem priming (e.g., dealer–DEAL) is one of the most established effects in visual word identification, it is less clear whether primes and targets sharing a suffix (e.g., kindness–WILDNESS) also yield facilitation. In a new take on this issue, we show that prime nonwords facilitate lexical decisions to target words ending with the same suffix (sheeter–TEACHER) compared to a condition where the critical suffix was substituted by another one (sheetal–TEACHER) or by an unrelated nonmorphological ending (sheetub– TEACHER). We also show that this effect is genuinely morphological, as no priming emerged in noncomplex items with the same orthographic characteristics (sportel–BROTHEL vs. sportic–BROTHEL vs. sportur–BROTHEL). In a further experiment, we took advantage of these results to assess whether suffixes are recognized in a position-specific fashion. Masked suffix priming did not emerge when the relative order of stems and suffixes was reversed in the prime nonwords—ersheet did not yield any time saving in the identification of teacher as compared to either alsheet or obsheet. We take these results to show that –er was not identified as a morpheme in ersheet, thus indicating that suffix identification is position specific. This conclusion is in line with data on interference effects in nonword rejection and strongly constrains theoretical proposals on how complex words are identified. In particular, because these findings were reported in a masked priming paradigm, they suggest that positional constraints operate early, most likely at a prelexical level of morpho-orthographic analysis.

Morphological Processing of Printed Nouns and Verbs: Cross Class Priming Effects

Despite grammatical class being a fundamental organising principle of the human mental lexicon, recent morphological models of visual word identification remain silent as to whether and how it is represented in the lexical system. The present study addresses this issue by investigating cross-class morphological priming (i.e., the effect obtained when nouns prime verbs sharing the same root or vice versa) to clarify whether morphological stems subserving the formation of both nouns and verbs (e.g., depart-) have a unique, grammatical class–independent representation. Experiments 1 and 2 suggest this to be the case, as they show that morphological priming crosses grammatical class boundaries in overt paradigm conditions. Experiment 3 shows that, in masked priming conditions, cross-class facilitation emerges both for genuine derivations and pseudo-related pairs with a homographic stem (e.g., port-e, doors, and port-are, to carry), which is taken to suggest that grammatical class–free stem representations are located at a pre-lexical level of morphological processing.