Prominent neurobiological models of language follow the widely accepted assumption that language comprehension requires two principal mechanisms: a lexicon storing the sound-to-meaning mapping of words, primarily involving bilateral temporal regions, and a combinatorial processor for syntactically structured items, such as phrases and sentences, localized in a left-lateralized network linking left inferior frontal gyrus (LIFG) and posterior temporal areas. However, recent research showing that the processing of simple phrasal sequences may engage only bilateral temporal areas, together with the claims of distributional approaches to grammar, raise the question of whether frequent phrases are stored alongside individual words in temporal areas. In this fMRI study, we varied the frequency of words and of short and long phrases in English. If frequent phrases are indeed stored, then only less frequent items should generate selective left frontotemporal activation, because memory traces for such items would be weaker or not available in temporal cortex. Complementary univariate and multivariate analyses revealed that, overall, simple words (verbs) and long phrases engaged LIFG and temporal areas, whereas short phrases engaged bilateral temporal areas, suggesting that syntactic complexity is a key factor for LIFG activation. Although we found a robust frequency effect for words in temporal areas, no frequency effects were found for the two phrasal conditions. These findings support the conclusion that long and short phrases are analyzed, respectively, in the left frontal network and in a bilateral temporal network but are not retrieved from memory in the same way as simple words during spoken language comprehension.

The processing of words containing inflectional affixes triggers morphophonological parsing and affix-related grammatical information processing. Increased perceptual complexity related to stem-affix parsing is hypothesized to create predominantly domain-general processing demands, whereas grammatical processing primarily implicates domain-specific linguistic demands. Exploiting the properties of Russian morphology and syntax, we designed an fMRI experiment to separate out the neural systems supporting these two demand types, contrasting inflectional complexity, syntactic (phrasal) complexity, and derivational complexity in three comparisons: (a) increase in parsing demands while controlling for grammatical complexity (inflections vs. phrases), (b) increase in grammatical processing demands, and (c) combined demands of morphophonological parsing and grammatical processing (inflections and phrases vs. derivations). Left inferior frontal and bilateral temporal areas are most active when the two demand types are combined, with inflectional and phrasal complexity contrasting strongly with derivational complexity (which generated only bilateral temporal activity). Increased stem-affix parsing demands alone did not produce unique activations, whereas grammatical structure processing activated bilateral superior and middle temporal areas. Selective left frontotemporal language system engagement for short phrases and inflections seems to be driven by simultaneous and interdependent domain-general and domain-specific processing demands.

Current research suggests that language comprehension engages two joint but functionally distinguishable neurobiological processes: a distributed bilateral system, which supports general perceptual and interpretative processes underpinning speech comprehension, and a left hemisphere (LH) frontotemporal system, selectively tuned to the processing of combinatorial grammatical sequences, such as regularly inflected verbs in English [Marslen-Wilson, W. D., & Tyler, L. K. Morphology, language and the brain: The decompositional substrate for language comprehension. Philosophical Transactions of the Royal Society: Biological Sciences, 362, 823–836, 2007]. Here we investigated how English derivationally complex words engage these systems, asking whether they selectively activate the LH system in the same way as inflections or whether they primarily engage the bilateral system that support nondecompositional access. In an fMRI study, we saw no evidence for selective activation of the LH frontotemporal system, even for highly transparent forms like bravely . Instead, a combination of univariate and multivariate analyses revealed the engagement of a distributed bilateral system, modulated by factors of perceptual complexity and semantic transparency. We discuss the implications for theories of the processing and representation of English derivational morphology and highlight the importance of neurobiological constraints in understanding these processes.

Spoken word recognition involves the activation of multiple word candidates on the basis of the initial speech input—the “cohort”—and selection among these competitors. Selection may be driven primarily by bottom–up acoustic–phonetic inputs or it may be modulated by other aspects of lexical representation, such as a word's meaning [Marslen-Wilson, W. D. Functional parallelism in spoken word-recognition. Cognition, 25, 71–102, 1987]. We examined these potential interactions in an fMRI study by presenting participants with words and pseudowords for lexical decision. In a factorial design, we manipulated (a) cohort competition (high/low competitive cohorts which vary the number of competing word candidates) and (b) the word's semantic properties (high/low imageability). A previous behavioral study [Tyler, L. K., Voice, J. K., & Moss, H. E. The interaction of meaning and sound in spoken word recognition. Psychonomic Bulletin & Review, 7, 320–326, 2000] showed that imageability facilitated word recognition but only for words in high competition cohorts. Here we found greater activity in the left inferior frontal gyrus (BA 45, 47) and the right inferior frontal gyrus (BA 47) with increased cohort competition, an imageability effect in the left posterior middle temporal gyrus/angular gyrus (BA 39), and a significant interaction between imageability and cohort competition in the left posterior superior temporal gyrus/middle temporal gyrus (BA 21, 22). In words with high competition cohorts, high imageability words generated stronger activity than low imageability words, indicating a facilitatory role of imageability in a highly competitive cohort context. For words in low competition cohorts, there was no effect of imageability. These results support the behavioral data in showing that selection processes do not rely solely on bottom–up acoustic–phonetic cues but rather that the semantic properties of candidate words facilitate discrimination between competitors.

The left inferior frontal gyrus (LIFG) has long been claimed to play a key role in language function. However, there is considerable controversy as to whether regions within LIFG have specific linguistic or domain-general functions. Using fMRI, we contrasted linguistic and task-related effects by presenting simple and morphologically complex words while subjects performed a lexical decision (LD) task or passively listened (PL) without making an overt response. LIFG Brodmann's area 47 showed greater activation in LD than PL, whereas LIFG Brodmann's area 44 showed greater activation to complex compared with simple words in both tasks. These results dissociate task-driven and obligatory language processing in LIFG and suggest that PL is the paradigm of choice for probing the core aspects of the neural language system.

The role of morphological structure in word recognition raises issues about the nature and structure of the language system. One major issue is whether morphological factors provide an independent principle for lexical organization and processing, or whether morphological effects can be reduced to the joint contribution of form and meaning. The independence of form, meaning, and morphological structure can be directly investigated using derivationally complex words, because derived words can share form but need not share meaning (e.g., archer-arch ). We used an event-related functional magnetic resonance imaging paradigm to investigate priming between pairs of words that potentially shared a stem, where this link was either semantically transparent (e.g., bravely-brave ) or opaque (e.g., archer-arch ). These morphologically related pairs were contrasted with identity priming (e.g., mist-mist ) and priming for pairs of words that shared only form (e.g., scandal-scan ) or meaning (e.g., accuse-blame ). Morphologically related words produced significantly reduced activation in left frontal regions, whether the pairs were semantically transparent or opaque. The effect was not found for any of the control conditions (identity, form, or meaning). Morphological effects were observed separately from processing form and meaning and we propose that they reflect segmentation of complex derived words, a process triggered by surface morphological structure of complex words.

An important method for studying how the brain processes familiar stimuli is to present the same item on more than one occasion and measure how responses change with repetition. Here we use repetition priming in a sparse functional magnetic resonance imaging (fMRI) study to probe the neuroanatomical basis of spoken word recognition and the representations of spoken words that mediate repetition priming effects. Participants made lexical decisions to words and pseudowords spoken by a male or female voice that were presented twice, with half of the repetitions in a different voice. Behavioral and neural priming was observed for both words and pseudowords and was not affected by voice changes. The fMRI data revealed an elevated response to words compared to pseudowords in both posterior and anterior temporal regions, suggesting that both contribute to word recognition. Both reduced and elevated activation for second presentations (repetition suppression and enhancement) were observed in frontal and posterior regions. Correlations between behavioral priming and neural repetition suppression were observed in frontal regions, suggesting that repetition priming effects for spoken words reflect changes within systems involved in generating behavioral responses. Based on the current results, these processes are sufficiently abstract to display priming despite changes in the physical form of the stimulus and operate equivalently for words and pseudowords.

The regular and irregular past tense has become a focus for recent debates about the structure of the language processing system, asking whether language functions are subserved by different neural and functional mechanisms or whether all processes can be accommodated within a single unified system. A critical claim of leading single mechanism accounts is that the relationship between an irregular stem and its past tense form is primarily semantic and not morphological in nature. This predicts an obligatory relationship between semantic performance and access to the irregular past tense, such that a semantic deficit necessarily leads to impairments on the irregulars. We tested this claim in a series of studies probing the comprehension and production of regular and irregular past tense forms in four semantic dementia patients, all of whom had profound semantic deficits. In two elicitation tasks and one auditory priming study, we found that three out of the four patients did not have a deficit for the irregular past tense, in spite of their semantic deficits. This argues against the view that the relationship between irregular past tense forms and their stems is primarily semantic, and more generally against the single system claim that morphological structure can be captured solely based on phonological and semantic relationships.

Neuropsychological research showing that the regular (“jump–jumped”) and irregular (“drive/drove”) past tense inflectional morphology can dissociate following brain damage has been important in testing claims about the cognitive and neural status of linguistic rules. These dissociations have been interpreted as evidence for two different computational systems—a rule-based system underlying the processing of regulars and the irregulars being individually listed in the mental lexicon. In contrast, connectionist accounts claim that these dissociations can be modeled within a single system. Combining behavioral data from patients with detailed information about their neuropathology can, in principle, provide strong constraints on accounts of the past tense. In this study, we tested five nonfluent aphasic patients, all of whom had extensive left hemisphere (LH) damage involving the left inferior frontal gyrus and underlying structures, and four patients with semantic deficits following herpes simplex encephalitis (HSE) who had extensive damage to the inferior temporal cortex. These patients were tested in experiments probing past tense processing. In a large priming study, the nonfluent patients showed no priming for the regular past tense but significant priming for the irregulars (whereas controls show priming for both). In contrast, the HSE patients showed significantly impaired performance for the irregulars in an elicitation task. These patterns of behavioral data and neuropathology suggest that two separable but interdependent systems underlie processing of the regular and irregular past tense.