Morphology, language and the brain: the decompositional substrate for language comprehension

Abstract

This paper outlines a neurocognitive approach to human language, focusing on inflectional morphology and grammatical function in English. Taking as a starting point the selective deficits for regular inflectional morphology of a group of non-fluent patients with left hemisphere damage, we argue for a core decompositional network linking left inferior frontal cortex with superior and middle temporal cortex, connected via the arcuate fasciculus. This network handles the processing of regularly inflected words (such as joined or treats), which are argued not to be stored as whole forms and which require morpho-phonological parsing in order to segment complex forms into stems and inflectional affixes. This parsing process operates early and automatically upon all potential inflected forms and is triggered by their surface phonological properties. The predictions of this model were confirmed in a further neuroimaging study, using event-related functional magnetic resonance imaging (fMRI), on unimpaired young adults. The salience of grammatical morphemes for the language system is highlighted by new research showing that similarly early and blind segmentation also operates for derivationally complex forms (such as darkness or rider). These findings are interpreted as evidence for a hidden decompositional substrate to human language processing and related to a functional architecture derived from non-human primate models.

Figure 1

Ventral and dorsal auditory object processing streams in the macaque brain. The dorsal stream (red) connects caudolateral (CL) and caudomedial (CM) regions in auditory cortex to prefrontal cortex (PFC) either directly or via posterior parietal cortex (PP). The ventral stream (green) connects mediolateral (ML) and anterolateral (AL) regions in auditory cortex to PFC, via parabelt cortex (PB) and areas T2/T3 in the anterior superior temporal gyrus (reprinted with permission from Rauschecker & Tian 2000, copyright 2000 National Academy of Sciences).

Figure 2

Classical Broca–Wernicke neurological diagram of the human language system. Note the absence of a ventral processing stream.

Figure 3

Priming effects (the difference in milliseconds between responses to targets following related versus unrelated prime words) for non-fluent patients and controls in an auditory–auditory semantic priming task for three types of prime word—regularly inflected past tenses (regular past); uninflected stems (stem); and irregularly inflected past tenses (irregular past). For details, see Longworth et al. (2005).

Figure 4

Per cent error rates (for different pairs only) for non-fluent patients in a same–different judgement task, comparing real regular, pseudo-regular and additional phoneme pairs in real word and non-word conditions (data replotted from Tyler et al. 2002a).

Figure 5

Structural correlates of regular inflection. (a) Three-dimensional reconstruction of a T1-weighted MRI image showing brain areas where variations in signal density (for grey and white matter) correlate with priming for regularly inflected words at three significance levels: p<0.001 (green); p<0.01 (blue); and p<0.05 (red) voxel thresholds. The clusters shown survived correction at p<0.05 cluster level adjusted for the entire brain. The statistical peak (−55, 36, −1) is in the LIFG (BA 47), and the cluster extends superiorly into BA 45. At lower thresholds, the cluster extends from Broca's to Wernicke's areas and includes the arcuate fasciculus. (b) The scatter plot showing the relationship between variations in signal intensity at the most significant voxel, and individual behavioural scores in the regular past tense condition and the non-morphological phonological overlap condition (reprinted with permission from Tyler et al. 2005a, copyright 2005 National Academy of Sciences).

Figure 6

fMRI data: significant activations for the contrast of real regulars minus real irregulars. Significant clusters were found in the right superior temporal gyrus (RSTG), left superior temporal gyrus (LSTG), left anterior cingulate cortex (LACC), and left inferior frontal gyrus (LIFG) (data redrawn from Tyler et al. 2005b).

Figure 7

fMRI data: significant activations for the contrast of regulars (real, non-word) minus additional phoneme (real, non-word). Clusters were found in the RSTG, LSTG and LIFG (data redrawn from Tyler et al. 2005b).

Figure 8

fMRI data: significant activations for the contrast of regular non-words minus additional phoneme non-words. Clusters were found in the LIFG (data redrawn from Tyler et al. 2005b).