Just as words can rhyme, the signs of a signed language can share structural properties, such as location. Linguistic description at this level is termed phonology. We report that a left-lateralised fronto-parietal network is engaged during phonological similarity judgements made in both English (rhyme) and British Sign Language (BSL; location). Since these languages operate in different modalities, these data suggest that the neural network supporting phonological processing is, to some extent, supramodal. Activation within this network was however modulated by language (BSL/English), hearing status (deaf/hearing), and age of BSL acquisition (native/non-native). The influence of language and hearing status suggests an important role for the posterior portion of the left inferior frontal gyrus in speech-based phonological processing in deaf people. This, we suggest, is due to increased reliance on the articulatory component of speech when the auditory component is absent. With regard to age of first language acquisition, non-native signers activated the left inferior frontal gyrus more than native signers during the BSL task, and also during the task performed in English, which both groups acquired late. This is the first neuroimaging demonstration that age of first language acquisition has implications not only for the neural systems supporting the first language, but also for networks supporting languages learned subsequently.

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This fMRI study explored the functional neural organisation of seen speech in congenitally deaf native signers and hearing non-signers. Both groups showed extensive activation in perisylvian regions for speechreading words compared to viewing the model at rest. In contrast to earlier findings, activation in left middle and posterior portions of superior temporal cortex, including regions within the lateral sulcus and the superior and middle temporal gyri, was greater for deaf than hearing participants. This activation pattern survived covarying for speechreading skill, which was better in deaf than hearing participants. Furthermore, correlational analysis showed that regions of activation related to speechreading skill varied with the hearing status of the observers. Deaf participants showed a positive correlation between speechreading skill and activation in the middle/posterior superior temporal cortex. In hearing participants, however, more posterior and inferior temporal activation (including fusiform and lingual gyri) was positively correlated with speechreading skill. Together, these findings indicate that activation in the left superior temporal regions for silent speechreading can be modulated by both hearing status and speechreading skill.

Hearing developmental dyslexics and profoundly deaf individuals both have difficulties processing the internal structure of words (phonological processing) and learning to read. In hearing non-impaired readers, the development of phonological representations depends on audition. In hearing dyslexics, many argue, auditory processes may be impaired. In congenitally profoundly deaf individuals, auditory speech processing is essentially absent. Two separate literatures have previously reported enhanced activation in the left inferior frontal gyrus in both deaf and dyslexic adults when contrasted with hearing non-dyslexics during reading or phonological tasks. Here, we used a rhyme judgement task to compare adults from these two special populations to a hearing non-dyslexic control group. All groups were matched on non-verbal intelligence quotient, reading age and rhyme performance. Picture stimuli were used since this requires participants to generate their own phonological representations, rather than have them partially provided via text. By testing well-matched groups of participants on the same task, we aimed to establish whether previous literatures reporting differences between individuals with and without phonological processing difficulties have identified the same regions of differential activation in these two distinct populations. The data indicate greater activation in the deaf and dyslexic groups than in the hearing non-dyslexic group across a large portion of the left inferior frontal gyrus. This includes the pars triangularis, extending superiorly into the middle frontal gyrus and posteriorly to include the pars opercularis, and the junction with the ventral precentral gyrus. Within the left inferior frontal gyrus, there was variability between the two groups with phonological processing difficulties. The superior posterior tip of the left pars opercularis, extending into the precentral gyrus, was activated to a greater extent by deaf than dyslexic participants, whereas the superior posterior portion of the pars triangularis extending into the ventral pars opercularis, was activated to a greater extent by dyslexic than deaf participants. Whether these regions play differing roles in compensating for poor phonological processing is not clear. However, we argue that our main finding of greater inferior frontal gyrus activation in both groups with phonological processing difficulties in contrast to controls suggests greater reliance on the articulatory component of speech during phonological processing when auditory processes are absent (deaf group) or impaired (dyslexic group). Thus, the brain appears to develop a similar solution to a processing problem that has different antecedents in these two populations.

How are signed languages processed by the brain? This review briefly outlines some basic principles of brain structure and function and the methodological principles and techniques that have been used to investigate this question. We then summarize a number of different studies exploring brain activity associated with sign language processing especially as compared to speech processing. We focus on lateralization: is signed language lateralized to the left hemisphere (LH) of native signers, just as spoken language is lateralized to the LH of native speakers, or could sign processing involve the right hemisphere to a greater extent than speech processing? Experiments that have addressed this question are described, and some problems in obtaining a clear answer are outlined.In order to understand how the brain processes signed language, we need a reasonable road map of the brainits general structure and the probable functions of different regions. The study of brain injury (lesions) has provided a well-established methodology for inferring relations of structure and function in the brain. The consequences of brain lesions depend on which part of the brain is damaged. For example, a person with damage to the front of the left side of the brain might be unable to speak. However, someone else, with damage to the back of the right side of the brain, might be able to produce structured language utterances but may have lost some spatial abilities and be unable to see more than one thing at a time. Patterns like this appear to be highly systematic over many individual patients. One straightforward inference is that, in most people, the front of the left side of the brain is required for producing language, whereas the back of the right half of the brain is needed for visuospatial processing. That is, these two functions are ''localized'' and cannot be readily undertaken by other regions. Since the mid-19 century, the likely relationships between particular brain regions and their functions have been inferred from descriptions of such systematic patterns of individual brain injuries. Such research has provided a basic map of the brain upon which subsequent research has built. A Tour of the BrainAs in all vertebrates, the human brain consists of two near-identical hemispheres, reflected on each other. The gray cortex (cortex 5 crust in Latin) comprises minute and densely packed nerve cells, to a depth of 0.5-1 cm. The corded band that connects the two hemispheres comprises many tightly bunched white fibers. Such fibers can also be seen beneath the cortex and run out to the spinal cord. These are bundles of individual nerve axons that carry information between nerve cells. Long axon fibers are organized in bundles (Latin-fasces/fasciculi), like cables that carry electricity, TV, or telephone information under urban roads. The glossy white protective sheath of myelin insulatesWe are grateful to two anonymous reviewers and Cheryl Capek for their comments on earlier versions of the manuscript. We thank Adam Schembri for sugges...