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Phonology and syntax usually show a left-hemisphere bias, whereas semantic processing depends on a more bilateral system with task-dependent asymmetries. None is confined absolutely to one cerebral hemisphere, and lateralisation of an experimental contrast is not equivalent to localisation of a representation. Phonology concerns structured sublexical units, semantics concerns meaning, and syntax concerns relations that organise words into sentences. These processes interact, so tasks must isolate the relevant contrast rather than merely require language.

Historical lesion observations motivated left language dominance, but their anatomical interpretation needs care. Modern MRI of Paul Broca's preserved cases showed damage extending beyond the conventional cortical region named after him. Severe impaired speech after a left lesion therefore supports lateralisation without proving that a small superficial patch alone constitutes the speech system. Large vascular lesions can also damage connections between surviving regions.^[ref]

For phonological processing, Robert J. Zatorre and colleagues used PET while participants judged different attributes of the same speech material. Phonetic discrimination increased left inferior-frontal activity, whereas pitch discrimination recruited right prefrontal regions; acoustically matched speech engaged secondary auditory cortices bilaterally. Holding the signal relatively constant while changing the decision supports functional differentiation beyond a contrast between unrelated stimuli. It does not show that all phonological representations reside in inferior frontal cortex: articulatory recoding, attention and decision demands can contribute to the phonetic contrast.^[ref]

The distinction matters because speech perception begins with bilateral auditory analysis, whereas selecting, sequencing or rehearsing phonological forms can have a stronger left bias. Impairment of repetition after damage to a dorsal language pathway and comprehension after other damage constrain different transformations. A deficit in producing a phoneme is not automatically a deficit in recognising it. Familiarity with a sound pattern and its linguistic role can also alter the task, so acoustic speed or frequency alone cannot define phonology.

Semantic processing illustrates why a leftward verbal-task result can conceal bilateral representational resources. In Gorana Pobric's rTMS experiment, perturbing either temporal pole slowed semantic associations for words and pictures while a perceptual comparison was spared; stimulation of a control site did not produce the same semantic effect. This supports a contribution from both anterior temporal lobes across input formats. It does not require identical response profiles throughout both lobes or imply that semantic retrieval and selection are entirely bilateral.^[ref]

Verbal naming places additional demands on lexical retrieval and speech production. Consequently, stronger left activity in naming than in passive picture viewing can reflect access and output as well as conceptual knowledge. Conversely, right-sided involvement in nonverbal meaning does not establish a separate nonlinguistic semantic store. Comparisons should separate semantic association from perceptual similarity and control both response requirements and difficulty. Lesion and perturbation results help adjudicate whether an activated region contributes to the task.

For syntax, lesion studies and functional imaging converge on a predominantly left frontotemporal network, rather than a single grammar centre. Evelina Fedorenko's individual-localisation study showed language-selective and domain-general regions next to one another within the anatomical territory called Broca's area. Averaging across people can merge them. Thus, an inferior-frontal response to a hard sentence may reflect language processing, domain-general control or both; independent functional localizers and nonlinguistic controls improve interpretation.^[ref]

Corianne Rogalsky's lesion study compared sentence-picture matching and plausibility judgments in 66 patients. The distribution of syntactic-comprehension impairment implicated a wider left-hemisphere system and challenged a simple equation between Broca's area damage and agrammatic comprehension. Convergence across tasks is useful because picture matching also demands visual analysis and response selection, while plausibility can recruit semantic expectations. Neither behavioural label makes the task syntactically pure.^[ref]

Sentence comprehension in Japanese and Japanese Sign Language also showed left-dominant frontal and temporoparietal responses across sensory conditions. This directly addresses hemispheric comparisons, although comprehension versus nonword detection is not a pure assay of syntax.^[ref]

A quantitative laterality index can summarise an explicitly defined response. For nonnegative left and right measurements $L,R$ with $L+R>0$,It ranges from $-1$ to $1$, but depends on baseline, threshold and region selection. Responses $(20,10)$ and $(10,5)$ have the same index $1/3$ despite different total responses. If left activity falls from $20$ to $10$ while right activity remains $10$, the index becomes zero without any right recruitment. These are illustrative values, not imaging observations. Negative contrasts need a separately justified convention.

The most convincing evidence therefore combines within-person process contrasts, unilateral perturbations or lesions, and converging measurements of behaviour and neural responses. It also describes individual variation instead of assuming that every participant has the population-average pattern. The justified conclusion is graded functional asymmetry within interacting networks: strongest left biases for many phonological and syntactic operations, bilateral conceptual contributions, and substantial dependence on task and representation level.

Sign languages recruit much of the left-dominant frontotemporal language network while also engaging modality-dependent visual, parietal and motor systems. The evidence supports shared organisation across modalities more strongly than it proves that the same neural populations contain fully amodal phonological, semantic and syntactic codes. Natural sign languages have conventional sublexical structure and grammar; they are not simply gestures illustrating spoken words.

Brain-lesion evidence provides an important starting point. In Gregory Hickok's study, 19 lifelong signers with unilateral lesions completed single-sign and simple/complex sentence comprehension tasks. Left temporal involvement was associated with substantial impairment across tasks; right-lesion and other groups performed well on the simpler conditions, with more difficulty on complex material. The result argues against assigning language to the right hemisphere merely because signs are visual and spatial. Complex-command performance can also reflect memory, spatial processing or general comprehension demands.^[ref]

Functional imaging identifies both common and distinct components. Karen Emmorey and colleagues compared sign and speech within native hearing ASL–English bilinguals using PET for picture naming and fMRI for sentence comprehension. Shared language-related responses coexisted with greater parietal recruitment for signing and different temporal responses for speech. The within-person comparison reduces confounding between hearing status and language modality. It also makes clear that motor output, visual movement and auditory feedback contribute to the total activation pattern.^[ref]

An amodal representation is stronger than a common anatomical location: it should capture relevant information independently of how that information enters or leaves the system. A shared inferior-frontal mean response can arise from common control demands, different intermingled populations or the same computation over differently formatted representations. This is the distinction expressed by regional activation does not identify a neural code. Conjunction of significant maps is useful evidence for shared territory, but cannot by itself establish identity of the represented information.

For phonology, the relevant units need not be sounds. A sign can be distinguished by handshape, location and movement, just as speech forms can differ in sublexical features. Mairéad MacSweeney's study compared BSL location judgments with English rhyme judgments using picture-based tasks. A partly shared left frontoparietal network was modulated by language, hearing status and age of first acquisition. Thus sublexical form analysis can share network resources across modalities, while the differences argue against an entirely invariant implementation. Delayed access to a first language must be separated from deafness itself.^[ref]

A stronger phonological test would match response selection and difficulty, contrast legal with carefully controlled sublexical combinations, and include nonlinguistic hand-movement judgments. Even successful cross-modal decoding would need interpretation: sign handshape and spoken sound have no automatic one-to-one physical mapping. An abstract combinatorial relation could be shared without every modality-specific feature having an amodal counterpart.

For semantics, signs and spoken words can access the same conceptual relations while having different forms. Emmorey's within-bilingual comprehension experiment used detection of semantic anomalies and identified overlapping frontotemporal responses with modality-dependent components. This supports common meaning-related processing, although anomaly detection also requires monitoring and a decision. A more specific test would train a decoder on one modality and test concept identity in the other, holding out stimuli and separating meaning from the picture, response or decision used to measure it.^[ref]

For syntax, Kuniyoshi L. Sakai's comparison of Japanese Sign Language and Japanese sentences found common left-dominant frontal and temporoparietal recruitment for comprehension. Comparing sentence comprehension with sentential nonword detection improved specificity beyond passive sign viewing. Different participant groups and potentially different demands nevertheless prevent inferring identical neuronal syntax codes. Spatial referent tracking, prosody and grammatical relations may draw on partly overlapping resources rather than a single modality-free operation.^[ref]

The best design combines native Deaf signers, hearing native signers and hearing nonsigners where appropriate, with careful measurement of first-language access and proficiency. Nonlinguistic biological motion, matched visual complexity, meaningful versus meaningless sequences and matched response demands help identify what each contrast removes. Individual functional localisation, lesion convergence and cross-modal multivariate tests are stronger together than any activation map alone. A failed decoder does not establish separate representations if measurement noise or anatomical alignment prevents generalisation.