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A natural sign language central to many studies of deaf and hearing native signers. It is not structurally identical to English despite frequent bilingual use.

Audiometry measures aspects of hearing under specified stimulus and response conditions. A tone-detection threshold depends on frequency, presentation route and the task. It is a psychophysical measurement rather than a complete description of every auditory ability.

Understanding language presented through hearing. Acoustic and phonological analysis add demands beyond those shared with written or nonverbal semantic tasks.

Auditory feedback is information obtained by hearing the sound produced by one's own action. During vocal control, it can be compared with an intended or learned sound, allowing later production to be adjusted. Hearing another individual's demonstration supplies an example to learn; hearing the consequences of one's own production supplies feedback. The two experiences have different causal roles.

A controlled birdsong experiment manipulated that distinction. Adult Bengalese finches wore custom headphones; a cage microphone fed sound-processing hardware that shifted the pitch of their own songs before playback. The birds changed their produced pitch in the compensatory direction, and pitch returned toward baseline when the perturbation was removed. Stable adult song therefore remained subject to active correction under these conditions.^[ref]

For an original numerical illustration, a playback system that multiplies frequency by $2^{1/12}$ turns a $500$-hertz component into about $529.7$ hertz. To cancel that multiplicative shift exactly, production would need to be about $500/2^{1/12}=471.9$ hertz. These values illustrate the direction of an ideal compensation, not a claim that the birds achieved complete correction.

Altered auditory feedback can thus probe a control mechanism without simply disabling hearing. Timing, pitch and loudness are distinct perturbations, and a response to one manipulation does not establish identical control rules for every species or vocal behavior.

Auditory phonetics investigates how listeners hear and interpret the physical sounds of speech. The input is an acoustic signal, but the outcome is a perceptual response: identifying a sound, discriminating two recordings or judging their similarity. It connects acoustic phonetics to hearing and to learned phonetic categories.

A simple controlled listening experiment presents pairs of recorded sounds in a randomized order and asks whether they differ. A separate identification task asks which category each recording belongs to. The tasks answer different questions: a listener can detect a physical difference without assigning the recordings to different phonemes. Praat supports forced-choice listening tasks, including same–different and AXB discrimination designs.^[ref]

The sound history presented to a listener can also matter. In Jessica Maye's infant study with collaborators, different exposure distributions along a speech-sound continuum produced different later discrimination patterns. The result concerns the tested exposure and task, rather than a complete account of how a language's sound system develops.^[ref]

A disproportionate difficulty recognizing spoken language despite better access through other routes. Hearing, phonological discrimination, reading and nonverbal semantic performance must be compared before attributing failure to a speech-input mechanism.

A category of severe auditory impairment or absent hearing. Onset, residual hearing, language experience and task demands matter in interpreting compensatory performance.

The ear is the sensory apparatus associated with hearing and, in its inner-ear components, balance. The outer and middle ear transmit sound mechanically toward the inner ear, where specialized sensory biological cells transduce movement into neural signals. Sound transmission and vestibular sensing are related anatomically but are different functions.

The eastern phoebe, Sayornis phoebe, is a North American flycatcher belonging to Tyranni, the suboscine branch of Passeriformes. It often watches for flying insects from a low perch, then makes a short flight to capture prey. Sheltered ledges, including those on bridges and buildings, provide nesting sites. Feeding behavior and nest placement help identify the living species alongside its appearance and voice.^[ref]

Its vocal development provides a controlled test of learning. In a 1991 study, four nestlings collected at 10–12 days were hand-reared and deafened at about 35 days, before they began singing. Their later songs were judged normal in comparison with wild males and intact laboratory birds. The tested birds could therefore develop the species-typical song without auditory feedback from their own voices.^[ref]

The result is narrower than saying that the brain or experience has no role in singing. It tests whether intact hearing during this developmental interval is necessary for the measured song pattern. Comparing it with a songbird experiment in which removing auditory experience disrupts song makes the experimental question clear: similar adult behavior can arise through different developmental requirements.

A rule converting image properties into acoustic features. One column-scanning example assigns vertical row to $f_i$, horizontal column to scan time, and brightness to amplitude $A(I_{ij})$, yielding $s_j(t)={\sum }_{i}A(I_{ij})\sin (2\pi f_{i}t+{\varphi }_i)$. Distinct orthogonal tones can preserve column coefficients in ideal conditions, while finite windows, masking and limited hearing resolution constrain perception.

A researcher who compared auditory verbal hallucinations in clinical and nonclinical groups. Detailed assessment of the experiences tests which features overlap and which relate to clinical difficulties. The work illustrates why hearing a voice, considered in isolation, does not specify a person’s level of distress, functioning or need for care.^[ref]

Calling Wernicke's aphasia receptive identifies a prominent clinical difficulty, but is inadequate as a complete description or mechanism. Auditory comprehension is typically impaired, yet fluent output can contain errors in sound structure and meaning, neologisms and poor monitoring, while repetition is also impaired. Fluency describes the ease and quantity of speech, not its informational accuracy. The syndrome is therefore neither an isolated loss of receiving words nor a clean counterpart to an exclusively expressive disorder elsewhere.

The distinction between a syndrome and a computation is crucial. A comprehension failure could arise because the listener cannot analyze the sound pattern, cannot access relevant knowledge, cannot select the contextually relevant meaning, or cannot maintain the sentence and response alternatives. These processes can interact. A brain lesion label such as Wernicke's area, named after Carl Wernicke, does not specify which transformation failed, and clinical classifications group patients whose exact brain lesion extent and remaining abilities can differ.

Holly Robson and colleagues directly compared Wernicke-type patients with groups showing semantic dementia and semantic aphasia. Their case-series results supported combined acoustic and phonological analysis deficits together with semantic control impairment in Wernicke's aphasia. The Wernicke group had difficulties on nonverbal as well as verbal comprehension tests, while additional acoustic and phonological difficulties helped explain their particularly poor auditory performance. This is more informative than equating all poor comprehension with destruction of a single semantic store.^[ref]

The experiment motivates a layered account. Spoken input requires analysis of rapidly changing acoustic structure and its relation to phonological representations. Meaning then depends on stored knowledge and control over its use. A disproportionately severe spoken-word deficit can therefore coexist with semantic difficulty that is also detectable in pictures. Conversely, comparatively better picture performance does not establish entirely intact conceptual knowledge: task difficulty, cue availability and the required decisions must be matched before drawing that conclusion.

Speech production is affected for related reasons. Selecting a word requires access to conceptual knowledge and lexical access, and producing its sound pattern requires organization of phonological representations. Fluent but inaccurate output, including paraphasia, cannot simply be classified as preserved expression. Repetition also requires auditory analysis, temporary retention and a mapping to output; it does not supply an independent test of a single receptive faculty. The mechanisms impairing understanding can consequently affect speech without producing the effortful, reduced output associated with some other aphasic presentations.

Evidence from semantic disorders further separates damaged knowledge from difficulty controlling access. Elizabeth Jefferies and colleagues compared comprehension-impaired stroke patients with semantic dementia patients at broadly matched overall impairment. The dementia group showed stronger cross-task/item consistency and familiarity effects; stroke patients were more sensitive to task demands and benefited from cues in ways consistent with impaired semantic control. These were not interchangeable forms of “receptive aphasia.” Nor were all stroke patients in that study necessarily Wernicke cases, so the result provides a comparison framework rather than a license to assign one mechanism to every Wernicke patient.^[ref]

Anatomical evidence also resists a one-area account. Nina F. Dronkers and colleagues studied sentence comprehension in 64 chronic left-hemisphere stroke patients, with comparison groups, and related performance to brain lesions. The task required choosing the drawing matching an auditory sentence. Multiple temporal, parietal and frontal regions were implicated rather than a single posterior temporal center explaining all comprehension. Because the task also required sentence retention, visual selection and interpretation, its brain lesion associations should not be read as a pure map of word meaning.^[ref]

The distinction from auditory verbal agnosia is useful. A relatively input-specific spoken-language impairment can leave reading and other access to knowledge much better preserved. By contrast, multimodal semantic impairment cannot be explained solely by failure to hear phonological distinctions. A careful assessment should therefore compare auditory discrimination, spoken and written language comprehension, nonverbal semantic matching, naming and repetition, while checking hearing, vision and general task demands. Anomia alone is not proof that the object or concept is unknown, and a naming failure is not equivalent to failed comprehension.

There are limits to both behavioral and anatomical inference. Stroke affects vascularly linked tissue and connections, making neighboring regions difficult to separate statistically. Chronic patients may have reorganized, while case-series samples can omit people unable to complete the tests. An association between brain lesion location and test failure supports a network contribution under those conditions, not a complete computational explanation. Likewise, introducing “semantic control” is useful only if it predicts sensitivity to cues, competition and task demands rather than merely renaming every inconsistent answer.

The receptive label is defensible as shorthand for a dominant presenting impairment, but false if it implies preserved production or a unitary input-only brain lesion. The stronger explanation distinguishes acoustic analysis, phonological analysis, representations and control within a connected language system. This preserves what the clinical syndrome tells us while using patient comparisons to explain why comprehension, repetition and fluent speech can fail together in different proportions.

Practice contributes substantially to compensatory performance after sensory loss, but evidence for selective crossmodal recruitment and causal changes in neural contribution goes beyond an account based solely on accumulated task repetitions. Practice and plasticity are not mutually exclusive: practice can induce neural change, while deprivation can change which circuits practice recruits.

The first requirement is to specify the claimed enhancement. Better tactile discrimination, peripheral visual localization and visual motion detection are different outcomes. Accuracy can improve because of sensory precision, attention, memory, a decision criterion or a slower response. There is no inference from success on one selected task to a universal sharpening of the remaining senses. Sensory loss also differs in onset, completeness, cause and duration. Early blindness, late blindness and temporary blindfolding cannot be treated as equivalent experiments.

A practice account predicts learning with repeated use, specificity to trained inputs and possible transfer to related tasks. A deprivation account predicts changes associated with absent input even when practice is matched. Both can operate, and a statistical difference between groups cannot by itself separate them: daily experience, motivation and selection into a study can differ along with sensory status. Years of Braille use, for example, are not randomly assigned. Comparing blind and sighted experts is more informative than comparing one expert group with novices, but expertise matching does not remove every difference in lifetime experience.

A controlled human study by Lotfi B. Merabet and colleagues used five days of complete blindfolding in normally sighted adults during intensive tactile training, with non-blindfolded training controls. Tactile stimulation increasingly recruited occipital cortex in the deprived group. Disrupting occipital function with repetitive transcranial magnetic stimulation impaired Braille-character discrimination in the blindfolded group on day five, but not in controls; the differential disruption was absent 24 hours after blindfold removal. The imaging response also reversed rapidly.^[ref] The imaging and stimulation arms used different participants, so one should not claim a within-person correlation between their measures.

This combination is more informative than cortical activation alone. Both groups received training, yet the deprived group temporarily depended on a different cortical contribution. Rapid reversibility is compatible with altered weighting or unmasking of existing connections; it does not require wholesale growth of new long-range pathways in five days. The result establishes a functional contribution under the tested state, not that occipital recruitment is sufficient for every tactile gain. Stimulation can affect a network, and the study cannot directly equate sudden reversible deprivation with lifelong blindness. Nor is the persistence of learned skill after the imaging effect disappears paradoxical: skill can remain while the supporting network changes.

A complementary causal result comes from Stephen G. Lomber and colleagues' experiments in congenitally deaf cats. Compared with hearing cats, these animals showed enhanced peripheral visual localization and motion detection on particular tasks. Reversible cooling of posterior auditory cortex removed the localization advantage, whereas cooling a dorsal auditory region removed the motion advantage.^[ref] Cooling provides an experimental contrast between a functioning and temporarily deactivated circuit. The task-specific double dissociation argues for functionally differentiated recruitment, rather than one undifferentiated increase in effort. The evidence is from cats and does not quantify the contribution of practice versus deprivation in humans.

A double dissociation is powerful but not magical. To interpret it, one must check that deactivation does not merely impair movement, alertness or basic visibility, and that tasks are not placed at incomparable difficulty levels. The existence of a recruited circuit identifies part of the implementation, not the entire developmental cause. Extensive experience could help establish that circuit's new role. Thus the strict opposition “neural reorganization or practice” confuses a mechanism with one possible source of the mechanism.

A discriminating program would measure performance before training, randomly assign normally sighted participants to matched training with or without temporary deprivation where appropriate, and follow learning and recovery. For established sensory loss, longitudinal observations and carefully matched comparison groups can constrain onset and experience effects without pretending that sensory loss can be freely randomized. Untrained tasks, different body sites or stimulus dimensions, delayed tests and speed–accuracy measurements distinguish broad transfer from repeated response memorization. Peripheral sensitivity should be checked before attributing every difference to cortex. These are design recommendations, not invented completed studies.

Measurement also needs a mechanistic bridge. Fitting psychometric functions can separate threshold and lapse changes; signal detection theory can distinguish sensitivity from a shifted decision criterion under explicit assumptions. Imaging identifies candidate recruitment, while causal disruption tests contribution. Neither activation magnitude nor a lower threshold alone establishes a “better” brain, and an adaptation useful for one task may create costs elsewhere. Residual input, age of onset and the sensory dimension previously calibrated by the missing modality can all constrain outcomes. Enhancement and impairment can coexist in the same person.

The evidence therefore supports an interaction among altered input, experience and circuit availability. Some compensation is task practice, some reflects state-dependent or developmental changes in neural recruitment, and the strongest explanation specifies how those factors cooperate rather than assigning every gain to only one.

Basal ganglia abnormalities plausibly contribute to some developmental speech and language disorders, especially through motor sequencing and learning, but the evidence does not establish a single basal-ganglia cause across diagnoses. A causal account must specify the disorder, the affected computation and how an alteration in a circuit produces that impairment. Childhood apraxia of speech, developmental language disorder and stuttering are different phenotypes; difficulty with speech output does not by itself identify a grammatical deficit.

The basal ganglia participate in loops with cerebral cortex and thalamus. Such circuits could support selecting, initiating and learning sequences, including the rapid articulatory transitions needed for speech. A plausible mechanism is impaired acquisition or reliable initiation of a learned motor sequence, rather than storage of every word or syntactic rule in the striatum. This gives testable predictions about skill learning, nonword repetition and sensitivity to external timing cues. General plausibility, however, does not establish which circuit is responsible in an individual child.

The KE family supplied a particularly informative genetic case. Cecilia S. L. Lai and colleagues identified an FOXP2 alteration segregating with severe speech and language impairment, together with an independent case in which a chromosome translocation disrupted the same gene. The phenotype included major problems in coordinated orofacial sequencing alongside broader language difficulties. This strengthens a causal connection between the gene and development of the phenotype. It does not isolate the anatomical route: a transcription factor can alter several developing systems, and the family is not a representative sample of every developmental language disorder.^[ref]

Kate Watkins's MRI study compared affected family members, unaffected relatives and controls. Voxel-based morphometry identified abnormalities in multiple motor- and speech-related regions; targeted volumetry confirmed reduced caudate volume bilaterally. Relationships with oral praxis and nonword repetition made the caudate result functionally relevant. The unaffected relatives provide a valuable comparison, but an anatomical correlation cannot establish that caudate reduction alone mediates the gene's effects. Multiple abnormalities and correlated behavioural measures make that inference underdetermined.^[ref]

Animal experiments move beyond an association between a human scan and a symptom. Matthias Groszer and colleagues studied mice with a corresponding alteration of the FOXP2 orthologue. Heterozygotes had relatively preserved baseline motor abilities but impaired motor-skill learning, with abnormal plasticity in striatal and cerebellar circuits. The distinction between baseline performance and improvement with practice supports a learning-related mechanism. Yet a mouse motor task is not human syntax or learned speech, and the cerebellar findings themselves argue against an exclusively basal-ganglia interpretation.^[ref]

Stuttering offers a partially independent line of evidence. Soo-Eun Chang's study combined resting-state fMRI and probabilistic tractography in children aged 3–9. Differences involved basal-ganglia–thalamocortical and auditory–motor networks, including weaker connectivity between putamen and supplementary motor regions. Studying young children reduces the extent to which decades of compensatory speaking strategies can explain the result. Nevertheless, the measurements were taken after symptoms began. Correlated activity at rest is not direct synaptic coupling, and tractography does not demonstrate the direction or necessity of a physiological connection.^[ref]

Improved fluency with an external pace would be compatible with compensation for impaired internal timing, but it would not uniquely identify a basal-ganglia abnormality. External cues also change attention, anticipation, auditory feedback and task structure. Similarly, a group difference in a procedural-learning task can result from poor motor execution, working memory or comprehension of instructions. The critical comparison separates learning rate from initial accuracy and uses nonverbal tasks, matched feedback and retained skill after a delay.

Several causal alternatives remain. A shared developmental influence may alter both caudate structure and cortical language function. Reduced language practice may itself change network measurements. A cortical or white-matter abnormality may produce downstream striatal differences. An experimentally supported gene-to-phenotype relationship does not automatically identify every intermediate step in a proposed gene-to-basal-ganglia-to-language chain.

A stronger programme would combine prospective measurements before or close to onset, repeated follow-up of persistence and recovery, detailed speech versus language phenotyping, and independent motor-learning measures. Analyses should control age, hearing, motion, general motor ability and broader development, and assess multiple network regions. Selective perturbations in appropriate models can test mechanisms, while their behavioural scope must remain explicit. The strongest claim is that distributed corticostriatal learning circuits are one important contributor in some phenotypes; neither a universal basal-ganglia lesion model nor a language-specific function of FOXP2 follows from the available evidence.

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.

A category of sensory input and processing, such as vision, hearing or gustation. Receptor type, pathway and experienced quality provide related classifications, but should not be assumed to define identical boundaries.

Researcher studying the evolution of hearing. Nummela and collaborators investigated Eocene whales, connecting changes in ear anatomy with the transition to hearing in an aquatic environment.^[ref]