Kimihiro Nakamura

The left ventral premotor cortex (PMv) has not been fully acknowledged as a key component of the brain’s network for reading. This region, corresponding to Brodmann area (BA) 6, is cytoarchitecturally distinct from adjacent Broca’s area (BA44/45) and primary motor cortex (BA4) and specifically involved in articulatory codes for speech production. The left PMv is known to receive direct white matter projections from posterior brain regions responsible for visual and phonological processing. Consequently, the region plays a significant role in reading when the expert visual word-form system in the left occipitotemporal cortex is underdeveloped or compromised in early literacy development in childhood or in neurologic disorders in adulthood. Recent functional neuroimaging and brain stimulation studies further suggest that those direct neural pathways connecting the left PMv with the occipital cortex are fully functioning in literate adults, rapidly generating speech motor codes at a very early stage of reading, and faster than thought previously. This region may therefore serve as an alternative and promising cortical target for future neuromodulation research aimed at boosting cognitive components involved in reading.

The nonfluent/agrammatic variant of primary progressive aphasia (nfav-PPA) and primary progressive apraxia of speech (PPAOS) are neurodegenerative syndromes that raise diagnostic challenges related to several issues. First, there are two divergent conceptions, one stipulating that (i) nfav-APP and PPAOS are distinct entities, and the other (ii) that PPAOS has to be integrated into the nfav-APP spectrum. A second related issue concerns the consideration of phonological dimensions, lying at the language interface with speech, which could potentially help overcome the nfva-PPA/PPAOS controversy. Third, there is a lack of internationally validated clinical tests assessing apraxia of speech and syntactic abilities with sufficient specificity and sensitivity. This narrative review discusses these issues taking into account clinical, neurocognitive and neurobiological dimensions. It proposes a conceptual-integrative framework conciliating competing nfav-APP/PPAOS accounts while suggesting a graded continuum with subdivisions, related to neurodegenerative expansion throughout language/speech production systems, ranging from syntactic to phonological to phonetic-articulatory impairments. A second controversy in the field of PPA arises from divergent conceptions of semantic PPA (sv-PPA), defined by primary damage to verbal semantics, and of semantic dementia (SD) characterized by multimodal semantic impairments. The current consensus criteria of PPA have deconstructed the initial SD conception by absorbing it into sv-PPA, hence leaving mixed and some non-verbal semantic phenotypes nosologically orphaned. Again, the article proposes a conceptual and integrative model, built on findings from clinical research and cognitive neuroscience, suggesting a graded continuum with subdivisions spanning from verbal to different non-verbal semantic impairments including social-semantic/behavioral phenotypes.

Research indicates that the anterior temporal lobes (ATLs) in the left and right hemispheres constitute a supramodal hub for verbal and nonverbal semantic information. However, it remains unclear whether (1) left and right ATLs are respectively specialized for verbal and nonverbal semantic processing (functional specialization models) or (2) bilateral ATLs equally contribute to multimodal semantic processing (unitary “hub-and-spoke” models). The present study examined the question using repetitive transcranial magnetic stimulation (rTMS) with an implicit semantic priming paradigm minimizing top-down influences from cognitive control mechanisms. After receiving rTMS over the left ATL, right ATL or vertex, healthy human adults of either sex made real-versus-unreal judgments about visual words or objects each preceded by semantically related or unrelated primes. While a similar amount of semantic priming survived vertex stimulation for both words and objects, we found that left ATL stimulation, but not right ATL stimulation, eliminated semantic priming for words, suggesting that the verbal semantics is represented in the left ATL. In contrast, rTMS over the ATL eliminated semantic priming for objects irrespective of the side of stimulation. This latter finding suggests a twofold mechanism whereby semantic processing of visual objects depends on semantic computations in the right ATL as well as automatic activation of semantic word representations in the left ATL generating “verbal label” feedback. The observed functional asymmetry of the semantic ATL hub overall supports functional specialization models enriched by verbal label feedback accounts, unveiling that verbal semantic knowledge plays a guiding role in the semantic analysis of visual objects.

Aims and objectives: Bilinguals recognize second language (L2) words more quickly when their forms are similar to their first language (L1) counterparts, known as cognates, due to cross-language orthographic and phonological similarities. However, it remains unclear how orthography functions independently from phonology because these two representations are difficult to separate in phonetic alphabetic scripts. To address this, we investigated cognates in Mandarin Chinese and Japanese, both of which employ logographic scripts where orthography does not directly correspond to phonology as it does in alphabetic scripts. Methodology: In a masked translation priming experiment, highly proficient Chinese–Japanese bilinguals performed a semantic categorization task on L2 Japanese words with four prime–target types: orthographically identical cognates, nonidentical cognates, noncognates, and unrelated pairs. We manipulated the prime durations (33 and 50 ms) to assess the orthographic components in masked priming. Data and analysis: We analyzed reaction times for semantic categorizations on L2 target words using a linear mixed-effects model. Findings/conclusions: The results showed that reaction times were shorter for L2 targets primed by L1 identical cognates than by noncognates at both prime durations, indicating a distinct processing advantage for identical cognates. For nonidentical cognates, facilitation appeared only with a 33-ms prime duration, suggesting that longer primes activate language-specific orthographic markers. Importantly, the results indicate that these orthographic markers are incorporated in the word-recognition system of bilinguals’ mental lexicon and influence the degree of language activation, particularly in the processing of logographic cognates by Chinese–Japanese bilinguals. Originality: This study is the first to demonstrate that orthographic markers in the mental lexicon of Chinese–Japanese bilinguals modulate the nonselective language activation during cognate processing. Significance: Our findings highlight the critical role of orthography in cognate processing, independent of phonological mediation.

Phonological knowledge plays a pivotal role in many aspects of language processing, but it remains controversial whether it is required for writing. In the present study, we examined the issue by focusing on written production in an opaque logographic script (kanji) with highly irregular pronunciation rules, which allowed for a rigorous test of whether or not phonology contributes to writing. Using a phonological priming paradigm in two experiments, we measured response latency while participants orally named target pictures or wrote down their names in kanji. Each target was preceded by a phonographic character (kana) which represented the same sound (mora) as the beginning of the target name or a different mora. By manipulating the degree of phonological overlap between primes and target names (i.e., morae, consonants and vowels), we found that only the moraic overlap could speed up word production in logographic writing (Experiment 1). In contrast, naming response was facilitated by mora-overlap as well as vowel-overlap. This between-task difference in phonological encoding suggests that phonological codes for spoken production do not necessarily precede orthographic access during logographic writing. In Experiment 2, we further found that the facilitatory effects of moraic information did not differ in magnitude between writing and naming when primes were masked and presented more briefly, suggesting a net component of bottom-up phonological activation which contributes to logographic writing. Collectively, we propose that orthographic codes of kanji are accessed directly from semantics, whereas phonology plays a non-specific modulatory role to enhance neurocognitive systems involved in writing.

Handwriting is thought to impede vocabulary learning in sighted adults because the motor execution of writing interferes with efficient audiovisual processing during encoding. However, the motor memory of writing may facilitate adult word learning when visual sensory inputs are severely restricted. Using functional MRI, we show that late-blind participants, but not sighted participants, learned novel words by recruiting the left dorsal premotor cortex known as Exner’s writing area and its functional coupling with the left hippocampus. During later recall, the phonological and semantic contents of these words are represented in the activation patterns of the left hippocampus as well as in those of left frontotemporal language areas. These findings suggest that motor codes of handwriting help blind participants maintain word-form representations during learning and retrieval. We propose that such reliance on the motor system reflects a broad architecture of the cerebral language network which encompasses the limb motor system as a hardwired component.