David W. Gow

LEXICAL AND PRELEXICAL INFLUENCES ON WORD SEGMENTATION: EVIDENCE FROM PRIMING

The authors examined the interaction of acoustic and lexical information in lexical access and segmentation. The cross-modal lexical priming technique was used to determine which word meanings listeners access at the offsets of oronyms (e.g., tulips or two lips) presented in connected speech. In Experiment 1, participants showed priming by the meaning of tulips when presented with two lips. In Experiment 2, priming by the meaning of the 2nd word was found in such sequences (e.g., lips in two lips). Finally, Experiment 3 demonstrated that listeners do not show priming by lips when it is pronounced as part of tulips. The results of these experiments show that listeners sometimes access words other than those intended by speakers and may simultaneously access words associated with several parses of ambiguous sequences. Furthermore, the results suggest that acoustic marking of word onsets places constraints on the success of lexical access. To account for these results, the authors propose a new model of lexical access and segmentation.

Analysis of lesions by MRI in stroke patients with acoustic‐phonetic processing deficits

Article abstract—we tested 10 aphasic stroke patients for the ability to discriminate and identify English phonemes. All patients underwent MRI and had their scans analyzed morphometrically. Patients with impairments in acoustic-phonetic processing tended to have lesions involving the left posterior supramarginal gyrus and the bordering parietal operculum, an observation further supported by regression and correlation analyses. These results are interpreted as evidence that the region including the left posterior supramarginal gyrus and parietal operculum plays a significant role in acoustic-phonetic processing.

Does English Coronal Place Assimilation Create Lexical Ambiguity

The purpose of this study was to determine how potential lexical ambiguity produced by place assimilation is resolved. Four cross-modal form priming experiments using primes in sentential contexts were performed. In the first 2, prime items had underlyingly coronal offsets (e.g., right) with assimilated noncoronal place. The primes were judged to be perceptually ambiguous (between right and ripe) in Experiment 1 and noncoronal (ripe) in Experiment 2 in off-line testing. In Experiment 3, primes were replaced with corresponding underlyingly noncoronal items (ripe). In all 3 experiments, participants showed selective priming for the underlying form of the prime. A 4th form priming experiment using the gated tokens of priming stimuli used in Experiment 2 examined the role of postlexical context on this process. In this experiment, participants showed priming for both underlying and surface forms of the prime.

Feature parsing: feature cue mapping in spoken word recognition.

For listeners to recognize words, they must map temporally distributed phonetic feature cues onto higher order phonological representations. Three experiments are reported that were performed to examine what information listeners extract from assimilated segments (e.g., place-assimilated tokens ofcone that resemblecomb) and how they interpret it. Experiment 1 employed form priming to demonstrate that listeners activate the underlying form of CONE, but not of its neighbor (COMB). Experiment 2 employed phoneme monitoring to show that the same assimilated tokens facilitate the perception of postassimilation context. Together, the results of these two experiments suggest that listeners recover both the underlying place of the modified item and information about the subsequent item from the same modified segment. Experiment 3 replicated Experiment 1, using different postassimilation contexts to demonstrate that context effects do not reflect familiarity with a given assimilation process. The results are discussed in the context of general auditory grouping mechanisms.

Lexical influences on speech perception: a Granger causality analysis of MEG and EEG source estimates.

Behavioral and functional imaging studies have demonstrated that lexical knowledge influences the categorization of perceptually ambiguous speech sounds. However, methodological and inferential constraints have so far been unable to resolve the question of whether this interaction takes the form of direct top-down influences on perceptual processing, or feedforward convergence during a decision process. We examined top-down lexical influences on the categorization of segments in a /s/-/integral/ continuum presented in different lexical contexts to produce a robust Ganong effect. Using integrated MEG/EEG and MRI data we found that, within a network identified by 40 Hz gamma phase locking, activation in the supramarginal gyrus associated with wordform representation influences phonetic processing in the posterior superior temporal gyrus during a period of time associated with lexical processing. This result provides direct evidence that lexical processes influence lower level phonetic perception, and demonstrates the potential value of combining Granger causality analyses and high spatiotemporal resolution multimodal imaging data to explore the functional architecture of cognition.

Induction of long-term plasticity in human swallowing motor cortex following repetitive cortical stimulation

OBJECTIVE The excitability of corticobulbar projections to swallowing musculature undergoes remarkable long-term increases after short periods of pharyngeal stimulation. The aim of this study was to investigate the excitability of swallowing motor cortex following repetitive transcranial magnetic stimulation (rTMS). METHODS Twelve healthy subjects were given 100 rTMS pulses over motor cortex at frequencies of 1, 5 and 10 Hz at an intensity of 80% threshold for corticobulbar activation. The excitability of the corticobulbar projection was assessed before and after rTMS and compared both to sham stimulation and to the corticospinal projection. RESULTS Stimulation at 5 Hz, but not 1 Hz, 10 Hz or sham stimulation increased the excitability of the corticobulbar projection to the pharynx, reaching a peak 60 min after rTMS (Delta increase: 65%, P=0.016). Excitability in the projection from the opposite hemisphere also increased, suggesting the presence of inter-hemispheric interactions, whereas excitability in the projection to thenar muscles was unchanged. CONCLUSIONS Corticobulbar and corticospinal projections may differ in response to rTMS, implying differences in relative thresholds of inhibitory and excitatory elements in hand versus swallowing cortex. SIGNIFICANCE This might be a useful approach in the motor rehabilitation of dysphagic stroke patients who have damage to sensory projections to the swallowing cortex.

Characterising the central mechanisms of sensory modulation in human swallowing motor cortex

OBJECTIVE Pharyngeal stimulation can induce remarkable increases in the excitability of swallowing motor cortex, which is associated with short-term improvements in swallowing behaviour in dysphagic stroke patients. However, the mechanism by which this input induces cortical change remains unclear. Our aims were to explore the stimulus-induced facilitation of the cortico-bulbar projections to swallowing musculature and examine how input from the pharynx interacts with swallowing motor cortex. METHODS In 8 healthy subjects, a transcranial magnetic stimulation (TMS) paired-pulse investigation was performed comprising a single conditioning electrical pharyngeal stimulus (pulse width 0.2 ms, 240 V) followed by cortical TMS at inter-stimulus intervals (ISI) of 10-100 ms. Pharyngeal sensory evoked potentials (PSEP) were also measured over the vertex. In 6 subjects whole-brain magnetoencephalography (MEG) was further acquired following pharyngeal stimulation. RESULTS TMS evoked pharyngeal motor evoked potentials were facilitated by the pharyngeal stimulus at ISI between 50 and 80 ms (Delta mean increase: 47+/-6%, P < 0.05). This correlated with the peak latency of the P1 component of the PSEP (mean 79.6+/-8.5 ms). MEG confirmed that the equivalent P1 peak activities were localised to caudolateral sensory and motor cortices (BA 4, 1, 2). CONCLUSIONS Facilitation of the cortico-bulbar pathway to pharyngeal stimulation relates to coincident afferent input to sensorimotor cortex. SIGNIFICANCE These findings have mechanistic importance on how pharyngeal stimulation may increase motor excitability and provide guidance on temporal windows for future manipulations of swallowing motor cortex.

Novel POLG1 mutations associated with neuromuscular and liver phenotypes in adults and children.

Background: The POLG1 gene encodes the catalytic subunit of DNA polymerase gamma, essential for mitochondrial DNA replication and repair. Mutations in POLG1 have been linked to a spectrum of clinical phenotypes, and may account for up to 25% of all adult presentations of mitochondrial disease. Methods and results: We present 14 patients, with characteristic features of mitochondrial disease including progressive external ophthalmoplegia (PEO) and Alpers–Huttenlocher syndrome and laboratory findings indicative of mitochondrial dysfunction, including cytochrome c oxidase (COX) deficiency and multiple deletions or depletion of the mitochondrial DNA. Four novel POLG1 missense substitutions (p.R597W, p.L605R, p.G746S, p.A862T), are described, together with the first adult patient with a recently described polymerase domain mutation (p.R1047W). All novel changes were rare in a control population and affected highly conserved amino acids. Conclusion: The addition of these substitutions—including the first report of a dinucleotide mutation (c.1814_1815TT>GC)—to the growing list of defects further confirms the importance of POLG1 mutations as the underlying abnormality in a range of neurological presentations.

Mapping metabolic brain activation during human volitional swallowing: a positron emission tomography study using [18F]fluorodeoxyglucose.

We have previously shown that labelled water positron emission tomography (H215O PET) can be used to identify regional cerebral blood flow (rCBF) changes in the human brain during volitional swallowing. (18F) fluorodeoxyglucose (FDG PET), by comparison, uses a glucose analogue to quantitatively measure regional cerebral glucose metabolism (rCMRglc) rather than rCBF. The main advantage of FDG PET is improved spatial resolution, and because of its pharmacodynamic properties, activation can be performed external to the scanner, allowing subjects to assume more physiologic positions. We therefore conducted a study of the brains metabolic response while swallowing in the erect seated position, using FDG PET. Eight healthy male volunteers were studied with a randomised 2 scan paradigm of rest or water swallowing at 20-second intervals for 30 minutes. Data were analysed with SPM99 using multisubject conditions and covariates design. During swallowing, analysis identified increased rCMRglc (P<0.01) in the following areas: left sensorimotor cortex, cerebellum, thalamus, precuneus, anterior insula, left and right lateral postcentral gyrus, and left and right occipital cortex. Decreased rCMRglc were also seen in the right premotor cortex, right and left sensory and motor association cortices, left posterior insula and left cerebellum. Thus, FDG PET can be applied to measure the brain metabolic activity associated with volitional swallowing and has the advantage of normal task engagement. This has implications for future activation studies in patients, especially those suffering swallowing problems after brain injury.

Parallel versus serial processing dependencies in the perisylvian speech network: A Granger analysis of intracranial EEG data

In this work, we apply Granger causality analysis to high spatiotemporal resolution intracranial EEG (iEEG) data to examine how different components of the left perisylvian language network interact during spoken language perception. The specific focus is on the characterization of serial versus parallel processing dependencies in the dominant hemisphere dorsal and ventral speech processing streams. Analysis of iEEG data from a large, 64-electrode grid implanted over the left perisylvian region in a single right-handed patient showed a consistent pattern of direct posterior superior temporal gyrus influence over sites distributed over the entire ventral pathway for words, non-words, and phonetically ambiguous items that could be interpreted either as words or non-words. For the phonetically ambiguous items, this pattern was overlayed by additional dependencies involving the inferior frontal gyrus, which influenced activation measured at electrodes located in both ventral and dorsal stream speech structures. Implications of these results for understanding the functional architecture of spoken language processing and interpreting the role of the posterior superior temporal gyrus in speech perception are discussed.