Oral-movement abilities and articulatory characteristics of brain-injured adults

The superior precentral gyrus of the insula does not appear to be functionally specialized for articulation

Broca (Broca P. Bull Soc Anat Paris 36: 330-357, 1861) influentially argued that posterior left inferior frontal gyrus supports speech articulation. According to an alternative proposal (e.g., Dronkers NF. Nature 384: 159-161, 1996; Wise RJ, Greene J, Buchel C, Scott SK. Lancet 353: 1057-1061, 1999; Baldo JV, Wilkins DP, Ogar J, Willock S, Dronkers NF. Cortex 47: 800-807, 2011), a region in the anterior insula [specifically, the superior precentral gyrus of the insula (SPGI)] is the seat of articulatory abilities. Moreover, Dronkers and colleagues have argued that the SPGI is functionally specialized for (complex) speech articulation. Here, we evaluate this claim using individual-subject functional MRI (fMRI) analyses (e.g., Fedorenko E, Hsieh PJ, Nieto-Castanon A, Whitfield-Gabrieli S, Kanwisher N. J Neurophysiol 104: 1177-1194, 2010). We find that the SPGI responds weakly, if at all, during articulation (parts of Broca's area respond 3-4 times more strongly) and does not show a stronger response to higher articulatory demands. This holds regardless of whether the SPGI is defined functionally (by selecting the most articulation-responsive voxels in the vicinity of the SPGI in each subject individually) or anatomically (by using masks drawn on each individual subject's anatomy). Critically, nonspeech oral movements activate the SPGI more strongly than articulation, especially under the anatomical definition of the SPGI. In line with Hillis et al. (Hillis AE, Work M, Barker PB, Jacobs MA, Breese EL, Maurer K. Brain 127: 1479-1487, 2004; also Trupe L, Varma DD, Gomez Y, Race D, Leigh R, Hillis AE, Gottesman RF. Stroke 44: 740-744, 2013), we argue that previous links between the SPGI, and perhaps anterior insula more generally, and articulation may be due to its high base rate of ischemic damage (and activation in fMRI; Yarkoni T, Poldrack RA, Nichols TE, Van Essen DC, Wager TD. Nat Methods 8: 665-670, 2011), combined with its proximity to regions that more directly support speech articulation, such as the precentral gyrus or the posterior aspects of the inferior frontal gyrus (Richardson JD, Fillmore P, Rorden C, Lapointe LL, Fridriksson J. Brain Lang 123: 125-130, 2012), and thus susceptibility to joint damage.

Nonverbal oral apraxia in primary progressive aphasia and apraxia of speech

OBJECTIVE

The goal of this study was to explore the prevalence of nonverbal oral apraxia (NVOA), its association with other forms of apraxia, and associated imaging findings in patients with primary progressive aphasia (PPA) and progressive apraxia of speech (PAOS).

METHODS

Patients with a degenerative speech or language disorder were prospectively recruited and diagnosed with a subtype of PPA or with PAOS. All patients had comprehensive speech and language examinations. Voxel-based morphometry was performed to determine whether atrophy of a specific region correlated with the presence of NVOA.

RESULTS

Eighty-nine patients were identified, of which 34 had PAOS, 9 had agrammatic PPA, 41 had logopenic aphasia, and 5 had semantic dementia. NVOA was very common among patients with PAOS but was found in patients with PPA as well. Several patients exhibited only one of NVOA or apraxia of speech. Among patients with apraxia of speech, the severity of the apraxia of speech was predictive of NVOA, whereas ideomotor apraxia severity was predictive of the presence of NVOA in those without apraxia of speech. Bilateral atrophy of the prefrontal cortex anterior to the premotor area and supplementary motor area was associated with NVOA.

CONCLUSIONS

Apraxia of speech, NVOA, and ideomotor apraxia are at least partially separable disorders. The association of NVOA and apraxia of speech likely results from the proximity of the area reported here and the premotor area, which has been implicated in apraxia of speech. The association of ideomotor apraxia and NVOA among patients without apraxia of speech could represent disruption of modules shared by nonverbal oral movements and limb movements.

Speech versus nonspeech: different tasks, different neural organization

This article reviews the extant studies of the relation of oromotor nonspeech activities to speech production. The relevancy of nonspeech oral motor behaviors to speech motor performance in assessment and treatment is challenged on several grounds. First, contemporary motor theory suggests that movement control is task specific. In other words, it is tied to the unique goals, sources of information, and characteristics of varying motor acts. Documented differences in movement characteristics for speech production versus nonspeech oral motor tasks support this claim. Second, advantages of training nonspeech oral motor tasks versus training speech production are not supported by current principles of motor learning and neural plasticity. Empirical data supports experience-specific training. Finally, functional imaging studies document differences in activation patterns for speech compared with nonspeech oral motor tasks in neurologically healthy individuals.

Philosophy of research in motor speech disorders

The primary objective of this position paper is to assess the theoretical and empirical support that exists for the Mayo Clinic view of motor speech disorders in general, and for oromotor, nonverbal tasks as a window to speech production processes in particular. Literature both in support of and against the Mayo clinic view and the associated use of oromotor, nonverbal tasks, is reviewed, along with literature from normal speech production and neurophysiology. This literature is organized and analysed to show that theoretical and experimental support for the Mayo view is weak at best, and that the frequent appeal to oromotor, nonverbal tasks is misguided. We conclude that studies of speech production in motor speech disorders, rather than oromotor nonverbal control or of neurological signs, is the correct approach for advancing the field.

Isolated and sequenced oral motor posture production under different input modalities by left-hemisphere damaged adults

The purpose of this investigation was to use a well-refined system of oral movement notation to examine whether the quality of performances for production of elemental oral motor postures as isolated targets was similar to performances of the same postures produced as members of a sequence by left-hemisphere damaged (LHD) and normal adults. The effect of input modality was also examined using three conditions--imitation, photograph and command. Result suggested that "additional movements" and "errors of spatial alignment" prominently characterized both isolated and sequenced postural production errors made by both LHD and normal adults. The frequency of occurrence of these behaviours, however, was dramatically higher among the LHD subjects. Moreover, the quality of productions of oral postures deteriorated in sequences of increasing length among the LHD subjects but not among normal adults. No differential effect for mode of input was found for the LHD subjects but was for the normal adults. Results suggested that the inferior performances of LHD subjects on isolated gesture production are due to exaggerated augmentations and spatial targeting deficits. Sequence production was affected negatively by the same behaviours as well as a disability for "phasing" subcomponents of discrete movements as reflected by a significant increase in "complex" errors on the individual gestures comprising the sequence.

Phonemic behavior of aphasic subjects without dysarthria or apraxia of speech

Thirty adult aphasic subjects without apraxia of speech or dysarthria were tested for their ability to produce phonemes in single test words and in spontaneous contextual speech. Results indicated that 75% of the total phonemic errors were due to a whole-word phenomenon apparently associated with faulty processing of the word rather than faulty production of the phoneme. True phonemic errors comprised 25% of the total errors or about 2% of all responses. Phoneme substitutions were by far the most frequent error (61%). Of the 30 subjects, 28 made no phonemic errors in spontaneous contextual speech. Aphasic behavior is not characterized by significant breakdown of articulatory performance. Observed patterns of error do not clearly support a phonemic regression hypothesis.