Statistical learning in patients in the minimally conscious state

When listening to an unknown language, listeners could learn the transitional probability between syllables and group frequently co-occurred syllables into a whole unit. Such statistical learning ability has been demonstrated for both pre-verbal infants and adults, even during passive listening. Here, we investigated whether statistical learning occurred in patients in minimally conscious state (MCS) and patients emerged from the minimally conscious state (EMCS) using electroencephalography (EEG). We presented to participants an isochronous sequence of syllables, which were composed of either 2-word real phrases or 2-word artificial phrases that were defined by the transitional probability between words. An inter-trial phase coherence (ITPC) analysis revealed that the phrase-rate EEG response was weakened in EMCS patients compared with healthy individuals, and was even more severely weakened in MCS patients. Although weak, the phrase-rate response or its harmonics remained statistically significant in MCS patients, suggesting that the statistical learning ability was preserved in MCS patients. The word-rate response was also weakened with a decreased level of consciousness. The harmonics of the word-rate response, however,were more salient in MCS than EMCS patients in the alpha and beta bands. Together with previous studies, the current results suggest that MCS patients retain residual learning ability, which can potentially be harnessed to induce neural plasticity, and that different frequency bands are differentially related to the consciousness level.

The Influence of Auditory Attention on Rhythmic Speech Tracking: Implications for Studies of Unresponsive Patients

Language comprehension relies on integrating words into progressively more complex structures, like phrases and sentences. This hierarchical structure-building is reflected in rhythmic neural activity across multiple timescales in E/MEG in healthy, awake participants. However, recent studies have shown evidence for this “cortical tracking” of higher-level linguistic structures also in a proportion of unresponsive patients. What does this tell us about these patients’ residual levels of cognition and consciousness? Must the listener direct their attention toward higher level speech structures to exhibit cortical tracking, and would selective attention across levels of the hierarchy influence the expression of these rhythms? We investigated these questions in an EEG study of 72 healthy human volunteers listening to streams of monosyllabic isochronous English words that were either unrelated (scrambled condition) or composed of four-word-sequences building meaningful sentences (sentential condition). Importantly, there were no physical cues between four-word-sentences. Rather, boundaries were marked by syntactic structure and thematic role assignment. Participants were divided into three attention groups: from passive listening (passive group) to attending to individual words (word group) or sentences (sentence group). The passive and word groups were initially naïve to the sentential stimulus structure, while the sentence group was not. We found significant tracking at word- and sentence rate across all three groups, with sentence tracking linked to left middle temporal gyrus and right superior temporal gyrus. Goal-directed attention to words did not enhance word-rate-tracking, suggesting that word tracking here reflects largely automatic mechanisms, as was shown for tracking at the syllable-rate before. Importantly, goal-directed attention to sentences relative to words significantly increased sentence-rate-tracking over left inferior frontal gyrus. This attentional modulation of rhythmic EEG activity at the sentential rate highlights the role of attention in integrating individual words into complex linguistic structures. Nevertheless, given the presence of high-level cortical tracking under conditions of lower attentional effort, our findings underline the suitability of the paradigm in its clinical application in patients after brain injury. The neural dissociation between passive tracking of sentences and directed attention to sentences provides a potential means to further characterise the cognitive state of each unresponsive patient.

From computing transition probabilities to word recognition in sleeping neonates, a two-step neural tale

Extracting statistical regularities from the environment is a primary learning mechanism, which might support language acquisition. While it is known that infants are sensitive to transition probabilities between syllables in continuous speech, the format of the encoded representation remains unknown. Here we used electrophysiology to investigate how 31 full-term neonates process an artificial language build by the random concatenation of four pseudo-words and which information they retain. We used neural entrainment as a marker of the regularities the brain is tracking in the stream during learning. Then, we compared the evoked-related potentials (ERP) to different triplets to further explore the format of the information kept in memory. After only two minutes of familiarization with the artificial language, we observed significant neural entrainment at the word rate over left temporal electrodes compared to a random stream, demonstrating that sleeping neonates automatically and rapidly extracted the word pattern. ERPs significantly differed between triplets starting or not with the correct first syllable in the test phase, but no difference was associated with later violations in transition probabilities, revealing a change in the representation format between segmentation and memory processes. If the transition probabilities were used to segment the stream, the retained representation relied on syllables' ordinal position, but still without a complete representation of the words at this age. Our results revealed a two-step learning strategy, probably involving different brain regions.

Syllables in Sync Form a Link: Neural Phase-locking Reflects Word Knowledge during Language Learning

Language is composed of small building blocks, which combine to form larger meaningful structures. To understand language, we must process, track, and concatenate these building blocks into larger linguistic units as speech unfolds over time. An influential idea is that phase-locking of neural oscillations across different levels of linguistic structure provides a mechanism for this process. Building on this framework, the goal of the current study was to determine whether neural phase-locking occurs more robustly to novel linguistic items that are successfully learned and encoded into memory, compared to items that are not learned. Participants listened to a continuous speech stream composed of repeating nonsense words while their EEG was recorded and then performed a recognition test on the component words. Neural phase-locking to individual words during the learning period strongly predicted the strength of subsequent word knowledge, suggesting that neural phase-locking indexes the subjective perception of specific linguistic items during real-time language learning. These findings support neural oscillatory models of language, demonstrating that words that are successfully perceived as functional units are tracked by oscillatory activity at the matching word rate. In contrast, words that are not learned are processed merely as a sequence of unrelated syllables and thus not tracked by corresponding word-rate oscillations.

Temporal and Episodic Analyses of the Story of Cinderella in Latent Aphasia

Purpose The purpose of this study was to improve our understanding of the language characteristics of people with latent aphasia using measures that examined temporal (i.e., real-time) and episodic organization of discourse production. Method Thirty AphasiaBank participants were included (10 people with latent aphasia, 10 people with anomic aphasia, and 10 neurotypical control participants). Speech material of Cinderella narratives was analyzed with Praat software. We devised a protocol that coded the presence and duration of all speech segments, dysfluencies such as silent and filled pauses, and other speech behaviors. Using these durations, we generated a range of temporal measures such as speech, articulation, and pure word rates. Narratives were also coded into episodes, which provided information about the discourse macrostructure abilities of the participants. Results The latent aphasia group differed from controls in number of words produced, silent pause duration, and speech rate, but not articulation rate or pure word rate. Episodic organization of the narratives was similar in these 2 groups. The latent and anomic aphasia groups were similar in most measures, apart from articulation rate, which was lower in the anomic group. The anomic aphasia group also omitted more episodes than the latent aphasia group. Conclusions The differences between latent aphasia and neurotypical controls can be attributed to a processing speed deficit. We propose that this deficit results in an impaired ability to process information from multiple cognitive domains simultaneously.

Propofol and Thiopental Do Not Interfere with Regional Cerebral Blood Flow Response at Sedative Concentrations

Background Anesthetics may affect the regional cerebral blood flow (rCBF) response associated with increased brain activity in humans. rCBF was measured as auditory stimulus rate was increased during propofol and thiopental administration. Methods After informed consent, 10 right-handed male volunteer participants (aged 33.5 +/- 10.4 yr, weighing 74.5 +/- 8.4 kg) received thiopental (n = 4) or propofol (n = 6) intravenously at stepwise target concentrations of propofol 1.2 and 2.5-3, or thiopental 4 and 7-9 microg/ml, representing sedative and hypnotic drug concentrations. The latter made volunteers unresponsive to voice or mild stimulation. Quantitative positron emission tomographic brain images were obtained at 0, 20, and 40 auditory words per minute at each drug concentration. Using SPM99 analysis, 10-mm spherical regions of interest were identified by peak covariation of word rate with rCBF across all conditions and drug concentrations. Individual mean rCBF responses in these and primary auditory cortex (Heschl's gyri) were obtained. Results Significant increases in rCBF with auditory word rate occurred in temporal lobes bilaterally at baseline (significance, T = 4.95). There was no change in this response during sedation (T = 5.60). During unresponsiveness seven of 10 participants had a diminished response in the left temporal lobe (T = 3.18). Global CBF, corrected for changes in PCO2 (3% .mmHg PCO2), was reduced 15% by sedation and 27% during unresponsiveness. Conclusion The presence of propofol or thiopental does not affect the rCBF response to increasing stimulus rate during consciousness. Thus, changes in rCBF activation patterns with sedative concentrations of these drugs represent effects on brain activity itself. The neuroanatomical targets of drug effect on memory and attention may be revealed by changes in rCBF patterns associated with these cognitive activities.

The Verbal Environment of a Language-Impaired Child

A child with a language handicap presents a potential conflict for the adults with whom he interacts. In terms of age and physical development, he may present cues that ordinarily elicit complex speech from adults. His language performance, however, may be comparable to that of a much younger child. This study involves a detailed analysis of the speech addressed to a 4.7-year-old language-impaired boy and to his normal-speaking female cousin (4.6 years) by the boy’s mother, father, and baby-sitter. Each adult interacted with each child in a five-minute free play and a five-minute story-telling situation. Quantitative measures of the speech of the adults included word rate, disfluencies, type-token ratio, and mean length of response. There was a strong tendency to present the language-impaired child with fluent, short, and simple sentence patterns, as compared to his peer. Qualitative analyses of sentence types again showed that simplifications were made in speech to the boy, and that the adults used sequential sets of sentences with the language-handicapped child that were similar to those used by mothers of normal, but much younger children.