Training of ultra-fast speech comprehension induces functional reorganization of the central-visual system in late-blind humans

Verbal and Spatial Working Memory Capacity in Blind Adults and the Possible Influence of Age at Blindness Onset: A Systematic Review and Meta-analysis

The loss of a sense, such as vision, forces individuals to adapt to their environment and its demands in a variety of ways. In the case of blindness, significant neurofunctional and cognitive changes have been documented. However, there is no clear consensus on the differences in performance between adult blind participants and sighted controls in cognitive processes such as working memory (WM). Two variables are important, including the cognitive task used to measure working memory and the age at which vision loss occurs. This review is aimed at exploring potential disparities in verbal and spatial WM performance between blind and sighted adults, as well as understanding how these differences may be influenced by the age of vision loss. A systematic search across PsycArticles, PsycInfo, Medline, and Web of Science databases identified 21 pertinent studies. The studies were categorized, and effect sizes were calculated through meta-analysis, distinguishing between verbal (auditory simple forward and backward span, complex span, and n-back) and visuospatial WM tasks (adapted Corsi-block and simple storage tasks, imagery tasks, and complex storage tasks). Visual sensory loss induces adaptations affecting WM function in blind participants. In the verbal domain, improved phonological processing and/or serial item position encoding might facilitate WM retrieval. In contrast, in spatial WM, an over-reliance on serial processing may hinder strategic grouping in blind individuals. This review highlights the need to further explore the role of age at the time of vision loss. Although evidence suggests that adaptations to serial processing may be more pronounced in early development, particularly in comparison to those who become blind in adulthood, the available data are limited. The study calls for further research to deepen our understanding of cognitive adaptations and their temporal dynamics in response to vision loss.

Impairing one sensory modality enhances another by reconfiguring peptidergic signalling in Caenorhabditis elegans

Animals that lose one sensory modality often show augmented responses to other sensory inputs. The mechanisms underpinning this cross-modal plasticity are poorly understood. We probe such mechanisms by performing a forward genetic screen for mutants with enhanced O₂ perception in Caenorhabditis elegans. Multiple mutants exhibiting increased O₂ responsiveness concomitantly show defects in other sensory responses. One mutant, qui-1, defective in a conserved NACHT/WD40 protein, abolishes pheromone-evoked Ca²⁺ responses in the ADL pheromone-sensing neurons. At the same time, ADL responsiveness to pre-synaptic input from O₂-sensing neurons is heightened in qui-1, and other sensory defective mutants, resulting in enhanced neurosecretion although not increased Ca²⁺ responses. Expressing qui-1 selectively in ADL rescues both the qui-1 ADL neurosecretory phenotype and enhanced escape from 21% O₂. Profiling ADL neurons in qui-1 mutants highlights extensive changes in gene expression, notably of many neuropeptide receptors. We show that elevated ADL expression of the conserved neuropeptide receptor NPR-22 is necessary for enhanced ADL neurosecretion in qui-1 mutants, and is sufficient to confer increased ADL neurosecretion in control animals. Sensory loss can thus confer cross-modal plasticity by changing the peptidergic connectome.

Cortical phase locking to accelerated speech in blind and sighted listeners prior to and after training

Cross-correlation of magnetoencephalography (MEG) with time courses derived from the speech signal has shown differences in phase-locking between blind subjects able to comprehend accelerated speech and sighted controls. The present training study contributes to disentangle the effects of blindness and training. Both subject groups (baseline: n = 16 blind, 13 sighted; trained: 10 blind, 3 sighted) were able to enhance speech comprehension up to ca. 18 syllables per second. MEG responses phase-locked to syllable onsets were captured in five pre-defined source locations comprising left and right auditory cortex (A1), right visual cortex (V1), left inferior frontal gyrus (IFG) and left pre-supplementary motor area. Phase locking in A1 was consistently increased while V1 showed opposite training effects in blind and sighted subjects. Also the IFG showed some group differences indicating enhanced top-down strategies in sighted subjects while blind subjects may have a more fine-grained bottom-up resolution for accelerated speech.

Top-down, contextual entrainment of neuronal oscillations in the auditory thalamocortical circuit

Our results indicate that nonhuman primates detect complex repeating acoustic sequences in a continuous auditory stream, which is an important precursor for human speech learning and perception. We demonstrate that oscillatory entrainment, known to support the attentive perception of rhythmic stimulus sequences, can occur for rhythms defined solely by stimulus context rather than physical boundaries. As opposed to acoustically driven entrainment by rhythmic tone sequences demonstrated previously, this form of entrainment relies on the brain’s ability to group auditory inputs based on their statistical regularities. The internally initiated, context-driven modulation of excitability in the medial pulvinar prior to A1 supports the notion of top-down entrainment.

Prior studies have shown that repetitive presentation of acoustic stimuli results in an alignment of ongoing neuronal oscillations to the sequence rhythm via oscillatory entrainment by external cues. Our study aimed to explore the neural correlates of the perceptual parsing and grouping of complex repeating auditory patterns that occur based solely on statistical regularities, or context. Human psychophysical studies suggest that the recognition of novel auditory patterns amid a continuous auditory stimulus sequence occurs automatically halfway through the first repetition. We hypothesized that once repeating patterns were detected by the brain, internal rhythms would become entrained, demarcating the temporal structure of these repetitions despite lacking external cues defining pattern on- or offsets. To examine the neural correlates of pattern perception, neuroelectric activity of primary auditory cortex (A1) and thalamic nuclei was recorded while nonhuman primates passively listened to streams of rapidly presented pure tones and bandpass noise bursts. At arbitrary intervals, random acoustic patterns composed of 11 stimuli were repeated five times without any perturbance of the constant stimulus flow. We found significant delta entrainment by these patterns in the A1, medial geniculate body, and medial pulvinar. In A1 and pulvinar, we observed a statistically significant, pattern structure-aligned modulation of neuronal firing that occurred earliest in the pulvinar, supporting the idea that grouping and detecting complex auditory patterns is a top-down, context-driven process. Besides electrophysiological measures, a pattern-related modulation of pupil diameter verified that, like humans, nonhuman primates consciously detect complex repetitive patterns that lack physical boundaries.

Reduced Performance During a Sentence Repetition Task by Continuous Theta-Burst Magnetic Stimulation of the Pre-supplementary Motor Area

The pre-supplementary motor area (pre-SMA) is engaged in speech comprehension under difficult circumstances such as poor acoustic signal quality or time-critical conditions. Previous studies found that left pre-SMA is activated when subjects listen to accelerated speech. Here, the functional role of pre-SMA was tested for accelerated speech comprehension by inducing a transient “virtual lesion” using continuous theta-burst stimulation (cTBS). Participants were tested (1) prior to (pre-baseline), (2) 10 min after (test condition for the cTBS effect), and (3) 60 min after stimulation (post-baseline) using a sentence repetition task (formant-synthesized at rates of 8, 10, 12, 14, and 16 syllables/s). Speech comprehension was quantified by the percentage of correctly reproduced speech material. For high speech rates, subjects showed decreased performance after cTBS of pre-SMA. Regarding the error pattern, the number of incorrect words without any semantic or phonological similarity to the target context increased, while related words decreased. Thus, the transient impairment of pre-SMA seems to affect its inhibitory function that normally eliminates erroneous speech material prior to speaking or, in case of perception, prior to encoding into a semantically/pragmatically meaningful message.

Cross-modal synaptic plasticity in adult primary sensory cortices

Sensory loss leads to widespread adaptation of brain circuits to allow an organism to navigate its environment with its remaining senses, which is broadly referred to as cross-modal plasticity. Such adaptation can be observed even in the primary sensory cortices, and falls into two distinct categories: recruitment of the deprived sensory cortex for processing the remaining senses, which we term 'cross-modal recruitment', and experience-dependent refinement of the spared sensory cortices referred to as 'compensatory plasticity.' Here we will review recent studies demonstrating that cortical adaptation to sensory loss involves LTP/LTD and homeostatic synaptic plasticity. Cross-modal synaptic plasticity is observed in adults, hence cross-modal sensory deprivation may be an effective way to promote plasticity in adult primary sensory cortices.

Network Modeling for Functional Magnetic Resonance Imaging (fMRI) Signals during Ultra-Fast Speech Comprehension in Late-Blind Listeners

In many functional magnetic resonance imaging (fMRI) studies blind humans were found to show cross-modal reorganization engaging the visual system in non-visual tasks. For example, blind people can manage to understand (synthetic) spoken language at very high speaking rates up to ca. 20 syllables/s (syl/s). FMRI data showed that hemodynamic activation within right-hemispheric primary visual cortex (V1), bilateral pulvinar (Pv), and left-hemispheric supplementary motor area (pre-SMA) covaried with their capability of ultra-fast speech (16 syllables/s) comprehension. It has been suggested that right V1 plays an important role with respect to the perception of ultra-fast speech features, particularly the detection of syllable onsets. Furthermore, left pre-SMA seems to be an interface between these syllabic representations and the frontal speech processing and working memory network. So far, little is known about the networks linking V1 to Pv, auditory cortex (A1), and (mesio-) frontal areas. Dynamic causal modeling (DCM) was applied to investigate (i) the input structure from A1 and Pv toward right V1 and (ii) output from right V1 and A1 to left pre-SMA. As concerns the input Pv was significantly connected to V1, in addition to A1, in blind participants, but not in sighted controls. Regarding the output V1 was significantly connected to pre-SMA in blind individuals, and the strength of V1-SMA connectivity correlated with the performance of ultra-fast speech comprehension. By contrast, in sighted controls, not understanding ultra-fast speech, pre-SMA did neither receive input from A1 nor V1. Taken together, right V1 might facilitate the “parsing” of the ultra-fast speech stream in blind subjects by receiving subcortical auditory input via the Pv (= secondary visual pathway) and transmitting this information toward contralateral pre-SMA.

Experience-related structural changes of degenerated occipital white matter in late-blind humans - a diffusion tensor imaging study

Late-blind humans can learn to understand speech at ultra-fast syllable rates (ca. 20 syllables/s), a capability associated with hemodynamic activation of the central-visual system. Thus, the observed functional cross-modal recruitment of occipital cortex might facilitate ultra-fast speech processing in these individuals. To further elucidate the structural prerequisites of this skill, diffusion tensor imaging (DTI) was conducted in late-blind subjects differing in their capability of understanding ultra-fast speech. Fractional anisotropy (FA) was determined as a quantitative measure of the directionality of water diffusion, indicating fiber tract characteristics that might be influenced by blindness as well as the acquired perceptual skills. Analysis of the diffusion images revealed reduced FA in late-blind individuals relative to sighted controls at the level of the optic radiations at either side and the right-hemisphere dorsal thalamus (pulvinar). Moreover, late-blind subjects showed significant positive correlations between FA and the capacity of ultra-fast speech comprehension within right-hemisphere optic radiation and thalamus. Thus, experience-related structural alterations occurred in late-blind individuals within visual pathways that, presumably, are linked to higher order frontal language areas.

The shape of the human language-ready brain

Our core hypothesis is that the emergence of our species-specific language-ready brain ought to be understood in light of the developmental changes expressed at the levels of brain morphology and neural connectivity that occurred in our species after the split from Neanderthals–Denisovans and that gave us a more globular braincase configuration. In addition to changes at the cortical level, we hypothesize that the anatomical shift that led to globularity also entailed significant changes at the subcortical level. We claim that the functional consequences of such changes must also be taken into account to gain a fuller understanding of our linguistic capacity. Here we focus on the thalamus, which we argue is central to language and human cognition, as it modulates fronto-parietal activity. With this new neurobiological perspective in place, we examine its possible molecular basis. We construct a candidate gene set whose members are involved in the development and connectivity of the thalamus, in the evolution of the human head, and are known to give rise to language-associated cognitive disorders. We submit that the new gene candidate set opens up new windows into our understanding of the genetic basis of our linguistic capacity. Thus, our hypothesis aims at generating new testing grounds concerning core aspects of language ontogeny and phylogeny.