Temporal dynamics of oscillatory activity during nonlexical language decoding: Evidence from Morse code and magnetoencephalography.

Understanding encoded languages, such as written script or Morse code, requires nonlexical and lexical processing components that act in a parallel and interactive fashion. Decoding written script-as for example in reading-is typically very fast, making the investigation of the lexical and nonlexical components and their underlying neural mechanisms challenging. In the current study, we aimed to accomplish this problem by using Morse code as a model for language decoding. The decoding of Morse code is slower and thus allows a better and more fine-grained investigation of the lexical and nonlexical components of language decoding. In the current study, we investigated the impact of various components of nonlexical decoding of Morse code using magnetoencephalography. For this purpose, we reconstructed the time-frequency responses below 40 Hz in brain regions significantly involved in Morse code decoding and word comprehension that were identified in a previous study. Event-related reduction in beta- and alpha-band power were found in left inferior frontal cortex and angular gyrus, respectively, while event-related theta-band power increase was found at frontal midline. These induced oscillations reflect working-memory encoding, long-term memory retrieval as well as demanding cognitive control, respectively. In sum, by using Morse code and MEG, we were able to identify a cortical network underlying language decoding in a time- and frequency-resolved manner.

Human movements do not look the same in a tilted world: Gravitational constraints influence the perception of biological motion.

We investigated whether gravitational constraints influence the interaction of visual, proprioceptive and vestibular cues for Biological Motion Perception (BMP). Participants were asked to distinguish between plausible and random point-light movements, while passively placed in either an upright or a tilted body orientation. Manipulating the body orientation with respect to gravity leads to different gravitational signals transmitted by the visual, proprioceptive, and vestibular systems. Participants were overall faster in distinguishing plausible point-light movements than random movements. Critically, response times for biologically plausible point-light movements - but not for random movements - were significantly prolonged in the tilted body orientation. Our results suggest that BMP depends not only on the spatial-temporal cues embedded in point-light movements but also rely on the congruency between current gravitational signals detected by the sensory systems and our previous knowledge of terrestrial gravity. STATEMENT OF RELEVANCE: As humankind is preparing for a new space age, understanding how gravity influences behaviour and cognition has never been more pressing. All living organisms have evolved to survive in a terrestrial gravitational field. Although we cannot consciously feel gravity, it has an impact in our life: it affects how we move and interact with the external environment. The sensory signals from the vestibular system are continuously combined with visual and proprioceptive cues to help us in maintaining a stable representation of the world. Here we placed participants in a tilted body orientation and were able to determine that a conflict between prior gravitational knowledge and what was actively sensed about gravity affected human Biological Movement Perception. Humans suffer changes in perception under non-terrestrial gravity conditions that may potentially compromise performance during space exploration.

Saccade suppression of displacements, but not of contrast, depends on context.

Saccades let the visual scene sweep with high speed across the retina, thus producing a massive motion stimulus. Yet, in natural vision, we never perceive motion that is produced by saccades. The absence of perisaccadic motion perception might be caused by a transient reduction of visual sensitivity at the time of saccade initiation, so-called saccadic suppression. Saccade suppression occurs for contrast, displacement, and motion stimuli. Saccade suppression of displacements has been shown to be context sensitive. After performing saccades in sessions without perisaccadic stimulation, saccade suppression magnitude is drastically decreased (Zimmermann, 2020). Here, we aimed to test whether saccade suppression of contrast is similarly modulated by context. To this end, we projected stimuli on a homogeneously white wall such that we could establish a ganzfeld-like environment that, depending on the experimental session, did or did not contain any visible contrast stimuli. We first successfully replicated the context sensitivity of saccade suppression of displacements. Then, we tested context sensitivity of contrast suppression by asking subjects to perform several saccades either across the uniform white wall or across a background consisting of a sinusoidal grating. In contrast to perisaccadic context sensitivity for displacement suppression, we did not find context sensitivity for suppression of contrast.

U-shaped Relation between Prestimulus Alpha-band and Poststimulus Gamma-band Power in Temporal Tactile Perception in the Human Somatosensory Cortex.

Neuronal oscillations are a ubiquitous phenomenon in the human nervous system. Alpha-band oscillations (8-12 Hz) have been shown to correlate negatively with attention and performance, whereas gamma-band oscillations (40-150 Hz) correlate positively. Here, we studied the relation between prestimulus alpha-band power and poststimulus gamma-band power in a suprathreshold tactile discrimination task. Participants received two electrical stimuli to their left index finger with different SOAs (0 msec, 100 msec, intermediate SOA, intermediate SOA ± 10 msec). The intermediate SOA was individually determined so that stimulation was bistable, and participants perceived one stimulus in half of the trials and two stimuli in the other half. We measured neuronal activity with magnetoencephalography (MEG). In trials with intermediate SOAs, behavioral performance correlated inversely with prestimulus alpha-band power but did not correlate with poststimulus gamma-band power. Poststimulus gamma-band power was high in trials with low and high prestimulus alpha-band power and low for intermediate prestimulus alpha-band power (i.e., U-shaped). We suggest that prestimulus alpha activity modulates poststimulus gamma activity and subsequent perception: (1) low prestimulus alpha-band power leads to high poststimulus gamma-band power, biasing perception such that two stimuli were perceived; (2) intermediate prestimulus alpha-band power leads to low gamma-band power (interpreted as inefficient stimulus processing), consequently, perception was not biased in either direction; and (3) high prestimulus alpha-band power leads to high poststimulus gamma-band power, biasing perception such that only one stimulus was perceived.

Rapid temporal recalibration to visuo-tactile stimuli.

For a comprehensive understanding of the environment, the brain must constantly decide whether the incoming information originates from the same source and needs to be integrated into a coherent percept. This integration process is believed to be mediated by temporal integration windows. If presented with temporally asynchronous stimuli for a few minutes, the brain adapts to this new temporal relation by recalibrating the temporal integration windows. Such recalibration can occur even more rapidly after exposure to just a single trial of asynchronous stimulation. While rapid recalibration has been demonstrated for audio-visual stimuli, evidence for rapid recalibration of visuo-tactile stimuli is lacking. Here, we investigated rapid recalibration in the visuo-tactile domain. Subjects received visual and tactile stimuli with different stimulus onset asynchronies (SOA) and were asked to report whether the visuo-tactile stimuli were presented simultaneously. Our results demonstrate visuo-tactile rapid recalibration by revealing that subjects' simultaneity reports were modulated by the temporal order of stimulation in the preceding trial. This rapid recalibration effect, however, was only significant if the SOA in the preceding trial was smaller than 100 ms, while rapid recalibration could not be demonstrated for SOAs larger than 100 ms. Since rapid recalibration in the audio-visual domain has been demonstrated for SOAs larger than 100 ms, we propose that visuo-tactile recalibration works at shorter SOAs, and thus faster time scales than audio-visual rapid recalibration.

Beta oscillations define discrete perceptual cycles in the somatosensory domain.

Whether seeing a movie, listening to a song, or feeling a breeze on the skin, we coherently experience these stimuli as continuous, seamless percepts. However, there are rare perceptual phenomena that argue against continuous perception but, instead, suggest discrete processing of sensory input. Empirical evidence supporting such a discrete mechanism, however, remains scarce and comes entirely from the visual domain. Here, we demonstrate compelling evidence for discrete perceptual sampling in the somatosensory domain. Using magnetoencephalography (MEG) and a tactile temporal discrimination task in humans, we find that oscillatory alpha- and low beta-band (8-20 Hz) cycles in primary somatosensory cortex represent neurophysiological correlates of discrete perceptual cycles. Our results agree with several theoretical concepts of discrete perceptual sampling and empirical evidence of perceptual cycles in the visual domain. Critically, these results show that discrete perceptual cycles are not domain-specific, and thus restricted to the visual domain, but extend to the somatosensory domain.

Prestimulus Alpha Power Influences Tactile Temporal Perceptual Discrimination and Confidence in Decisions.

Recent studies have demonstrated that prestimulus alpha-band activity substantially influences perception of near-threshold stimuli. Here, we studied the influence of prestimulus alpha power fluctuations on temporal perceptual discrimination of suprathreshold tactile stimuli and subjects' confidence regarding their perceptual decisions. We investigated how prestimulus alpha-band power influences poststimulus decision-making variables. We presented electrical stimuli with different stimulus onset asynchronies (SOAs) to human subjects, and determined the SOA for which temporal perceptual discrimination varied on a trial-by-trial basis between perceiving 1 or 2 stimuli, prior to recording brain activity with magnetoencephalography. We found that low prestimulus alpha power in contralateral somatosensory and occipital areas predicts the veridical temporal perceptual discrimination of 2 stimuli. Additionally, prestimulus alpha power was negatively correlated with confidence ratings in correctly perceived trials, but positively correlated for incorrectly perceived trials. Finally, poststimulus event-related fields (ERFs) were modulated by prestimulus alpha power and reflect the result of a decisional process rather than physical stimulus parameters around ∼150 ms. These findings provide new insights into the link between spontaneous prestimulus alpha power fluctuations, temporal perceptual discrimination, decision making, and decisional confidence. The results suggest that prestimulus alpha power modulates perception and decisions on a continuous scale, as reflected in confidence ratings.

Reduced occipital alpha power indexes enhanced excitability rather than improved visual perception.

Several studies have demonstrated that prestimulus occipital alpha-band activity substantially influences subjective perception and discrimination of near-threshold or masked visual stimuli. Here, we studied the role of prestimulus power fluctuations in two visual phenomena called double-flash illusion (DFI) and fusion effect (FE), both consisting of suprathreshold stimuli. In both phenomena, human subjects' perception varies on a trial-by-trial basis between perceiving one or two visual stimuli, despite constant stimulation. In the FE, two stimuli correspond to veridical perception. In the DFI, two stimuli correspond to an illusory perception. This provides for a critical test of whether reduced alpha power indeed promotes veridical perception in general. We find that in both, DFI and FE, reduced prestimulus occipital alpha predicts the perception of two stimuli, regardless of whether this is veridical (FE) or illusory (DFI). Our results suggest that reduced alpha-band power does not always predict improved visual processing, but rather enhanced excitability. In addition, for the DFI, enhanced prestimulus occipital gamma-band power predicted the perception of two visual stimuli. These findings provide new insights into the role of prestimulus rhythmic activity for visual processing.

Interactions between visual and motor areas during the recognition of plausible actions as revealed by magnetoencephalography.

Several studies have shown activation of the mirror neuron system (MNS), comprising the temporal, posterior parietal, and sensorimotor areas when observing plausible actions, but far less is known on how these cortical areas interact during the recognition of a plausible action. Here, we recorded neural activity with magnetoencephalography while subjects viewed point-light displays of biologically plausible and scrambled versions of actions. We were interested in modulations of oscillatory activity and, specifically, in coupling of oscillatory activity between visual and motor areas. Both plausible and scrambled actions elicited modulations of θ (5-7 Hz), α (7-13 Hz), ß (13-35 Hz), and γ (55-100 Hz) power within visual and motor areas. When comparing between the two actions, we observed sequential and spatially distinct increases of γ (∼65 Hz), ß (∼25 Hz), and α (∼11 Hz) power between 0.5 and 1.3 s in parieto-occipital, sensorimotor, and left temporal areas. In addition, significant clusters of γ (∼65 Hz) and α/ß (∼15 Hz) power decrease were observed in right temporal and parieto-occipital areas between 1.3 and 2.0 s. We found ß-power in sensorimotor areas to be positively correlated on a trial-by-trial basis with parieto-occipital γ and left temporal α-power for the plausible but not for the scrambled condition. These results provide new insights in the neuronal oscillatory activity of the areas involved in the recognition of plausible action movements and their interaction. The power correlations between specific areas underscore the importance of interactions between visual and motor areas of the MNS during the recognition of a plausible action.

The angular gyrus serves as an interface between the non-lexical reading network and the semantic system: evidence from dynamic causal modeling.

Understanding encoded language, such as written words, requires multiple cognitive processes that act in a parallel and interactive fashion. These processes and their interactions, however, are not fully understood. Various conceptual and methodical approaches including computational modeling and neuroimaging have been applied to better understand the neural underpinnings of these complex processes in the human brain. In this study, we tested different predictions of cortical interactions that derived from computational models for reading using dynamic causal modeling. Morse code was used as a model for non-lexical decoding followed by a lexical-decision during a functional magnetic resonance examination. Our results suggest that individual letters are first converted into phonemes within the left supramarginal gyrus, followed by a phoneme assembly to reconstruct word phonology, involving the left inferior frontal cortex. To allow the identification and comprehension of known words, the inferior frontal cortex then interacts with the semantic system via the left angular gyrus. As such, the left angular gyrus is likely to host phonological and semantic representations and serves as a bidirectional interface between the networks involved in language perception and word comprehension.

Audio-visual congruency alters power and coherence of oscillatory activity within and between cortical areas.

Dynamic communication between functionally specialized, but spatially distributed areas of the brain is essential for effective brain functioning. A candidate mechanism for effective neuronal communication is oscillatory neuronal synchronization. Here, we used magnetoencephalography (MEG) to study the role of oscillatory neuronal synchronization in audio-visual speech perception. Subjects viewed congruent audio-visual stimuli of a speaker articulating the vowels /a/ or /o/. In addition, we presented modified, incongruent versions in which visual and auditory signals mismatched. We identified a left hemispheric network for processing congruent audio-visual speech as well as network interaction between areas: low frequency (4-12 Hz) power was suppressed for congruent stimuli at auditory onset around auditory cortex, while power in the high gamma (120-140 Hz)-band was enhanced in the Broca's area around auditory offset. In addition, beta-power (20-30 Hz) was suppressed in supramarginal gyrus for incongruent stimuli. Interestingly, coherence analysis revealed a functional coupling between auditory cortex and Broca's area for congruent stimuli demonstrated by an increase of coherence. In contrast, coherence decreased for incongruent stimuli, suggesting a decoupling of auditory cortex and Broca's area. In addition, the increase of coherence was positively correlated with the increase of high gamma-power. The results demonstrate that oscillatory power in several frequency bands correlates with the processing of matching audio-visual speech on a large spatio-temporal scale. The findings provide evidence that coupling of neuronal groups can be mediated by coherence in the theta/alpha band and that low frequency coherence and high frequency power modulations are correlated in audio-visual speech perception.

Fluctuations of prestimulus oscillatory power predict subjective perception of tactile simultaneity.

Oscillatory activity is modulated by sensory stimulation but can also fluctuate in the absence of sensory input. Recent studies have demonstrated that such fluctuations of oscillatory activity can have substantial influence on the perception of subsequent stimuli. In the present study, we employed a simultaneity task in the somatosensory domain to study the role of prestimulus oscillatory activity on the temporal perception of 2 events. Subjects received electrical stimulations of the left and right index finger with varying stimulus onset asynchronies (SOAs) and reported their subjective perception of simultaneity, while brain activity was recorded with magnetoencephalography. With intermediate SOAs (30 and 45 ms), subjects frequently misperceived the stimulation as simultaneously. We compared neuronal oscillatory power in these conditions and found that power in the high beta band (∼20 to 40 Hz) in primary and secondary somatosensory cortex prior to the electrical stimulation predicted subjects' reports of simultaneity. Additionally, prestimulus alpha-band power influenced perception in the condition SOA 45 ms. Our results indicate that fluctuations of ongoing oscillatory activity in the beta and alpha bands shape subjective perception of physically identical stimulation.

Perception of the touch-induced visual double-flash illusion correlates with changes of rhythmic neuronal activity in human visual and somatosensory areas.

A single brief visual stimulus accompanied by two brief tactile stimuli is frequently perceived incorrectly as two flashes, a phenomenon called double-flash illusion (DFI). We investigated whether the DFI is accompanied by changes in rhythmic neuronal activity, using magnetoencephalography in human subjects. Twenty-two subjects received visuo-tactile stimulation and reported the number of perceived visual stimuli. We sorted trials with identical physical stimulation according to the reported subjective percept and assessed differences in spectral power in somatosensory and occipital sensors. In DFI trials, occipital sensors displayed a contralateral enhancement of gamma-band (80-140 Hz) activity in response to stimulation. In somatosensory sensors, the DFI was associated with an increase of spectral power for low frequencies (5-17.5 Hz) around stimulation and a decrease of spectral power in the 22.5-30 Hz range between 450 and 750 ms post-stimulation. In summary, several components of rhythmic activity predicted variable subjective experience for constant physical stimulation. Notably, the enhanced occipital gamma-band activity during DFI was similar in time and frequency extent to the somatosensory gamma-band response to tactile stimulation. We speculate that the DFI might therefore occur when the somatosensory gamma-response is transmitted to visual cortex. This transmission might be supported by the observed modulations in low-frequency activity.

10 Hz tACS Over Somatosensory Cortex Does Not Modulate Supra-Threshold Tactile Temporal Discrimination in Humans.

Perception of physical identical stimuli can differ over time depending on the brain state. One marker of this brain state can be neuronal oscillations in the alpha band (8-12 Hz). A previous study showed that the power of prestimulus alpha oscillations in the contralateral somatosensory area negatively correlate with the ability to temporally discriminate between two subsequent tactile suprathreshold stimuli. That is, with high alpha power subjects were impaired in discriminating two stimuli and more frequently reported to perceive only one stimulus. While this previous study found correlative evidence for a role of alpha oscillations on tactile temporal discrimination, here, we aimed to study the causal influence of alpha power on tactile temporal discrimination by using transcranial alternating current stimulation (tACS). We hypothesized that tACS in the alpha frequency should entrain alpha oscillations and thus modulate alpha power. This modulated alpha power should alter temporal discrimination ability compared to a control frequency or sham. To this end, 17 subjects received one or two electrical stimuli to their left index finger with different stimulus onset asynchronies (SOAs). They reported whether they perceived one or two stimuli. Subjects performed the paradigm before (pre), during (peri), and 25 min after tACS (post). tACS was applied to the contralateral somatosensory-parietal area with either 10, 5 Hz or sham on three different days. We found no significant difference in discrimination abilities between 10 Hz tACS and the control conditions, independent of SOAs. In addition to choosing all SOAs as the independent variable, we chose individually different SOAs, for which we expected the strongest effects of tACS. Again, we found no significant effects of 10 Hz tACS on temporal discrimination abilities. We discuss potential reasons for the inability to modulate tactile temporal discrimination abilities with tACS.

Event-related desynchronization of mu and beta oscillations during the processing of novel tool names.

According to the embodied cognition framework, the formation of conceptual representations integrates the type of experience during learning. In this electroencephalographic study, we applied a linguistic variant of a training paradigm, in which participants learned to associate novel names to novel tools while either manipulating or visually exploring them. The analysis focused on event-related desynchronization (ERD) of oscillations in the mu and beta frequency range, which reflects activation of sensorimotor brain areas. After three training sessions, processing names of manipulated tools elicited a stronger ERD of the beta (18-25 Hz, 140-260 ms) and the lower mu rhythm (8-10 Hz, 320-440 ms) than processing names of visually explored tools, reflecting a possible reactivation of experiential sensorimotor information. Given the unexpected result that familiarized pseudo-words elicited an ERD comparable to names of manipulated tools, our findings could reflect a suppression of sensorimotor activity during the processing of objects with exclusively visual features.

Impaired Tactile Temporal Discrimination in Patients With Hepatic Encephalopathy.

The sensory system constantly receives stimuli from the external world. To discriminate two stimuli correctly as two temporally distinct events, the temporal distance or stimulus onset asynchrony (SOA) between the two stimuli has to exceed a specific threshold. If the SOA between two stimuli is shorter than this specific threshold, the two stimuli will be perceptually fused and perceived as one single stimulus. Patients with hepatic encephalopathy (HE) are known to show manifold perceptual impairments, including slowed visual temporal discrimination abilities as measured by the critical flicker frequency (CFF). Here, we hypothesized that HE patients are also impaired in their tactile temporal discrimination abilities and, thus, require a longer SOA between two tactile stimuli to perceive the stimuli as two temporally distinct events. To test this hypothesis, patients with varying grades of HE and age-matched healthy individuals performed a tactile temporal discrimination task. All participants received two tactile stimuli with varying SOA applied to their left index finger and reported how many distinct stimuli they perceived ("1" vs. "2"). HE patients needed a significantly longer SOA (138.0 ± 11.3 ms) between two tactile stimuli to perceive the stimuli as two temporally distinct events than healthy controls (78.6 ± 13.1 ms; p < 0.01). In addition, we found that the temporal discrimination ability in the tactile modality correlated positively with the temporal discrimination ability in the visual domain across all participants (i.e., negative correlation between tactile SOA and visual CFF: r = -0.37, p = 0.033). Our findings provide evidence that temporal tactile perception is substantially impaired in HE patients. In addition, the results suggest that tactile and visual discrimination abilities are affected in HE in parallel. This finding might argue for a common underlying pathophysiological mechanism. We argue that the known global slowing of neuronal oscillations in HE might represent such a common mechanism.

Connecting occipital alpha band peak frequency, visual temporal resolution, and occipital GABA levels in healthy participants and hepatic encephalopathy patients.

Recent studies have proposed a connection between the individual alpha band peak frequency and the temporal resolution of visual perception in healthy human participants. This connection rests on animal studies describing oscillations in the alpha band as a mode of phasic thalamocortical information transfer for low-level visual stimuli, which critically relies on GABAergic interneurons. Here, we investigated the interplay of these parameters by measuring occipital alpha band peak frequency by means of magnetoencephalography, visual temporal resolution by means of behavioral testing, and occipital GABA levels by means of magnetic resonance spectroscopy. Importantly, we investigated a sample of healthy participants and patients with varying grades of hepatic encephalopathy, which are known to exhibit decreases in the investigated parameters, thus providing an increased parameter space. We found that occipital alpha band peak frequency and visual temporal resolution were positively correlated, i.e., higher occipital alpha band peak frequencies were on average related to a higher temporal resolution. Likewise, occipital alpha band peak frequency correlated positively with occipital GABA levels. However, correlations were significant only when both healthy participants and patients were included in the analysis, thereby indicating a connection of the measures on group level (instead of the individual level). These findings provide new insights into neurophysiological and neurochemical underpinnings of visual perception.

A new device for high-temperature in situ GISAXS measurements.

A heating stage originally designed for diffraction experiments is implemented into a Bruker NANOSTAR instrument for in situ grazing incidence small-angle x-ray scattering experiments. A controlled atmosphere is provided by a dome separating the sample environment from the evacuated scattering instrument. This dome is double shelled in order to enable cooling water to flow through it. A mesoporous silica film templated by a self-assembled block copolymer system is investigated in situ during step-wise heating in air. The GISAXS pattern shows the structural development of the ordered lattice of parallel cylindrical pores. The deformation of the elliptical pore-cross section perpendicular to the film surface was studied with increasing temperature. Moreover, the performance of the setup was tested by controlled in situ heating of a copper surface under controlled oxygen containing atmosphere.

A model of biological motion perception from configural form cues.

Biological motion perception is the compelling ability of the visual system to perceive complex human movements effortlessly and within a fraction of a second. Recent neuroimaging and neurophysiological studies have revealed that the visual perception of biological motion activates a widespread network of brain areas. The superior temporal sulcus has a crucial role within this network. The roles of other areas are less clear. We present a computational model based on neurally plausible assumptions to elucidate the contributions of motion and form signals to biological motion perception and the computations in the underlying brain network. The model simulates receptive fields for images of the static human body, as found by neuroimaging studies, and temporally integrates their responses by leaky integrator neurons. The model reveals a high correlation to data obtained by neurophysiological, neuroimaging, and psychophysical studies.

Beyond the Peak - Tactile Temporal Discrimination Does Not Correlate with Individual Peak Frequencies in Somatosensory Cortex.

The human sensory systems constantly receive input from different stimuli. Whether these stimuli are integrated into a coherent percept or segregated and perceived as separate events, is critically determined by the temporal distance of the stimuli. This temporal distance has prompted the concept of temporal integration windows or perceptual cycles. Although this concept has gained considerable support, the neuronal correlates are still discussed. Studies suggested that neuronal oscillations might provide a neuronal basis for such perceptual cycles, i.e., the cycle lengths of alpha oscillations in visual cortex and beta oscillations in somatosensory cortex might determine the length of perceptual cycles. Specifically, recent studies reported that the peak frequency (the frequency with the highest spectral power) of alpha oscillations in visual cortex correlates with subjects' ability to discriminate two visual stimuli. In the present study, we investigated whether peak frequencies in somatosensory cortex might serve as the correlate of perceptual cycles in tactile discrimination. Despite several different approaches, we were unable to find a significant correlation between individual peak frequencies in the alpha- and beta-band and individual discrimination abilities. In addition, analysis of Bayes factor provided evidence that peak frequencies and discrimination thresholds are unrelated. The results suggest that perceptual cycles in the somatosensory domain are not necessarily to be found in the peak frequency, but in other frequencies. We argue that studies based solely on analysis of peak frequencies might thus miss relevant information.