Lars Meyer

Dual‐Color STED Microscopy at 30‐nm Focal‐Plane Resolution

Owing to its sensitivity and noninvasiveness, far-field fluo-rescence microscopy would be almost ideal for biologicalimaging if the resolution of its established variants were notlimitedbydiffractiontoDr l=ð2n sinaÞ,withldenotingthewavelength of light, n the index of refraction, and a theaperture angle of the objective lens.

Early steps in the assembly of photoreceptor ribbon synapses in the mouse retina: The involvement of precursor spheres

The retinal photoreceptor ribbon synapse is a chemical synapse structurally and functionally specialized for the tonic release of neurotransmitter. It is characterized by the presynaptic ribbon, an electron‐dense organelle at the active zone covered by hundreds of synaptic vesicles. In conventional synapses, dense‐core transport vesicles carrying a set of active zone proteins are implicated in early steps of synapse formation. In photoreceptor ribbon synapses, synaptic spheres are suggested to be involved in ribbon synapse assembly, but nothing is known about the molecular composition of these organelles. With light, electron, and stimulated emission depletion microscopy and immunocytochemistry, we investigated a series of presynaptic proteins during photoreceptor synaptogenesis. The cytomatrix proteins Bassoon, Piccolo, RIBEYE, and RIM1 appear early in synaptogenesis. They are transported in nonmembranous, electron‐dense, spherical transport units, which we called precursor spheres, to the future presynaptic site. Other presynaptic proteins, i.e., Munc13, CAST1, RIM2, and an L‐type Ca2+ channel α1 subunit are not associated with the precursor spheres. They cluster directly at the active zone some time after the first set of cytomatrix proteins has arrived. By quantitative electron microscopy, we found an inverse correlation between the numbers of spheres and synaptic ribbons in the postnatally developing photoreceptor synaptic terminals. From these results, we suggest that the precursor spheres are the transport units for proteins of the photoreceptor ribbon compartment and are involved in the assembly of mature synaptic ribbons. J. Comp. Neurol. 512:814–824, 2009.

3D reconstruction of high-resolution STED microscope images.

Tackling biological problems often involves the imaging and localization of cellular structures on the nanometer scale. Although optical super‐resolution below 100 nm can be readily attained with stimulated emission depletion (STED) and photoswitching microscopy methods, attaining an axial resolution <100 nm with focused light generally required the use of two lenses in a 4Pi configuration or exceptionally bright photochromic fluorophores. Here, we describe a simple technical solution for 3D nanoscopy of fixed samples: biological specimens are fluorescently labeled, embedded in a polymer resin, cut into thin sections, and then imaged via STED microscopy with nanoscale resolution. This approach allows a 3D image reconstruction with a resolution <80 nm in all directions using available state‐of‐the art STED microscopes. Microsc. Res. Tech., 2008.

Left parietal alpha enhancement during working memory-intensive sentence processing.

Both functional magnetic resonance imaging (fMRI) and event-related brain potential (ERP) studies have shown that verbal working memory plays an important role during sentence processing. There is growing evidence from outside of sentence processing that human alpha oscillations (7-13 Hz) play a critical role in working memory. This study aims to link this to the sentence processing domain. Time-frequency analyses and source localization were performed on electroencephalography (EEG) data that were recorded during the processing of auditorily presented sentences involving either a short or a long distance between an argument (subject or object) and the respective sentence-final verb. We reasoned that oscillatory activity in the alpha band should increase during sentences with longer argument-verb distances, since decreased temporal proximity should result in increased memory demands. When verbal working memory-intensive long-dependency sentences were compared to short-dependency sentences, a sustained oscillatory enhancement at 10 Hz was found during storage prior to the sentence-final verb, turning into a transient power increase in the beta band (13-20 Hz) at the sentence-final verb. The sources of the alpha oscillations were localized to bilaterally occipital and left parietal cortices. Only the source activity in the left parietal cortex was negatively correlated with verbal working memory abilities. These findings indicate that the parsimonious role of alpha oscillations in domain-general working memory can be extended to language, that is, sentence processing. We suggest that the function of left parietal cortex underlying verbal working memory storage during sentence processing is to inhibit the premature release of verbal information that will subsequently be integrated.

Frontal–posterior theta oscillations reflect memory retrieval during sentence comprehension

Successful working-memory retrieval requires that items be retained as distinct units. At the neural level, it has been shown that theta-band oscillatory power increases with the number of to-be-distinguished items during working-memory retrieval. Here we hypothesized that during sentence comprehension, verbal-working-memory retrieval demands lead to increased theta power over frontal cortex, supposedly supporting the distinction amongst stored items during verbal-working-memory retrieval. Also, synchronicity may increase between the frontal cortex and the posterior cortex, with the latter supposedly supporting item retention. We operationalized retrieval by using pronouns, which refer to and trigger the retrieval of antecedent nouns from a preceding sentence part. Retrieval demand was systematically varied by changing the pronoun antecedent: Either, it was non-embedded in the preceding main clause, and thus easy-to-retrieve across a single clause boundary, or embedded in the preceding subordinate clause, and thus hard-to-retrieve across a double clause boundary. We combined electroencephalography (EEG), scalp-level time-frequency analysis, source localization, and source-level coherence analysis, observing a frontal-midline and broad left-hemispheric theta-power increase for embedded-antecedent compared to non-embedded-antecedent retrieval. Sources were localized to left-frontal, left-parietal, and bilateral-inferior-temporal cortices. Coherence analyses suggested synchronicity between left-frontal and left-parietal and between left-frontal and right-inferior-temporal cortices. Activity of an array of left-frontal, left-parietal, and bilateral-inferior-temporal cortices may thus assist retrieval during sentence comprehension, potentially indexing the orchestration of item distinction, verbal working memory, and long-term memory. Our results extend prior findings by mapping prior knowledge on the functional role of theta oscillations onto processes genuine to human sentence comprehension.

Distinguishing Neurocognitive Processes Reflected by P600 Effects: Evidence from ERPs and Neural Oscillations

Research on language comprehension using event-related potentials (ERPs) reported distinct ERP components reliably related to the processing of semantic (N400) and syntactic information (P600). Recent ERP studies have challenged this well-defined distinction by showing P600 effects for semantic and pragmatic anomalies. So far, it is still unresolved whether the P600 reflects specific or rather common processes. The present study addresses this question by investigating ERPs in response to a syntactic and pragmatic (irony) manipulation, as well as a combined syntactic and pragmatic manipulation. For the syntactic condition, a morphosyntactic violation was applied, whereas for the pragmatic condition, such as “That is rich”, either an ironic or literal interpretation was achieved, depending on the prior context. The ERPs at the critical word showed a LAN-P600 pattern for syntactically incorrect sentences relative to correct ones. For ironic compared to literal sentences, ERPs showed a P200 effect followed by a P600 component. In comparison of the syntax-related P600 to the irony-related P600, distributional differences were found. Moreover, for the P600 time window (i.e., 500–900 ms), different changes in theta power between the syntax and pragmatics effects were found, suggesting that different patterns of neural activity contributed to each respective effect. Thus, both late positivities seem to be differently sensitive to these two types of linguistic information, and might reflect distinct neurocognitive processes, such as reanalysis of the sentence structure versus pragmatic reanalysis.

Building by Syntax: The Neural Basis of Minimal Linguistic Structures

Abstract Language comes in utterances in which words are bound together according to a simple rule‐based syntactic computation (merge), which creates linguistic hierarchies of potentially infinite length—phrases and sentences. In the current functional magnetic resonance imaging study, we compared prepositional phrases and sentences—both involving merge—to word lists—not involving merge—to explore how this process is implemented in the brain. We found that merge activates the pars opercularis of the left inferior frontal gyrus (IFG; Brodmann Area [BA] 44) and a smaller region in the posterior superior temporal sulcus (pSTS). Within the IFG, sentences engaged a more anterior portion of the area (pars triangularis, BA 45)—compared with phrases—which showed activity peak in BA 44. As prepositional phrases, in contrast to sentences, do not contain verbs, activity in BA 44 may reflect structure‐building syntactic processing, while the involvement of BA 45 may reflect the encoding of propositional meaning initiated by the verb. The pSTS appears to work together with the IFG during thematic role assignment not only at the sentential level, but also at the phrasal level. The present results suggest that merge, the process of binding words together into syntactic hierarchies, is primarily supported by BA 44 in the IFG.

Spatiotemporal Dynamics of Argument Retrieval and Reordering: An fMRI and EEG Study on Sentence Processing

In sentence processing, it is still unclear how the neural language network successfully establishes argument–verb dependencies in its spatiotemporal neuronal dynamics. Previous work has suggested that the establishment of subject–verb and object–verb dependencies requires argument retrieval from working memory, and that dependency establishment in object-first sentences additionally necessitates argument reordering. We examine the spatiotemporal neuronal dynamics of the brain regions that subserve these sub-processes by crossing an argument reordering factor (i.e., subject-first versus object-first sentences) with an argument retrieval factor (i.e., short versus long argument–verb dependencies) in German. Using functional magnetic resonance imaging (fMRI), we found that reordering demands focally activate the left pars opercularis (Broca’s area), while storage and retrieval demands activated left temporo-parietal (TP) regions. In addition, when analyzing the time course of fMRI-informed equivalent current dipole sources in the EEG at the subcategorizing verb, we found that activity in the TP-region occurs relatively early (40–180 ms), followed by activity in Broca’s area (300–500 ms). These findings were matched by topographical correlation analyses of fMRI activations in EEG sensor space, showing that, in the scalp potential, TP-region activity surfaces as an early positivity and IFG activity as a later positivity in the scalp potential. These results provide fine-grained evidence for spatiotemporally separable sub-processes of argument retrieval and reordering in sentence processing.

Sentence processing and verbal working memory in a white-matter-disconnection patient

The Arcuate Fasciculus/Superior Longitudinal Fasciculus (AF/SLF) is the white-matter bundle that connects posterior superior temporal and inferior frontal cortex. Its causal functional role in sentence processing and verbal working memory is currently under debate. While impairments of sentence processing and verbal working memory often co-occur in patients suffering from AF/SLF damage, it is unclear whether these impairments result from shared white-matter damage to the verbal-working-memory network. The present study sought to specify the behavioral consequences of focal AF/SLF damage for sentence processing and verbal working memory, which were assessed in a single patient suffering from a cleft-like lesion spanning the deep left superior temporal gyrus, sparing most surrounding gray matter. While tractography suggests that the ventral fronto-temporal white-matter bundle is intact in this patient, the AF/SLF was not visible to tractography. In line with the hypothesis that the AF/SLF is causally involved in sentence processing, the patient׳s performance was selectively impaired on sentences that jointly involve both complex word orders and long word-storage intervals. However, the patient was unimpaired on sentences that only involved long word-storage intervals without involving complex word orders. On the contrary, the patient performed generally worse than a control group across standard verbal-working-memory tests. We conclude that the AF/SLF not only plays a causal role in sentence processing, linking regions of the left dorsal inferior frontal gyrus to the temporo-parietal region, but moreover plays a crucial role in verbal working memory, linking regions of the left ventral inferior frontal gyrus to the left temporo-parietal region. Together, the specific sentence-processing impairment and the more general verbal-working-memory impairment may imply that the AF/SLF subserves both sentence processing and verbal working memory, possibly pointing to the AF and SLF respectively supporting each.

The Neural Oscillations of Speech Processing and Language Comprehension: State of the Art and Emerging Mechanisms

Neural oscillations subserve a broad range of functions in speech processing and language comprehension. On the one hand, speech contains—somewhat—repetitive trains of air pressure bursts that occur at three dominant amplitude modulation frequencies, physically marking the linguistically meaningful progressions of phonemes, syllables and intonational phrase boundaries. To these acoustic events, neural oscillations of isomorphous operating frequencies are thought to synchronise, presumably resulting in an implicit temporal alignment of periods of neural excitability to linguistically meaningful spectral information on the three low‐level linguistic description levels. On the other hand, speech is a carrier signal that codes for high‐level linguistic meaning, such as syntactic structure and semantic information—which cannot be read from stimulus acoustics, but must be acquired during language acquisition and decoded for language comprehension. Neural oscillations subserve the processing of both syntactic structure and semantic information. Here, I synthesise a mapping from each linguistic processing domain to a unique set of subserving oscillatory mechanisms—the mapping is plausible given the role ascribed to different oscillatory mechanisms in different subfunctions of cortical information processing and faithful to the underlying electrophysiology. In sum, the present article provides an accessible and extensive review of the functional mechanisms that neural oscillations subserve in speech processing and language comprehension.