•The functional characterization of callosal connections is informed by anatomical data.•Callosal connections play a conditional driving role depending on the brain state and behavioral ...demands.•Callosal connections play a modulatory function, in addition to a driving role.•The corpus callosum participates in learning and interhemispheric transfer of sensorimotor habits.•The corpus callosum contributes to language processing and cognitive functions.
The brain operates through the synaptic interaction of distant neurons within flexible, often heterogeneous, distributed systems. Histological studies have detailed the connections between distant neurons, but their functional characterization deserves further exploration. Studies performed on the corpus callosum in animals and humans are unique in that they capitalize on results obtained from several neuroscience disciplines. Such data inspire a new interpretation of the function of callosal connections and delineate a novel road map, thus paving the way toward a general theory of cortico-cortical connectivity. Here we suggest that callosal axons can drive their post-synaptic targets preferentially when coupled to other inputs endowing the cortical network with a high degree of conditionality. This might depend on several factors, such as their pattern of convergence-divergence, the excitatory and inhibitory operation mode, the range of conduction velocities, the variety of homotopic and heterotopic projections and, finally, the state-dependency of their firing. We propose that, in addition to direct stimulation of post-synaptic targets, callosal axons often play a conditional driving or modulatory role, which depends on task contingencies, as documented by several recent studies.
Abstract
Quantum extreme learning machines (QELMs) aim to efficiently post-process the outcome of fixed — generally uncalibrated — quantum devices to solve tasks such as the estimation of the ...properties of quantum states. The characterisation of their potential and limitations, which is currently lacking, will enable the full deployment of such approaches to problems of system identification, device performance optimization, and state or process reconstruction. We present a framework to model QELMs, showing that they can be concisely described via single effective measurements, and provide an explicit characterisation of the information exactly retrievable with such protocols. We furthermore find a close analogy between the training process of QELMs and that of reconstructing the effective measurement characterising the given device. Our analysis paves the way to a more thorough understanding of the capabilities and limitations of QELMs, and has the potential to become a powerful measurement paradigm for quantum state estimation that is more resilient to noise and imperfections.
ABSTRACT
In addition to neuronal death and elimination of synapses, the production of transient, exuberant axons, and axonal branches is a general phenomenon in development across species and ...systems. To understand what drives the decision of which axons are maintained and which are eliminated, it is important to monitor the interaction of juvenile axons at their target. As old and more recent work show, unlike what is claimed by Ribeiro Gomez et al. (2019), in the cerebral cortex, both classes of axons branch in the white matter near the target; axons destined to be maintained massively invade the gray matter where they develop terminal arbors and synapses. Axons destined to elimination remain in the white matter although a few transient, exploratory branches can enter the cortex. Axonal behavior and fate seem dictated by positional information probably conveyed by thalamic afferents and activity. Unlike what is suggested by Ribeiro Gomez et al. (2019), axonal selection should not be confused with synaptic reduction, which is a later event with minor or no impact on the topography of the connection.
OBJECTIVES
Previous studies concerning ultrasound evaluation of the seminal vesicles (SV) were performed on a limited series of subjects, and considered few parameters, often only before ejaculation ...and without assessing the patients' sexual abstinence. The aim of this study was to evaluate the volume and the emptying characteristics of the SV and their possible correlations with scrotal and transrectal ultrasound features.
METHODS
The SV of 368 men seeking medical care for couple infertility were evaluated by ultrasound. All patients underwent, during the same ultrasound session, scrotal and transrectal evaluation, before and after ejaculation, and the ejaculate was subjected to semen analysis. A new parameter, SV ejection fraction, calculated as: (SV volume before ejaculation − SV volume after ejaculation)/SV volume before ejaculation × 100, was evaluated.
RESULTS
After adjusting for sexual abstinence and age, both pre-ejaculatory SV volume and SV ejection fraction were positively associated with ejaculate volume. As assessed by receiver operating characteristic curve, a cut-off for SV ejection fraction of 21.6% discriminates subjects with normal ejaculate volume (≥1.5 ml) and pH (≥7.2 ml) with both sensitivity and specificity equal to 75%. Subjects with SV ejection fraction of <21.6% more often had a higher post-ejaculatory SV volume and ejaculatory duct abnormalities. Furthermore, a higher post-ejaculatory SV volume was associated with a higher prostate volume and SV abnormalities. Higher epididymal and deferential diameters were also detected in subjects with a higher post-ejaculatory SV volume or reduced SV ejection fraction. No association between SV and testis ultrasound features or sperm parameters was observed. Associations with SV ejection fraction were confirmed in nested 1:1 case–control analysis.
CONCLUSIONS
The SV contribute significantly to the ejaculate volume. A new parameter, SV ejection fraction, could be useful in assessing SV emptying. A SV ejection fraction of <21.6% was associated with prostate–vesicular and epididymal ultrasound abnormalities.
The Zeitgeist favors an interpretation of schizophrenia as a condition of abnormal connectivity of cortical neurons, particularly in the prefrontal and temporal cortex. The available evidence points ...to reduced connectivity, a possible consequence of excessive synaptic pruning in development. A decreased thalamic input to the cerebral cortex appears likely, and developmental studies predict that this decrease should entail a secondary loss of both long- and short-range cortico-cortical connections, including connections between the hemispheres. Indeed, morphological, electrophysiological and neuropsychological studies over the last two decades suggest that the callosal connections are altered in schizophrenics. However, the alterations are subtle and sometimes inconsistent across studies, and need to be investigated further with new methodologies.
A
ll brain operations are implemented by networks of neurons. Unfortunately, the networks underlying even the most elementary brain operations remain elusive. This is due to the complexity of the ...networks, their heterogeneity, and to the multiple computations performed by the axons. Poffenberger’s paradigm is one example of a simple task aimed at characterizing the temporal properties of an interhemispheric network which has remained elusive to this day.
Extracting microanatomical information beyond the image resolution of MRI would provide valuable tools for diagnostics and neuroscientific research. A number of mathematical models already suggest ...microstructural interpretations of diffusion MRI (dMRI) data. Examples of such microstructural features could be cell bodies and neurites, e.g. the axon's diameter or their orientational distribution for global connectivity analysis using tractography, and have previously only been possible to access through conventional histology of post mortem tissue or invasive biopsies. The prospect of gaining the same knowledge non-invasively from the whole living human brain could push the frontiers for the diagnosis of neurological and psychiatric diseases. It could also provide a general understanding of the development and natural variability in the healthy brain across a population. However, due to a limited image resolution, most of the dMRI measures are indirect estimations and may depend on the whole chain from experimental parameter settings to model assumptions and implementation.
Here, we review current literature in this field and highlight the integrative work across anatomical length scales that is needed to validate and trust a new dMRI method. We encourage interdisciplinary collaborations and data sharing in regards to applying and developing new validation techniques to improve the specificity of future dMRI methods.
•Validation methods should challenge the interpretation of dMRI.•dMRI contain information beyond the specificity of single validation techniques.•Sharing data, methods and software are key for dMRI development.
The functional organization of the parieto-frontal system is crucial for understanding cognitive-motor behavior and provides the basis for interpreting the consequences of parietal lesions in humans ...from a neurobiological perspective. The parieto-frontal connectivity defines some main information streams that, rather than being devoted to restricted functions, underlie a rich behavioral repertoire. Surprisingly, from macaque to humans, evolution has added only a few, new functional streams, increasing however their complexity and encoding power. In fact, the characterization of the conduction times of parietal and frontal areas to different target structures has recently opened a new window on cortical dynamics, suggesting that evolution has amplified the probability of dynamic interactions between the nodes of the network, thanks to communication patterns based on temporally-dispersed conduction delays. This might allow the representation of sensory-motor signals within multiple neural assemblies and reference frames, as to optimize sensory-motor remapping within an action space characterized by different and more complex demands across evolution.
•Propose a method for quantitative evaluation of tractography algorithm performances.•Investigate the effect of the tractography method on connectivity estimation.•Study white matter bottleneck ...configurations in an ex-vivo macaque brain.•Provide a 59-areas tracer-based connectivity matrix and parcellation for the macaque.
Display omitted
Diffusion-weighted magnetic resonance imaging (DW-MRI) tractography is a non-invasive tool to probe neural connections and the structure of the white matter. It has been applied successfully in studies of neurological disorders and normal connectivity. Recent work has revealed that tractography produces a high incidence of false-positive connections, often from “bottleneck” white matter configurations. The rich literature in histological connectivity analysis studies in the macaque monkey enables quantitative evaluation of the performance of tractography algorithms. In this study, we use the intricate connections of frontal, cingulate, and parietal areas, well established by the anatomical literature, to derive a symmetrical histological connectivity matrix composed of 59 cortical areas. We evaluate the performance of fifteen diffusion tractography algorithms, including global, deterministic, and probabilistic state-of-the-art methods for the connectivity predictions of 1711 distinct pairs of areas, among which 680 are reported connected by the literature. The diffusion connectivity analysis was performed on a different ex-vivo macaque brain, acquired using multi-shell DW-MRI protocol, at high spatial and angular resolutions. Across all tested algorithms, the true-positive and true-negative connections were dominant over false-positive and false-negative connections, respectively. Moreover, three-quarters of streamlines had endpoints location in agreement with histological data, on average. Furthermore, probabilistic streamline tractography algorithms show the best performances in predicting which areas are connected. Altogether, we propose a method for quantitative evaluation of tractography algorithms, which aims at improving the sensitivity and the specificity of diffusion-based connectivity analysis. Overall, those results confirm the usefulness of tractography in predicting connectivity, although errors are produced. Many of the errors result from bottleneck white matter configurations near the cortical grey matter and should be the target of future implementation of methods.
PDF
