Working memory enhancement using real-time phase-tuned transcranial alternating current stimulation.

BACKGROUND: Prior work has shown that transcranial alternating current stimulation (tACS) of parietooccipital alpha oscillations (8 - 14 Hz) can modulate working memory (WM) performance as a function of the phase lag to endogenous oscillations. However, leveraging this effect using real-time phase-tuned tACS has not been feasible so far due to stimulation artifacts preventing continuous phase tracking. OBJECTIVES/HYPOTHESIS: We aimed to develop a system that tracks and adapts the phase lag between tACS and ongoing parietooccipital alpha oscillations in real-time. We hypothesized that such real-time phase-tuned tACS enhances working memory performance, depending on the phase lag. METHODS: We developed real-time phase-tuned closed-loop amplitude-modulated tACS (CLAM-tACS) targeting parietooccipital alpha oscillations. CLAM-tACS was applied at six different phase lags relative to ongoing alpha oscillations while participants (N = 21) performed a working memory task. To exclude that behavioral effects of CLAM-tACS were mediated by other factors such as sensory co-stimulation, a second group of participants (N = 25) received equivalent stimulation of the forehead. RESULTS: WM accuracy improved in a phase lag dependent manner (p = 0.0350) in the group receiving parietooccipital stimulation, with the strongest enhancement observed at 330° phase lag between tACS and ongoing alpha oscillations (p = 0.00273, d = 0.976). Moreover, across participants, modulation of frontoparietal alpha oscillations correlated both in amplitude (p = 0.0248) and phase (p = 0.0270) with the modulation of WM accuracy. No such effects were observed in the control group receiving frontal stimulation. CONCLUSIONS: Our results demonstrate the feasibility and efficacy of real-time phase-tuned CLAM-tACS in modulating both brain activity and behavior, thereby paving the way for further investigation into brain-behavior relationships and the exploration of innovative therapeutic applications.

The Prevalence and Association of Biofilms With Otitis Media With Effusion: A Systematic Review and Meta-Analysis.

PURPOSE: We aimed to identify the role of bacterial biofilms in the chronicity of otitis media with effusion and its resistance to antibiotics. We illustrated this role by reviewing, analyzing, and correlating the findings with the results of the included studies to reach clear evidence. METHODS: A comprehensive search of electronic databases (Scopus, PubMed, Web of Science, Cochrane, and GHL databases) was performed for all studies using the following strategy till April 2021 with the search terms: Biofilm and Middle ear effusion. We found 935 references, 421 were duplicates, and 514 were needed for further screening, and it was as follows: PubMed 215, Scopus 18, Cochrane 130, Web of Science 136, and GHL 15. RESULTS: The pooled prevalence of culture-positive effusions was estimated to be 40% (95% CI [28%, 53%]) of the total OME population. Overall, the prevalence of PCR-positive effusions was estimated to be 97% (95% CI [95%, 99%]) of the total OME population. The pooled prevalence of EM-positive effusions was estimated to be 82% (95% CI [69%, 95%]) of the total OME population. CONCLUSION: The data presented in this study coincide with the significant role of bacterial biofilms in the pathogenesis of chronic otitis media with effusion. The involvement of bacterial biofilm as a component of the OME pathogenic process can help us to explain why antimicrobial therapy is not always effective in the eradication of the disease process and, also explain the recurrence of middle ear effusion after treatment with tympanostomy tubes either with or without adenoidectomy.

Invasive fungal rhinosinusitis associated with COVID-19: Course changes and prognosis predictors / Rinosinusitis fúngica invasiva asociada con COVID-19: cambios clínicos y predictores de pronóstico

Background This study aimed to analyze the behavior of acute invasive fungal rhinosinusitis (AIFRS) associated with COVID-19 infection as there has been an increase in the rate of AIFRS cases in the last two years, and many reports connected this rising with the COVID-19 infection. We studied most factors that may impact the prognosis as a trial to find the most affecting factors to improve the outcomes. Methods It was a retrospective observational study that included cases from four tertiary referral institutions between November 2020 to February 2022. We included sixty-six patients who suffered from AIFRS associated with confirmed COVID-19. We observed the prognosis of all included patients with a six-month follow-up. We correlated the prognosis with many factors, such as demographic data, medical conditions, blood investigations, the features of fungal infections, and management. Results Forty-two patients (64%) survived after the AIFRS associated with COVID-19, and twenty-two patients (36%) died. High doses of corticosteroids with prolonged use were the main factors that affected the behavior of the AIFRS associated with COVID-19. HbA1c was a good predictor of the prognosis; a level less than 9.35% may indicate survival with 87.5% sensitivity. Conclusions According to this multi-center study, the mortality of the AIFRS associated with COVID-19 was high. The behavior was affected by glycemic control, the type of fungal species, and the type of antifungal therapy. Early surgical debridement, a combination of Amphotericin B with Voriconazole, and anticoagulants helped improve the prognosis. (AU)

Antecedentes Durante el periodo de la pandemia de COVID19, ha habido un aumento en la tasa de casos de rinosinusitis fúngica invasiva aguda (RSFIA), siendo cada vez más evidente la asociación entre ambas entidades. EL objetivo de este estudio ha sido analizar la evolución de los pacientes con rinosinusitis fúngica invasiva aguda asociado con la infección por COVID-19, analizando los factores determinantes en la evolución y pronóstico de la enfermedad. Métodos Fue un estudio observacional retrospectivo que incluyó casos de cuatro instituciones de referencia de tercer nivel entre noviembre de 2020 y febrero de 2022. Se incluyeron sesenta y seis pacientes que padecían RSFIA asociado a COVID-19 confirmado. Observamos el pronóstico de todos los pacientes incluidos con un seguimiento de seis meses. Correlacionamos el pronóstico con muchos factores, como los datos demográficos, las condiciones médicas, las investigaciones de sangre, las características de las infecciones fúngicas y el manejo. Resultados Cuarenta y dos pacientes (64%) sobrevivieron después de la RSFIA asociada a COVID-19, y veintidós pacientes (36%) fallecieron. Las dosis altas de corticoides con uso prolongado fueron los principales factores que afectaron el comportamiento de la RSFIA asociada a la COVID-19. HbA1c fue un buen predictor del pronóstico; un nivel inferior al 9,35 % puede indicar supervivencia con una sensibilidad del 87.5%. Conclusiones Según este estudio multicéntrico, la mortalidad de la RSFIA asociada a la COVID-19 fue alta. El comportamiento se vio afectado por el control glucémico, el tipo de especie fúngica y el tipo de terapia antifúngica. El desbridamiento quirúrgico temprano, una combinación de Amphotericin B con Voriconazole y anticoagulantes ayudaron a mejorar el pronóstico. (AU)

In vivo phase-dependent enhancement and suppression of human brain oscillations by transcranial alternating current stimulation (tACS).

Transcranial alternating current stimulation (tACS) can influence perception and behavior, with recent evidence also highlighting its potential impact in clinical settings, but its underlying mechanisms are poorly understood. Behavioral and indirect physiological evidence indicates that phase-dependent constructive and destructive interference between the applied electric field and brain oscillations at the stimulation frequency may play an important role, but in vivo validation during stimulation was unfeasible because stimulation artifacts impede single-trial assessment of brain oscillations during tACS. Here, we attenuated stimulation artifacts to provide evidence for phase-dependent enhancement and suppression of visually evoked steady state responses (SSR) during amplitude-modulated tACS (AM-tACS). We found that AM-tACS enhanced and suppressed SSR by 5.77 ± 2.95%, while it enhanced and suppressed corresponding visual perception by 7.99 ± 5.15%. While not designed to investigate the underlying mechanisms of this effect, our study suggests feasibility and superiority of phase-locked (closed-loop) AM-tACS over conventional (open-loop) AM-tACS to purposefully enhance or suppress brain oscillations at specific frequencies.

Invasive fungal rhinosinusitis associated with COVID-19: Course changes and prognosis predictors.

BACKGROUND: This study aimed to analyze the behavior of acute invasive fungal rhinosinusitis (AIFRS) associated with COVID-19 infection as there has been an increase in the rate of AIFRS cases in the last two years, and many reports connected this rising with the COVID-19 infection. We studied most factors that may impact the prognosis as a trial to find the most affecting factors to improve the outcomes. METHODS: It was a retrospective observational study that included cases from four tertiary referral institutions between November 2020 to February 2022. We included sixty-six patients who suffered from AIFRS associated with confirmed COVID-19. We observed the prognosis of all included patients with a six-month follow-up. We correlated the prognosis with many factors, such as demographic data, medical conditions, blood investigations, the features of fungal infections, and management. RESULTS: Forty-two patients (64%) survived after the AIFRS associated with COVID-19, and twenty-two patients (36%) died. High doses of corticosteroids with prolonged use were the main factors that affected the behavior of the AIFRS associated with COVID-19. HbA1c was a good predictor of the prognosis; a level less than 9.35% may indicate survival with 87.5% sensitivity. CONCLUSIONS: According to this multi-center study, the mortality of the AIFRS associated with COVID-19 was high. The behavior was affected by glycemic control, the type of fungal species, and the type of antifungal therapy. Early surgical debridement, a combination of Amphotericin B with Voriconazole, and anticoagulants helped improve the prognosis.

Non-invasive Imaging of Antisense Oligonucleotides in the Brain via In Vivo Click Chemistry.

PURPOSE: The treatment of complex neurological diseases often requires the administration of large therapeutic drugs, such as antisense oligonucleotide (ASO), by lumbar puncture into the intrathecal space in order to bypass the blood-brain barrier. Despite the growing number of ASOs in clinical development, there are still uncertainties regarding their dosing, primarily around their distribution and kinetics in the brain following intrathecal injection. The challenge of taking measurements within the delicate structures of the central nervous system (CNS) necessitates the use of non-invasive nuclear imaging, such as positron emission tomography (PET). Herein, an emergent strategy known as "pretargeted imaging" is applied to image the distribution of an ASO in the brain by developing a novel PET tracer, [18F]F-537-Tz. This tracer is able to undergo an in vivo "click" reaction, covalently binding to a trans-cyclooctene conjugated ASO. PROCEDURES: A novel small molecule tracer for pretargeted PET imaging of ASOs in the CNS is developed and tested in a series of in vitro and in vivo experiments, including biodistribution in rats and non-human primates. RESULTS: In vitro data and extensive in vivo rat data demonstrated delivery of the tracer to the CNS, and its successful ligation to its ASO target in the brain. In an NHP study, the slow tracer kinetics did not allow for specific binding to be determined by PET. CONCLUSION: A CNS-penetrant radioligand for pretargeted imaging was successfully demonstrated in a proof-of-concept study in rats, laying the groundwork for further optimization.

Breaking the boundaries of interacting with the human brain using adaptive closed-loop stimulation.

The human brain is arguably one of the most complex systems in nature. To understand how it operates, it is essential to understand the link between neural activity and behavior. Experimental investigation of that link requires tools to interact with neural activity during behavior. Human neuroscience, however, has been severely bottlenecked by the limitations of these tools. While invasive methods can support highly specific interaction with brain activity during behavior, their applicability in human neuroscience is limited. Despite extensive development in the last decades, noninvasive alternatives have lacked spatial specificity and yielded results that are commonly fraught with variability and replicability issues, along with relatively limited understanding of the neural mechanisms involved. Here we provide a comprehensive review of the state-of-the-art in interacting with human brain activity and highlight current limitations and recent efforts to overcome these limitations. Beyond crucial technical and scientific advancements in electromagnetic brain stimulation, new frontiers in interacting with human brain activity such as task-irrelevant sensory stimulation and focal ultrasound stimulation are introduced. Finally, we argue that, along with technological improvements and breakthroughs in noninvasive methods, a paradigm shift towards adaptive closed-loop stimulation will be a critical step for advancing human neuroscience.

Spontaneous representation of numerosity zero in a deep neural network for visual object recognition.

Conceiving "nothing" as a numerical value zero is considered a sophisticated numerical capability that humans share with cognitively advanced animals. We demonstrate that representation of zero spontaneously emerges in a deep learning neural network without any number training. As a signature of numerical quantity representation, and similar to real neurons from animals, numerosity zero network units show maximum activity to empty sets and a gradual decrease in activity with increasing countable numerosities. This indicates that the network spontaneously ordered numerosity zero as the smallest numerical value along the number line. Removal of empty-set network units caused specific deficits in the network's judgment of numerosity zero, thus reflecting these units' functional relevance. These findings suggest that processing visual information is sufficient for a visual number sense that includes zero to emerge and explains why cognitively advanced animals with whom we share a nonverbal number system exhibit rudiments of numerosity zero.

Advancing sensory neuroprosthetics using artificial brain networks.

Implementation of effective brain or neural stimulation protocols for restoration of complex sensory perception, e.g., in the visual domain, is an unresolved challenge. By leveraging the capacity of deep learning to model the brain's visual system, optic nerve stimulation patterns could be derived that are predictive of neural responses of higher-level cortical visual areas in silico. This novel approach could be generalized to optimize different types of neuroprosthetics or bidirectional brain-computer interfaces (BCIs).

A set of electroencephalographic (EEG) data recorded during amplitude-modulated transcranial alternating current stimulation (AM-tACS) targeting 10-Hz steady-state visually evoked potentials (SSVEP).

Transcranial alternating current stimulation (tACS) can affect perception, learning and cognition, but the underlying mechanisms are not well understood. A promising strategy to elucidate these mechanisms aims at applying tACS while electric or magnetic brain oscillations targeted by stimulation are recorded. However, reconstructing brain oscillations targeted by tACS remains a challenging problem due to stimulation artifacts. Besides lack of an established strategy to effectively supress such stimulation artifacts, there are also no resources available that allow for the development and testing of new and effective tACS artefact suppression algorithms, such as adaptive spatial filtering using beamforming or signal-space projection. Here, we provide a full dataset comprising encephalographic (EEG) recordings across six healthy human volunteers who underwent 10-Hz amplitude-modulated tACS (AM-tACS) during a 10-Hz steady-state visually evoked potential (SSVEP) paradigm. Moreover, data and scripts are provided related to the validation of a novel stimulation artefact suppression strategy, Stimulation Artifact Source Separation (SASS), removing EEG signal components that are maximally different in the presence versus absence of stimulation. Besides including EEG single-trial data and comparisons of 10-Hz brain oscillatory phase and amplitude recorded across three conditions (condition 1: no stimulation, condition 2: stimulation with SASS, condition 3: stimulation without SASS), also power spectra and topographies of SSVEP amplitudes across all three conditions are presented. Moreover, data is provided for assessing nonlinear modulations of the stimulation artifact in both time and frequency domains due to heartbeats. Finally, the dataset includes eigenvalue spectra and spatial patterns of signal components that were identified and removed by SASS for stimulation artefact suppression at the target frequency. Besides providing a valuable resource to assess properties of AM-tACS artifacts in the EEG, this dataset allows for testing different artifact rejection methods and offers in-depth insights into the workings of SASS.

Investigating new CT contrast agents: a phantom study exploring quantification and differentiation methods for high-Z elements using dual-energy CT.

OBJECTIVES: To develop a dual-energy CT method for differentiating and quantifying high-Z contrast elements and to evaluate the limitations based on element concentration and atomic number by using an anthropomorphic phantom study. METHODS: Mass spectrometry standards for iodine, barium, gadolinium, ytterbium, tantalum, gold, and bismuth were diluted from 10.0 to 0.3 mg/mL, placed inside 7-mL vials, and scanned with dual-energy CT using an abdominal phantom and cylindrical water-filled insert. This procedure was repeated with all seven high-Z elements at six isoattenuating values from 250 to 8 HU. Quantification accuracy was measured using a linear regression model and residual error analysis with 90% limits of agreement. The limit of detection for each element was evaluated using the limit of blank of water. Pairwise differentiation of isoattenuating vials was evaluated using AUC values and the difference in fit angles between the two elements. RESULTS: Each high-Z element had a unique concentration vector in a two-dimensional plot of Compton scattering versus photoelectric effect attenuations. Mean quantification values were within ± 0.1 mg/mL of the true values for each element with no proportional bias. Limits of detection ranged from 0.35 to 0.56 mg/mL. Pairwise differentiations were proportional to the isoattenuating HU and the angle between the linear fits with mean AUC values increasing from 0.61 to 0.98 at 8 to 250 HU, respectively. CONCLUSION: Dual-energy CT can differentiate and quantify isoattenuating high-Z elements. The high-attenuation characteristics and unique concentration vectors of ytterbium, tantalum, gold, and bismuth are well suited for new dual-energy CT contrast agents especially when simultaneously imaged with iodine, barium, or gadolinium. KEY POINTS: • Dual-energy CT can accurately quantify high-Z contrast elements and readily differentiate iodine, barium, and gadolinium from ytterbium, tantalum, gold, and bismuth. • The differentiation and quantification capabilities for high-Z contrast elements are largely unaffected by phantom size and transaxial location within the phantom. • Potential benefits of new CT contrast agents based on these high-Z elements include alternatives for patients with iodine sensitivity, high conspicuity at both 120 and 140 kVp, simultaneous imaging of two contrast agents, and reduced injection volume.

Stimulation artifact source separation (SASS) for assessing electric brain oscillations during transcranial alternating current stimulation (tACS).

Brain oscillations, e.g. measured by electro- or magnetoencephalography (EEG/MEG), are causally linked to brain functions that are fundamental for perception, cognition and learning. Recent advances in neurotechnology provide means to non-invasively target these oscillations using frequency-tuned amplitude-modulated transcranial alternating current stimulation (AM-tACS). However, online adaptation of stimulation parameters to ongoing brain oscillations remains an unsolved problem due to stimulation artifacts that impede such adaptation, particularly at the target frequency. Here, we introduce a real-time compatible artifact rejection algorithm (Stimulation Artifact Source Separation, SASS) that overcomes this limitation. SASS is a spatial filter (linear projection) removing EEG signal components that are maximally different in the presence versus absence of stimulation. This enables the reliable removal of stimulation-specific signal components, while leaving physiological signal components unaffected. For validation of SASS, we evoked brain activity with known phase and amplitude using 10 Hz visual flickers across 7 healthy human volunteers. 64-channel EEG was recorded during and in absence of 10 Hz AM-tACS targeting the visual cortex. Phase differences between AM-tACS and the visual stimuli were randomized, so that steady-state visually evoked potentials (SSVEPs) were phase-locked to the visual stimuli but not to the AM-tACS signal. For validation, distributions of single-trial amplitude and phase of EEG signals recorded during and in absence of AM-tACS were compared for each participant. When no artifact rejection method was applied, AM-tACS stimulation artifacts impeded assessment of single-trial SSVEP amplitude and phase. Using SASS, amplitude and phase of single trials recorded during and in absence of AM-tACS were comparable. These results indicate that SASS can be used to establish adaptive (closed-loop) AM-tACS, a potentially powerful tool to target various brain functions, and to investigate how AM-tACS interacts with electric brain oscillations.

Number detectors spontaneously emerge in a deep neural network designed for visual object recognition.

Humans and animals have a "number sense," an innate capability to intuitively assess the number of visual items in a set, its numerosity. This capability implies that mechanisms to extract numerosity indwell the brain's visual system, which is primarily concerned with visual object recognition. Here, we show that network units tuned to abstract numerosity, and therefore reminiscent of real number neurons, spontaneously emerge in a biologically inspired deep neural network that was merely trained on visual object recognition. These numerosity-tuned units underlay the network's number discrimination performance that showed all the characteristics of human and animal number discriminations as predicted by the Weber-Fechner law. These findings explain the spontaneous emergence of the number sense based on mechanisms inherent to the visual system.

Separating High-Z Oral Contrast From Intravascular Iodine Contrast in an Animal Model Using Dual-Layer Spectral CT.

RATIONALE AND OBJECTIVES: To show that water and iodine two-material decomposition images from dual-layer dual-energy spectral X-ray computed tomography (DECT) can be used to separate intravascular iodine contrast from simultaneously administered oral tantalum, tungsten, or rhenium contrast in an animal model. MATERIALS AND METHODS: In this Institutional Animal Care and Use Committee approved study, four female Fischer rats were given simultaneous intravenous and oral X-ray computed tomography contrast. Intravenous iodine contrast was administered via tail vein injection. Oral barium, tantalum, tungsten, or rhenium contrast was administered via gavage. The animals were imaged on a dual-layer DECT system at 120 kVp. Water and iodine two-material decomposition images (water equivalent and iodine equivalent images) were used for qualitative analysis. Computer simulations were performed using a customized DECT simulator to better understand why certain high-Z elements disappear in the iodine equivalent images and what is the theoretical range of elements with this property. RESULTS: The iodine and barium contrast appeared only in the iodine equivalent images and could not be differentiated from each other. However, the tantalum, tungsten, and rhenium contrast only appeared in the water equivalent images. This allowed iodine contrast in the bowel wall to be easily segmented from tantalum, tungsten, and rhenium contrast in the bowel lumen. Simulations confirmed that certain high-Z elements will have pixel values of ≤0 mg iodine/mL in the iodine equivalent images due to a K-edge effect associated with DECT systems. CONCLUSIONS: Dual-layer DECT can separate iodine from certain high-Z elements using water equivalent and iodine equivalent images with an increased element range compared to other DECT systems. This K-edge effect could promote the development and approval of new high-Z contrast agents for DECT.

Breast Tumor Microcalcification Induced by Bone Morphogenetic Protein-2: A New Murine Model for Human Breast Tumor Diagnosis.

Widespread use of screening mammography has recently increased the detection of breast microcalcifications. These nonpalpable microcalcifications with specific features in breast tissues are clinically considered an early indicator of breast carcinoma. Our goal in this study was to develop a murine breast microcalcification model for optimizing in vivo imaging. Recombinant human BMP-2 was expressed in E. coli, and the purified bioactive protein was used as inducing factor for the production of breast microcalcifications in a murine animal model. Syngeneic breast tumors were obtained by injection of MDA-MB-231 human breast cancer cells with Matrigel into the mammary fat pad of female nude mice. Different doses of bioactive rhBMP-2 were administered either as single or multiple intraperitoneal injections or directly into tumor on a weekly basis. Three weeks after the first injection of rhBMP-2, the microcalcification of breast tumor was detected by microcomputed tomography followed by intravenous injection of radiotracer [18F] Sodium fluoride for positron emission tomography imaging. Our findings indicate that rhBMP-2 induced microcalcifications of breast tumor by both systemic and direct injection of rhBMP-2 into tumors in a dose-dependent manner. Although little is known about the molecular mechanism of microcalcification, here we report a new murine model of human breast tumor induced microcalcification by rhBMP-2 to optimize in vivo imaging methods and to study the role of BMP-2 as a mediator of pathological mineralization and bone-like microcalcification formation in breast tumor. This BMP-2-induced microcalcification model may allow us to discriminate the type of microcalcification in tumors and to perform quantitative analysis on the calcification as a new detection strategy for early identification of pathological mineralization of breast tissues in women.

Pseudoenhancement effects on iodine quantification from dual-energy spectral CT systems: A multi-vendor phantom study regarding renal lesion characterization.

PURPOSE: To measure the effect of pseudoenhancement on spectral CT iodine quantification as a function of lesion size, lesion iodine level, background iodine level, helical versus axial scanning, and spectral CT scanner type in a phantom model. MATERIALS AND METHODS: A custom-built water-filled cylindrical phantom contained either six small vials (8 mm diameter) or six large vials (27 mm diameter) of aqueous iopamidol solutions (0, 0.5, 1.0, 2.0, 4.0 and 6.0 mg iodine/mL). The background iodine concentration was 0, 5, or 10 mg iodine/mL. Helical and axial scans were taken on three different dual-energy spectral CT scanners (two image-based and one projection-based) with the scan parameters consistent between the systems. ROIs were used to measure the average iodine concentration of the vials in the 36 individual scans. Linear fits of the true versus measured iodine values were used for pvalue statistical analysis. Having a y-intercept or slope p-value less than 0.05 implied statistically significant iodine quantification errors. RESULTS: Iodine quantification pseudoenhancement effects are inversely proportional to lesion size and lesion enhancement and are directly proportional to background attenuation level. No significant differences between helical and axial scans were observed. 100% and 88% of the slope and y-intercept p-values were below 0.05 for the two image-based systems, while 13% of the slope and y-intercept p-values were below 0.05 for the projection-based system. CONCLUSIONS: Pseudoenhancement can artificially increase spectral CT iodine quantification levels most notably for small low-enhancing lesions (<5.0 mg iodine/mL) surrounded by a high attenuating background (10 mg iodine/mL). In this study we found iodine quantification to be more accurate on projection-based spectral CT systems than image-based systems.

Interspike interval analysis and spikelets in presubicular head-direction cells.

Head-direction (HD) neurons are thought to provide the mammalian brain with an internal sense of direction. These cells, which selectively increase their firing when the animal's head points in a specific direction, use the spike rate to encode HD with a high signal-to-noise ratio. In the present work, we analyzed spike train features of presubicular HD cells recorded juxtacellularly in passively rotated rats. We found that HD neurons could be classified into two groups on the basis of their propensity to fire spikes at short interspike intervals. "Bursty" neurons displayed distinct spike waveforms and were weakly but significantly more modulated by HD compared with "nonbursty" cells. In a subset of HD neurons, we observed the occurrence of spikelets, small-amplitude "spike-like" events, whose HD tuning was highly correlated to that of the co-recorded juxtacellular spikes. Bursty and nonbursty HD cells, as well as spikelets, were also observed in freely moving animals during natural behavior. We speculate that spike bursts and spikelets might contribute to presubicular HD coding by enhancing its accuracy and transmission reliability to downstream targets. NEW & NOTEWORTHY We provide evidence that presubicular head-direction (HD) cells can be classified into two classes (bursty and nonbursty) on the basis of their propensity to fire spikes at short interspike intervals. Bursty cells displayed distinct electrophysiological properties and stronger directional tuning compared with nonbursty neurons. We also provide evidence for the occurrence of spikelets in a subset of HD cells. These electrophysiological features (spike bursts and spikelets) might contribute to the precision and robustness of the presubicular HD code.

Manipulating Hippocampal Place Cell Activity by Single-Cell Stimulation in Freely Moving Mice.

Learning critically depends on the ability to rapidly form and store non-overlapping representations of the external world. In line with their postulated role in episodic memory, hippocampal place cells can undergo a rapid reorganization of their firing fields upon contextual manipulations. To explore the mechanisms underlying such global remapping, we juxtacellularly stimulated 42 hippocampal neurons in freely moving mice during spatial exploration. We found that evoking spike trains in silent neurons was sufficient for creating place fields, while in place cells, juxtacellular stimulation induced a rapid remapping of their place fields to the stimulus location. The occurrence of complex spikes was most predictive of place field plasticity. Our data thus indicate that plasticity-inducing stimuli are able to rapidly bias place cell activity, simultaneously suppressing existing place fields. We propose that such competitive place field dynamics could support the orthogonalization of the hippocampal map during global remapping.

Testing the Efficacy of Single-Cell Stimulation in Biasing Presubicular Head Direction Activity.

To support navigation, the firing of head direction (HD) neurons must be tightly anchored to the external space. Indeed, inputs from external landmarks can rapidly reset the preferred direction of HD cells. Landmark stimuli have often been simulated as excitatory inputs from "visual cells" (encoding landmark information) to the HD attractor network; when excitatory visual inputs are sufficiently strong, preferred directions switch abruptly to the landmark location. In the present work, we tested whether mimicking such inputs via juxtacellular stimulation would be sufficient for shifting the tuning of individual presubicular HD cells recorded in passively rotated male rats. We recorded 81 HD cells in a cue-rich environment, and evoked spikes trains outside of their preferred direction (distance range, 11-178°). We found that HD tuning was remarkably resistant to activity manipulations. Even strong stimulations, which induced seconds-long spike trains, failed to induce a detectable shift in directional tuning. HD tuning curves before and after stimulation remained highly correlated, indicating that postsynaptic activation alone is insufficient for modifying HD output. Our data are thus consistent with the predicted stability of an HD attractor network when anchored to external landmarks. A small spiking bias at the stimulus direction could only be observed in a visually deprived environment in which both average firing rates and directional tuning were markedly reduced. Based on this evidence, we speculate that, when attractor dynamics become unstable (e.g., under disorientation), the output of HD neurons could be more efficiently controlled by strong biasing stimuli.SIGNIFICANCE STATEMENT The activity of head direction (HD) cells is thought to provide the mammalian brain with an internal sense of direction. To support navigation, the firing of HD neurons must be anchored to external landmarks, a process thought to be supported by associative plasticity within the HD system. Here, we investigated these plasticity mechanisms by juxtacellular stimulation of single HD neurons in vivo in awake rats. We found that HD coding is strongly resistant to external manipulations of spiking activity. Only in a visually deprived environment was juxtacellular stimulation able to induce a small activity bias in single presubicular neurons. We propose that juxtacellular stimulation can bias HD tuning only when competing anchoring inputs are reduced or not available.

Amplifying the sensitivity of zinc(II) responsive MRI contrast agents by altering water exchange rates.

Given the known water exchange rate limitations of a previously reported Zn(II)-sensitive MRI contrast agent, GdDOTA-diBPEN, new structural targets were rationally designed to increase the rate of water exchange to improve MRI detection sensitivity. These new sensors exhibit fine-tuned water exchange properties and, depending on the individual structure, demonstrate significantly improved longitudinal relaxivities (r1). Two sensors in particular demonstrate optimized parameters and, therefore, show exceptionally high longitudinal relaxivities of about 50 mM(-1) s(-1) upon binding to Zn(II) and human serum albumin (HSA). This value demonstrates a 3-fold increase in r1 compared to that displayed by the original sensor, GdDOTA-diBPEN. In addition, this study provides important insights into the interplay between structural modifications, water exchange rate, and kinetic stability properties of the sensors. The new high relaxivity agents were used to successfully image Zn(II) release from the mouse pancreas in vivo during glucose stimulated insulin secretion.