Inherent Crosstalk Research Articles

Resolution enhancement via guided filtering for spatial-frequency multiplexing single-shot high-speed imaging.

The frequency recognition algorithm for multiple exposures (FRAME) is a progressive single-shot high-speed videography technique that employs the spatial-frequency multiplexing concept to provide high temporal and spatial resolution. However, the inherent crosstalk from the zero-frequency component to the carrier-frequency component leads to resolution degradation and artifacts. To improve recovered frames' quality, we propose a FRAME reconstruction method using guided filters for a removal of the zero-frequency component, which can minimize the artifacts while enhance spatial resolution. A total variation (TV) denoising operation is involved to remove artifacts further to achieve optimized performances. Simulations and experiments were conducted to demonstrate the robust and efficient post-processing capability of the proposed method. With a two-frame experimental system, the results of a USAF 1951 resolution target reveal a 1.8-fold improvement in spatial resolution from 16 lp/mm to 28.5 lp/mm. For complex dynamic scenarios, the wide field of high-speed fuel spray was shot and the proposed method can resolve two droplets with a 30 μm distance which outperforms the traditional method.

Experimental Demonstration of Sub-THz Wireless Communications Using Multiplexing of Laguerre-Gaussian Beams When Varying two Different Modal Indices

The ability to multiplex multiple structured beams for sub-terahertz (sub-THz) wireless communications has been recently explored. The spatial orthogonality of structured beams enables mode-division multiplexing (MDM). In an MDM system, multiple data-carrying beams are transmitted and received simultaneously through a single aperture pair with low inherent crosstalk. This can increase data capacity and spectral efficiency of the system. Here, we experimentally demonstrate multiplexing orthogonal sub-THz Laguerre-Gaussian (<inline-formula><tex-math notation="LaTeX">${{\bf L}}{{{\bf G}}_{\ell,{{\bf p}}}}$</tex-math></inline-formula>) beams when varying two different modal indices (i.e., both <inline-formula><tex-math notation="LaTeX">$\ell $</tex-math></inline-formula> and <inline-formula><tex-math notation="LaTeX">${{\bf p}}$</tex-math></inline-formula>); this contrasts with prior reports when varying only one index (i.e., <inline-formula><tex-math notation="LaTeX">$\ell $</tex-math></inline-formula>) and could potentially provide a larger set of channels and beams in an MDM system. In our demonstration, specially designed phase patterns are used: (1) at the transmitter, to convert two THz Gaussian beams to two different LG beams, each carrying an independent data channel; and (2) at the receiver, to separate and convert the LG beams back to Gaussian-like THz beams. An 8-Gbit&#x002F;s quadrature-phase-shift-keying (QPSK) link containing two multiplexed LG modes over 40 cm is experimentally demonstrated. For the above link, three different LG modal sets are chosen (i.e., {<inline-formula><tex-math notation="LaTeX">${{\bf L}}{{{\bf G}}_{ - 2,0}}$</tex-math></inline-formula> and <inline-formula><tex-math notation="LaTeX">${{\bf L}}{{{\bf G}}_{1,1}}$</tex-math></inline-formula>} or {<inline-formula><tex-math notation="LaTeX">${{\bf L}}{{{\bf G}}_{3,0}}$</tex-math></inline-formula> and <inline-formula><tex-math notation="LaTeX">${{\bf L}}{{{\bf G}}_{0,1}}$</tex-math></inline-formula>} or {<inline-formula><tex-math notation="LaTeX">${{\bf L}}{{{\bf G}}_{2,1}}$</tex-math></inline-formula> and <inline-formula><tex-math notation="LaTeX">${{\bf L}}{{{\bf G}}_{0,1}}$</tex-math></inline-formula>}). All channels are recovered with bit errorrates (BERs) below the 7&#x0025; forward error correction (FEC) limit. The experimental results indicate that: (a) higher-order LG modes experience more divergence, larger size, and lower conversion efficiency, contributing to higher power loss; and (b) the modal coupling and crosstalk for different LG modes is &lt;&#x2212;12 dB, which could be due to the transmitter&#x002F;receiver misalignments as well as beam truncation by the limited-sized receiver aperture.

Research on technology of 1550nm all-fiber continuous wave coherent Doppler wind lidar

In order to obtain the telemetry data of wind field 80 m ahead of the wind turbine, an all-fiber continuous wave coherent Doppler lidar (CW-CDL) system is demonstrated in this paper. The CW-CDL of all-fiber structure has advantages of the compact structure, good stability and low cost. However, there is inherent crosstalk in the optical circulator of the optical fiber module, the crosstalk light and the reference light enter the photon detector together to produce phase induced intensity noise (PIIN). Aiming to effectively solve the noise, we proposed a polarization control method by combining a polarization beam splitter and a quarter wave plate to reduce the PIIN in the system. The wind speed Doppler signal at 80 meters is obtained on a sunny day, and the typical signal-to-noise ratio can reach 21.7 dB.

Lighting Simulation in 3D Printing

3D printing for rapid prototyping and production of unique objects is being actively developed. Consumer-grade printers are now commonly available for a range of purposes, while increasingly advanced techniques allow us to fabricate novel shapes, mechanical properties, and appearances. The printers’ capabilities have improved dramatically from printing single-material objects to producing detailed structures with pervoxel material variation. Since the 2010s, it is possible to fabricate full-colour 3D objects with resolutions of hundreds of DPI (voxels’ dimensions are in the order of 10 μm). Such capabilities are most prominent in printers based on the photo-polymer jetting process. Ideally, it should be possible now to produce photorealistic appearances or visually indistinguishable objects copies for, e.g., cultural-heritage applications.However, the resins used as print materials in commercial devices are inherently translucent, i.e., exhibit significant sub-surface scattering. This serves effective colour mixing in full-colour print processes, thus commercial printer drivers offer high-quality colour reproduction. At the same time, the resulting light diffusion leads to over-blurring and potential colour bleeding when printingspatially-varying colour textures. This translucent ‘crosstalk’ between surface points also strongly depends on the internal structure of the volume surrounding each surface point.Previously existing scattering-aware methods used simplified models for light diffusion and accepted the visual blur as an immutable property of the print medium.In this talk, we present the series of works conducted by a consortium of several institutes (Max-Planck Institute for Informatics, Germany; Charles University in Prague, Czech Republic; Institute of Science and Technology, Austria; University Col-lege London, United Kingdom; Universita della Svizzera Italiana, Switzerland; The Keldysh Institute of Applied Mathematics RAS, Russia). Our work counteracts heterogeneous scattering to obtain the impression of a crisp albedo texture on top of the 3D print, by optimizing for a fully volumetric material distribution that preserves the target appearance.We build our iterative method on top of a general Monte-Carlo simulation of heterogeneous scattering. We find out that a certain arrangement of materials expands the gamut of achievable appearances and makes it possible to produce sharp textures. This knowledge built-in into the volume-update step enables convergence justafter 10–15 iterations. We verify these findings using an established stochastic gradient-descent optimization for small canonical objects where it is feasible computationally.Expansion of our method to fabrication of arbitrary 3D objects with the translucent resins opens a set of problems of achievable colour combinations on the two sides of thin shapes, in the extreme convex and concave shapes. Physically correct lighting simulation enables exploration of these extreme cases where no ideal solution is possible. It turns out that a re-formulation of established gamut-mapping methods is needed for the medium with the inherent cross-talk properties such as the scattering resins of the modern full-colour 3D printers.Elaborating further ideas from we also propose a fast forward predictor of the object's surface appearance based on a neural network to replace the Monte-Carlo simulation in order to speed up the preparation of the model by 300 times. The achieved acceleration allows to reduce simulation time to minutes for a single, GPU-equipped workstation.Thismakesthe print preparation timings practical.

"Hiding" a low-intensity 50 Gbit/s QPSK free-space OAM beam using an orthogonal coaxial high-intensity 50 Gbit/s QPSK beam.

In this paper, we experimentally demonstrate an approach that "hides" a low-intensity 50 Gbit/s quadrature-phase-keying (QPSK) free-space optical beam when it coaxially propagates on the same wavelength with an orthogonal high-intensity 50 Gbit/s QPSK optical beam. Our approach is to coaxially transmit the strong and weak beams carrying different orthogonal spatial modes within a modal basis set, e.g., orbital angular momentum (OAM) modes. Although the weak beam has much lower power than that of the strong beam, and the beams are in the same frequency band and on the same polarization, the two beams can still be effectively demultiplexed with little inherent crosstalk at the intended receiver due to their spatial orthogonality. However, an eavesdropper may not readily identify the weak beam when simply analyzing the spatial intensity profile. The correlation coefficient between the intensity profiles of the strong beam and the combined strong and weak beams is measured to characterize the potential for "hiding" a weak beam when measuring intensity profiles. Such a correlation coefficient is demonstrated to be higher than 0.997 when the power difference between the strong fundamental Gaussian beam and the weak OAM beam is ∼8,∼10, and ∼10dB for the weak OAM -1,-2, and -3 beams, respectively. Moreover, a 50 Gbit/s QPSK data link having its Q factor above the 7% forward error correction limit is realized when the power of the weak OAM -3 beam is 30 dB lower than that of the strong fundamental Gaussian beam.

MR contingency supplement prior for joint estimation of activity and attenuation in non‐time‐of‐flight positron emission tomography/MR

Maximum likelihood reconstruction of activity and attenuation (MLAA) from emission data only suffered from the inherent cross-talk between the estimated attenuation and activity distributions. The authors proposed an improved MLAA algorithm by utilising tissue prior atlas (TPA) and a Gibbs prior as prior knowledge. TPA determines the plausible region for each of the typical attenuation coefficients; hence, it imposes statistical condition as a supplement for the exclusive magnetic resonance (MR) information on the reconstruction process of attenuation map. Therefore, along with the soft tissue distribution provided by the segmentation of MR images, an air mask and a bone probability map breakdown the MR low-signal class into four subclasses in order to favour recognition of air and bone. Estimations on attenuation coefficients are realised as a mixture of pseudo-Gaussian distributions. The proposed algorithm is evaluated using the simulated 3D emission data. The proposed MLAA-TPA algorithm is compared with the MR-MLAA algorithm proposed by Heuser et al. Their results demonstrate that the MR-MLAA algorithm performance depends heavily on the MR segmentation accuracy well handled by MLAA-TPA. The quantification results illustrated that the MLAA-TPA outperformed the MR-MLAA algorithm owing to reduction of misclassification and more precise tissue detection.

Least-squares reverse time migration using controlled-order multiple reflections

Reverse time migration (RTM) using multiples generates inherent crosstalk artifacts due to the interference among multiples of different orders. We have developed a method to remove such crosstalk. This approach first separates the recorded seismic data into primary reflections and multiples using the surface-related multiples elimination algorithm and then isolates the multiples into different orders. We can take any specified, say the [Formula: see text]th, order of multiples data as the incident wave and the next higher order multiples data, ([Formula: see text])th order, as the corresponding primary reflection data for imaging. We have applied the least-squares migration scheme to these two successive orders of multiples. Our method is denoted as least-squares RTM using controlled-order multiples (LSRTM-CM). Our numerical tests demonstrated that LSRTM-CM can significantly improve imaging quality compared with straightforward seismic imaging using multiples without multiples separation.

Adiabatic elimination-based coupling control in densely packed subwavelength waveguides.

The ability to control light propagation in photonic integrated circuits is at the foundation of modern light-based communication. However, the inherent crosstalk in densely packed waveguides and the lack of robust control of the coupling are a major roadblock toward ultra-high density photonic integrated circuits. As a result, the diffraction limit is often considered as the lower bound for ultra-dense silicon photonics circuits. Here we experimentally demonstrate an active control of the coupling between two closely packed waveguides via the interaction with a decoupled waveguide. This control scheme is analogous to the adiabatic elimination, a well-known procedure in atomic physics. This approach offers an attractive solution for ultra-dense integrated nanophotonics for light-based communications and integrated quantum computing.

A Hardwired HIV Latency Program

Biological circuits can be controlled by two general schemes: environmental sensing or autonomous programs. For viruses such as HIV, the prevailing hypothesis is that latent infection is controlled by cellular state (i.e., environment), with latency simply an epiphenomenon of infected cells transitioning from an activated to resting state. However, we find that HIV expression persists despite the activated-to-resting cellular transition. Mathematical modeling indicates that HIV's Tat positive-feedback circuitry enables this persistence and strongly controls latency. To overcome the inherent crosstalk between viral circuitry and cellular activation and to directly test this hypothesis, we synthetically decouple viral dependence on cellular environment from viral transcription. These circuits enable control of viral transcription without cellular activation and show that Tat feedback is sufficient to regulate latency independent of cellular activation. Overall, synthetic reconstruction demonstrates that a largely autonomous, viral-encoded program underlies HIV latency—potentially explaining why cell-targeted latency-reversing agents exhibit incomplete penetrance.

Next-Generation Fibers for Space-Division-Multiplexed Transmissions

This paper reviews the most recent advances on next-generation fibers for space-division-multiplexed transmissions. These fibers can be classified into different categories, depending on the way the inherent crosstalk issue is addressed, and have thus different characteristics and limitations. Comparisons of their performances and insights in their different potentials are also provided.

Wrist electrogoniometry: are current mathematical correction procedures effective in reducing crosstalk in functional assessment?

The recording of human movement is an essential requirement for biomechanical, clinical, and occupational analysis, allowing assessment of postural variation, occupational risks, and preventive programs in physical therapy and rehabilitation. The flexible electrogoniometer (EGM), considered a reliable and accurate device, is used for dynamic recordings of different joints. Despite these advantages, the EGM is susceptible to measurement errors, known as crosstalk. There are two known types of crosstalk: crosstalk due to sensor rotation and inherent crosstalk. Correction procedures have been proposed to correct these errors; however no study has used both procedures in clinical measures for wrist movements with the aim to optimize the correction. To evaluate the effects of mathematical correction procedures on: 1) crosstalk due to forearm rotation, 2) inherent sensor crosstalk; and 3) the combination of these two procedures. 43 healthy subjects had their maximum range of motion of wrist flexion/extension and ulnar/radials deviation recorded by EGM. The results were analyzed descriptively, and procedures were compared by differences. There was no significant difference in measurements before and after the application of correction procedures (P<0.05). Furthermore, the differences between the correction procedures were less than 5° in most cases, having little impact on the measurements. Considering the time-consuming data analysis, the specific technical knowledge involved, and the inefficient results, the correction procedures are not recommended for wrist recordings by EGM.

Gas biology: small molecular medicine

In this special issue of the Journal of Molecular Medicine, we present five review articles concerning small molecules that play big roles in physiology and medicine: the gases oxygen (O2), nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H2S). Of these, the essential requirement for O2 is well known to physicians, scientists, and laymen alike. However, it is only within the last two decades that we have begun to understand the molecular mechanisms by which every cell in our body senses the local O2 concentration and responds to reduced O2 availability (i.e., hypoxia) with changes in gene expression that are mediated by the hypoxia-inducible factors HIF-1 and HIF-2 [1]. Recent evidence suggest that NO, CO, and H2S function as signaling molecules that also play critical roles in regulating O2 homeostasis. Philip Marsden and colleagues (University of Toronto) describe the fascinating crosstalk between O2 sensing and NO signaling that occurs to regulate red blood cell levels and blood vessel tone, which together play critical roles in O2 delivery [2]. In particular, they discuss exciting data from their lab demonstrating that the physiological responses to anemic hypoxia (reduced O2 carrying capacity) and hypoxic hypoxia (reduced O2) are mechanistically distinct. This difference is dramatically illustrated by their finding that subjecting mice that lack neuronal NO synthase (also known as nNOS or NOS1) to anemic hypoxia resulted in an increased mortality rate, compared to wild-type mice, whereas these NOS1-deficient mice were protected against mortality during hypoxic hypoxia [3]. Their delineation of the molecular and cellular basis for these effects is truly elegant physiology. The crosstalk between O2 sensing and NO signaling is extensive and includes increased expression of inducible NOS (also known as iNOS or NOS2) under hypoxic conditions that is mediated by HIF-1 [4, 5] and the regulation of HIF-1 activity by S-nitrosylation of a cysteine residue in the HIF-1α subunit [6]. Puneet Anand and Jonathan Stamler (Case Western Reserve University) provide a global view of protein Snitrosylation and its effects on protein function [7]. They describe several different molecular mechanisms by which proteins are nitrosylated and counteracting mechanisms by which they are denitrosylated, which is analogous to the phosphorylation and dephosphorylation of proteins by kinases and phosphatases. The balance between nitrosylation and denitrosylation is dependent on the redox state of the cell, thus establishing inherent crosstalk between NO and reactive oxygen species. The authors discuss several examples of diseases associated with dysregulated S-nitrosylation, which may therefore represent a novel therapeutic target. Makoto Suetmatsu and colleagues (Keio University) discuss the biological role of CO, which is generated by heme oxygenases (HO1 and HO2) using heme and O2 as G. L. Semenza (*) Vascular Program, Institute for Cell Engineering; Departments of Pediatrics, Medicine, Oncology, Radiation Oncology, and Biological Chemistry, The Johns Hopkins University School of Medicine, Baltimore, MD, USA e-mail: gsemenza@jhmi.edu

Transcriptomic Coordination in the Human Metabolic Network Reveals Links between n-3 Fat Intake, Adipose Tissue Gene Expression and Metabolic Health

Understanding the molecular link between diet and health is a key goal in nutritional systems biology. As an alternative to pathway analysis, we have developed a joint multivariate and network-based approach to analysis of a dataset of habitual dietary records, adipose tissue transcriptomics and comprehensive plasma marker profiles from human volunteers with the Metabolic Syndrome. With this approach we identified prominent co-expressed sub-networks in the global metabolic network, which showed correlated expression with habitual n-3 PUFA intake and urinary levels of the oxidative stress marker 8-iso-PGF2α. These sub-networks illustrated inherent cross-talk between distinct metabolic pathways, such as between triglyceride metabolism and production of lipid signalling molecules. In a parallel promoter analysis, we identified several adipogenic transcription factors as potential transcriptional regulators associated with habitual n-3 PUFA intake. Our results illustrate advantages of network-based analysis, and generate novel hypotheses on the transcriptomic link between habitual n-3 PUFA intake, adipose tissue function and oxidative stress.

Simultaneous Reconstruction of Activity and Attenuation for PET/MR

Medical investigations targeting a quantitative analysis of the position emission tomography (PET) images require the incorporation of additional knowledge about the photon attenuation distribution in the patient. Today, energy range adapted attenuation maps derived from computer tomography (CT) scans are used to effectively compensate for image quality degrading effects, such as attenuation and scatter. Replacing CT by magnetic resonance (MR) is considered as the next evolutionary step in the field of hybrid imaging systems. However, unlike CT, MR does not measure the photon attenuation and thus does not provide an easy access to this valuable information. Hence, many research groups currently investigate different technologies for MR-based attenuation correction (MR-AC). Typically, these approaches are based on techniques such as special acquisition sequences (alone or in combination with subsequent image processing), anatomical atlas registration, or pattern recognition techniques using a data base of MR and corresponding CT images. We propose a generic iterative reconstruction approach to simultaneously estimate the local tracer concentration and the attenuation distribution using the segmented MR image as anatomical reference. Instead of applying predefined attenuation values to specific anatomical regions or tissue types, the gamma attenuation at 511 keV is determined from the PET emission data. In particular, our approach uses a maximum-likelihood estimation for the activity and a gradient-ascent based algorithm for the attenuation distribution. The adverse effects of scattered and accidental gamma coincidences on the quantitative accuracy of PET, as well as artifacts caused by the inherent crosstalk between activity and attenuation estimation are efficiently reduced using enhanced decay event localization provided by time-of-flight PET, accurate correction for accidental coincidences, and a reduced number of unknown attenuation coefficients. First results achieved with measured whole body PET data and reference segmentation from CT showed an absolute mean difference of 0.005 cm⁻¹ (< 20%) in the lungs, 0.0009 cm⁻¹ (< 2%) in case of fat, and 0.0015 cm⁻¹ (< 2%) for muscles and blood. The proposed method indicates a robust and reliable alternative to other MR-AC approaches targeting patient specific quantitative analysis in time-of-flight PET/MR.

Improved optical coding method for orthogonal ASK/DPSK modulation format

Orthogonal ASK/DPSK labeling, which encodes label information on optical carriers in a modulation format, is regarded as a competing scheme to subcarrier multiplexed optical labeling. However, the inherent cross-talk limits the extinction ratio (ER) of the payload and label sensitivity. In this paper, two optical coding methods-Manchester coding and 8B10B coding-which can remarkably improve the system quality to obtain acceptable ER are discussed. A novel optical packet encoding method-mark-insertion coding-is demonstrated to significantly reduce cross-talk between the amplitude shift keying (ASK) payload and the differential phase shift keying (DPSK) label through spectrum shaping. The performance of mark-insertion coding depends on the number of inserted ‘marks’. Finally, the transmission over a 40 km single mode fiber (SMF) is compared with optical label swapping for a 40 Gbit/s ASK payload and 2.5 Gbit/s DPSK label under these coding schemes. The experimental result is consistent with the theoretical analysis.

Improving goniometer accuracy by compensating for individual transducer characteristics

Flexible goniometers are useful for direct movement measurements. Crosstalk due to rotation between the endblocks is well known. However, even without any rotation, some crosstalk can occur. The objective of this study was to elucidate the effect of, and compensate for, the inherent crosstalk in biaxial goniometers, with specific relevance for applications with one dominating movement direction. Six biaxial goniometers (M110, Biometrics Ltd., Gwent, UK) were evaluated. A precision jig, for simulating pure flexion/extension angles, was constructed. Each sensor produced a consistent and specific crosstalk pattern, when tested over a ±100° range of motion. A procedure for correction for the inherent crosstalk of individual goniometer, based on polynomial adjust, is presented. The method for compensation, which reduced the root mean square error from, on average for the six goniometers, 3.7° (range 1.8–10.1°) to 0.35° (0.12–0.55°), might be required for obtaining valid goniometer measurements, e.g. of valgus/varus of the knee during gait flexion/extension movements.

Measurements of wrist and forearm positions and movements: effect of, and compensation for, goniometer crosstalk

Flexible biaxial goniometers are extensively used for measuring wrist positions and movements. However, they display an inherent crosstalk error. The aim was to evaluate the effect, of this error, on summary measures used for characterizing manual work. A goniometer and a torsiometer were combined into one device. An algorithm that effectively compensated for crosstalk was developed. Recordings from 25 women, performing five worktasks, were analyzed, both with and without compensation for crosstalk. The errors in the 10th, 50th and 90th percentiles of the flexion/extension distributions were small, on average <1°. The ulnar/radial deviation distributions were weakly dependent on forearm position. The flexion/extension velocity measures were, for the 50th and 90th percentiles, as well as the mean velocity, consistently underestimated by, on average, 3.9%. For ulnar/radial deviation, the velocity errors were less consistent. Mean power frequency, which is a measure of repetitiveness, was insensitive (error <1%) to crosstalk. The forearm supination/pronation angular distributions were wider, and the velocities higher, than for the wrists. Considering wrist/hand exposure in epidemiologic studies, as well as for establishing and surveillance of exposure limits for prevention of work-related upper extremity musculoskeletal disorders, the crosstalk error can, when considering other errors and sources to variation, be disregarded.

Goniometer measurement and computer analysis of wrist angles and movements applied to occupational repetitive work

In epidemiological studies of occupational musculoskeletal disorders there is a need for quantitative exposure measurements of the physical work load. In studies of neck and upper limb disorders the so-called cumulative trauma disorders (CTDs), in particular the carpal tunnel syndrome, the positions and movements of the wrist are of special interest. A biaxial flexible electrogoniometer was used to measure continuously, with a sampling rate of 20 Hz, wrist flexion/extension and abduction/adduction angles up to 27 min. Evaluation of the influence of rotation on the goniometer showed some inherent crosstalk, which, however, did not invalidate the results. For occupational repetitive work, 99.5% of the signal power was contained in the 0–5 Hz band. Two-dimensional angle distributions and power spectra gave comprehensive information about wrist postures and movements. Measures reflecting both static and dynamic properties were derived from time and frequency domains. These measures give quantitative information on different potential risk factors and are therefore useful in epidemiological studies. Mean power frequency (MPF) is suggested as a generalized measure of repetitiveness. The method was applied in field studies of fish processing industry workers with highly repetitive work and risk of CTD.

Local adaptations of two naturally occurring neuronal conductances, gK+(A) and gK+(Ca), allow for associative conditioning and contiguity judgements in artificial neural networks

Features of two potassium conductances implicated in the acquisition of conditioned reflexes, the slow calcium dependent conductance (gK + (Ca] and the fast transient conductance (gK + (A], were incorporated into a 6 x 6 element artificial neural network. Adaptive algorithms derived from observations of cortical neurons during associative learning changed gK + (A) in proportion to the product of this current and an EPSP-induced second messenger concentration, and changed gK + (Ca) as a function of a spike-induced second messenger concentration. This network concurrently acquired two distinct representations in response to presentation of stimuli: one resembled associative conditioning (defined in terms of its sensitivity to forward pairing vs. simultaneous or backward pairing); the other reflected contiguous pairings of stimuli. The acquisition of one representation did not markedly interfere with acquisition of the other. This network may accordingly serve as an example of a self-organizing system which minimizes the postulated inherent cross talk between functionally dissiminar representations (Minsky and Papert 1988).