A principle for learning egocentric-allocentric transformation

Numerous single-unit recording studies have found mammalian hippocampal neurons that fire selectively for the animal's location in space, independent of its orientation. The population of such neurons, commonly known as place cells, is thought to maintain an allocentric, or orientation-independent, internal representation of the animal's location in space, as well as mediating long-term storage of spatial memories. The fact that spatial information from the environment must reach the brain via sensory receptors in an inherently egocentric, or viewpoint-dependent, fashion leads to the question of how the brain learns to transform egocentric sensory representations into allocentric ones for long-term memory storage. Additionally, if these long-term memory representations of space are to be useful in guiding motor behavior, then the reverse transformation, from allocentric to egocentric coordinates, must also be learned. We propose that orientation-invariant representations can be learned by neural circuits that follow two learning principles: minimization of reconstruction error and maximization of representational temporal inertia. Two different neural network models are presented that adhere to these learning principles, the first by direct optimization through gradient descent and the second using a more biologically realistic circuit based on the restricted Boltzmann machine (Hinton, 2002; Smolensky, 1986). Both models lead to orientation-invariant representations, with the latter demonstrating place-cell-like responses when trained on a linear track environment.     

Noniterative compensation for the distance-dependent detector response and photon attenuation in SPECT imaging

A filtering approach is described, which accurately compensates for the 2D distance-dependent detector response, as well as for photon attenuation in a uniform attenuating medium. The filtering method is based on the frequency distance principle (FDP) which states that points in the object at a specific source-to-detector distance provide the most significant contribution to specified frequency regions in the discrete Fourier transform (DFT) of the sinogram. By modeling the detector point spread function as a 2D Gaussian function whose width is dependent on the source-to-detector distance, a spatially variant inverse filter can be computed and applied to the 3D DFT of the set of all sinogram slices. To minimize noise amplification the inverse filter is rolled off at high frequencies by using a previously published Wiener filter strategy. Attenuation compensation is performed with Bellini's method. It was observed that the tomographic point response, after distance-dependent filtering with the FDP, was approximately isotropic and varied substantially less with position than that obtained with other correction methods. Furthermore, it was shown that processing with this filtering technique provides reconstructions with minimal degradation in image fidelity.     

Arginine 206 of the C5a receptor is critical for ligand recognition and receptor activation by C-terminal hexapeptide analogs

C5a is a 74-amino-acid glycoprotein whose receptor is a member of the rhodopsin superfamily. While antagonists have been generated to many of these receptors, similar efforts directed at family members whose natural ligands are proteins have met with little success. The recent development of hexapeptide analogs of C5a has allowed us to begin elucidation of the molecular events that lead to activation by combining a structure/activity study of the ligand with receptor mutagenesis. Removal of the hexapeptide's C-terminal arginine reduces affinity by 100-fold and eliminates the ability of the ligand to activate the receptor. Both the guanidino side chain and the free carboxyl of the arginine participate in the interaction. The guanidino group makes the energy-yielding contact with the receptor, while the free carboxylate negates "electrostatic" interference with Arg-206 of the receptor. It is the apparent movement Arg-206 induced by this set of interactions that is responsible for activation, since conversion of Arg-206 to alanine eliminates the agonist activity of the hexapeptides. Surprisingly, activation is a nearly energy-neutral event and may reflect the binding process rather than the final resting site of the ligand.     

The genome of Treponema pallidum: new light on the agent of syphilis

Covert brain activity related to task-free, spontaneous (i.e. unrequested), emotional evaluation of human face images was analysed in 27-channel averaged event-related potential (ERP) map series recorded from 18 healthy subjects while observing random sequences of face images without further instructions. After recording, subjects self-rated each face image on a scale from "liked" to "disliked". These ratings were used to dichotomize the face images into the affective evaluation categories of "liked" and "disliked" for each subject and the subjects into the affective attitudes of "philanthropists" and "misanthropists" (depending on their mean rating across images). Event-related map series were averaged for "liked" and "disliked" face images and for "philanthropists" and "misanthropists". The spatial configuration (landscape) of the electric field maps was assessed numerically by the electric gravity center, a conservative estimate of the mean location of all intracerebral, active, electric sources. Differences in electric gravity center location indicate activity of different neuronal populations. The electric gravity center locations of all event-related maps were averaged over the entire stimulus-on time (450 ms). The mean electric gravity center for disliked faces was located (significant across subjects) more to the right and somewhat more posterior than for liked faces. Similar differences were found between the mean electric gravity centers of misanthropists (more right and posterior) and philanthropists. Our neurophysiological findings are in line with neuropsychological findings, revealing visual emotional processing to depend on affective evaluation category and affective attitude, and extending the conclusions to a paradigm without directed task.     

Electrophysiological evidence for a postperceptual locus of suppression during the attentional blink

When an observer detects a target in a rapid stream of visual stimuli, there is a brief period of time during which the detection of subsequent targets is impaired. In this study, event-related potentials (ERPs) were recorded from normal adult observers to determine whether this "attentional blink" reflects a suppression of perceptual processes or an impairment in postperceptual processes. No suppression was observed during the attentional blink interval for ERP components corresponding to sensory processing (the P1 and N1 components) or semantic analysis (the N400 component). However, complete suppression was observed for an ERP component that has been hypothesized to reflect the updating of working memory (the P3 component). Results indicate that the attentional blink reflects an impairment in a postperceptual stage of processing.     

Paramagnetic 1H-NMR relaxation probes of stereoselectivity in metalloporphyrin catalyzed olefin epoxidation

Enantioselective catalytic epoxidation of olefins is an important problem from both practical and mechanistic points of view. The origins of chiral induction by asymmetric porphyrin and salen complexes were investigated by FT-NMR T1 relaxation techniques. A new chiral vaulted porphyrin (1) that carries (S)-binaphthyl-L-alanine straps across both faces of the porphyrin macrocycle was synthesized and characterized. (R)-styrene oxide was obtained in > 90% ee in the initial stages of styrene epoxidation with F5PhIO catalyzed by 1-Fe(III)Cl. The transition state for olefin epoxidation with high-valent metal-oxo species was modeled by coordinating epoxides to paramagnetic copper complexes of the corresponding ligands. The epoxide enantiomer that better fit the chiral cavity of the catalyst, as revealed by T1 relaxation measurements, was also the major product of catalytic olefin epoxidation. These results are consistent with the "lock-and-key" mechanism of asymmetric catalysis by metalloporphyrins. The copper complex of a chiral salen ligand showed no differentiation in terms of T1 relaxation rates between the enantiomers of cis-beta-methylstyrene oxide in contrast to the high enantioselectivity observed for catalytic epoxidation.     

Periodic orbits: a new language for neuronal dynamics

A new nonlinear dynamical analysis is applied to complex behavior from neuronal systems. The conceptual foundation of this analysis is the abstraction of observed neuronal activities into a dynamical landscape characterized by a hierarchy of "unstable periodic orbits" (UPOs). UPOs are rigorously identified in data sets representative of three different levels of organization in mammalian brain. An analysis based on UPOs affords a novel alternative method of decoding, predicting, and controlling these neuronal systems.     

Model selection for covariance structures analysis in nursing research

Covariance structures analysis is often used in nursing research to appraise statistical models reflecting complex human health processes. The model selection approach in covariance structures analysis is designed to select the "best" model from a specified set of theoretically defensible, competing alternatives, all of which are viewed as approximations. Model selection criteria explicitly incorporate both model misfit in the population and sampling error to evaluate the set of models. The result is that interpretability of model parameters and goodness-of-fit are enhanced simultaneously. Relative merits of the model selection approach are identified in light of technical concerns, parsimony, and use of scientific theory in nursing.     

Juvenile Tillaux fracture simulating syndesmosis separation: a case report

Determination of proliferative activity of non-Hodgkin's lymphomas (NHL), aimed at improving the prediction of their clinical behavior, has gained considerable attention in the recent years. Flow cytometry has allowed rapid measurement of the cellular DNA content in terms of ploidy and proliferative activity. Flow cytometric DNA analysis was performed on paraffin embedded biopsy specimens taken from 125 patients with NHL. In 90 of them, proliferative index (PI) could be accurately measured and correlated with histology grade of the Working Formulation (WF). Intermediate and high grade NHL (54 patients) were analyzed together as HG-NHL. With the discrimination point for PI of 10%, the survival of high and low proliferative lymphomas was compared in the whole NHL group and within the WF prognostic groups. The median PI was 5% in LG (low grade) NHL and 10% in HG (high grade) NHL group. Acturial survival in NHL with high proliferative activity (39 patients) was 31% at 5 years and 15% at 10 years, and in NHL with low proliferative activity (51 patients) 53% and 18%, respectively (p = 0.002). In HG-NHL, survival at 5 years for low proliferative cases was 55% and for high proliferative cases 28% (p = 0.065), whereas in the LG-NHL group it was 54% and 28%, respectively (p = 0.059). The survival at 10 years was nearly equal in all groups. Proliferative index was associated with the overall survival of NHL in the whole group, as well as within the LG and HG prognostic categories. PI could differentiate more and less aggressive NHLs both within LG-NHL and HG-NHL. A tendency of survival curves toward continuous relapse was observed in low proliferative NHL and a tendency toward "plateau" in high proliferative NHL, irrespective of the histology grade.