Drifting assemblies for persistent memory: Neuron transitions and unsupervised compensation.

Change is ubiquitous in living beings. In particular, the connectome and neural representations can change. Nevertheless, behaviors and memories often persist over long times. In a standard model, associative memories are represented by assemblies of strongly interconnected neurons. For faithful storage these assemblies are assumed to consist of the same neurons over time. Here we propose a contrasting memory model with complete temporal remodeling of assemblies, based on experimentally observed changes of synapses and neural representations. The assemblies drift freely as noisy autonomous network activity and spontaneous synaptic turnover induce neuron exchange. The gradual exchange allows activity-dependent and homeostatic plasticity to conserve the representational structure and keep inputs, outputs, and assemblies consistent. This leads to persistent memory. Our findings explain recent experimental results on temporal evolution of fear memory representations and suggest that memory systems need to be understood in their completeness as individual parts may constantly change.

Dynamical Learning of Dynamics.

The ability of humans and animals to quickly adapt to novel tasks is difficult to reconcile with the standard paradigm of learning by slow synaptic weight modification. Here, we show that fixed-weight neural networks can learn to generate required dynamics by imitation. After appropriate weight pretraining, the networks quickly and dynamically adapt to learn new tasks and thereafter continue to achieve them without further teacher feedback. We explain this ability and illustrate it with a variety of target dynamics, ranging from oscillatory trajectories to driven and chaotic dynamical systems.

Altered Dynamics of Canonical Feedback Inhibition Predicts Increased Burst Transmission in Chronic Epilepsy.

Inhibitory interneurons, organized into canonical feedforward and feedback motifs, play a key role in controlling normal and pathological neuronal activity. We demonstrate prominent quantitative changes in the dynamics of feedback inhibition in a rat model of chronic epilepsy (male Wistar rats). Systematic interneuron recordings revealed a large decrease in intrinsic excitability of basket cells and oriens-lacunosum moleculare interneurons in epileptic animals. Additionally, the temporal dynamics of interneuron recruitment by recurrent feedback excitation were strongly altered, resulting in a profound loss of initial feedback inhibition during synchronous CA1 pyramidal activity. Biophysically constrained models of the complete feedback circuit motifs of normal and epileptic animals revealed that, as a consequence of altered feedback inhibition, burst activity arising in CA3 is more strongly converted to a CA1 output. This suggests that altered dynamics of feedback inhibition promote the transmission of epileptiform bursts to hippocampal projection areas.SIGNIFICANCE STATEMENT We quantitatively characterized changes of the CA1 feedback inhibitory circuit in a model of chronic temporal lobe epilepsy. This study shows, for the first time, that dynamic recruitment of inhibition in feedback circuits is altered and establishes the cellular mechanisms for this change. Computational modeling revealed that the observed changes are likely to systematically alter CA1 input-output properties leading to (1) increased seizure propagation through CA1 and (2) altered computation of synchronous CA3 input.

Bridging structure and function: A model of sequence learning and prediction in primary visual cortex.

Recent experiments have demonstrated that visual cortex engages in spatio-temporal sequence learning and prediction. The cellular basis of this learning remains unclear, however. Here we present a spiking neural network model that explains a recent study on sequence learning in the primary visual cortex of rats. The model posits that the sequence learning and prediction abilities of cortical circuits result from the interaction of spike-timing dependent plasticity (STDP) and homeostatic plasticity mechanisms. It also reproduces changes in stimulus-evoked multi-unit activity during learning. Furthermore, it makes precise predictions regarding how training shapes network connectivity to establish its prediction ability. Finally, it predicts that the adapted connectivity gives rise to systematic changes in spontaneous network activity. Taken together, our model establishes a new conceptual bridge between the structure and function of cortical circuits in the context of sequence learning and prediction.

Performance evaluation of the PelvoCheck CT/NG test kit for the detection of Chlamydia trachomatis and Neisseria gonorrhoeae.

OBJECTIVE: Assessment of the performance of the PelvoCheck CT/NG test (Greiner-Bio-One GmbH) to detect Chlamydia trachomatis (CT) and Neisseria gonorrhoeae (NG) in first-void urine (FVU) of females. DESIGN: A cross-sectional study to compare the PelvoCheck CT/NG with COBAS TaqMan CT Test V.2.0 (Roche) for the detection of CT and with an in-house porA-based PCR for the detection of NG in FVU specimens. In addition, pools of 5 FVU specimens containing only CT-negative or 1 CT-positive and 4 CT-negative samples were tested. Abbott RealTime CT/NG was used as an additional test to resolve discordant results. SETTING: Samples sent from six laboratories were tested at the University Medical Center Hamburg. PARTICIPANTS: Urine samples were from 1622 female patients attending gynaecological practices for chlamydia screening, another 120 urine samples were from patients pretested for NG at Synlab, Medical Service Center, Weiden GmbH. In addition, 50 urine samples spiked with various concentrations of reference material were used. RESULTS: For the detection of CT and NG, the sensitivity and specificity of the PelvoCheck CT/NG test were 98.8% and 100%, and 98.3% and 98.2%, respectively. The data obtained with the PelvoCheck CT/NG for pooled urine specimens resulted in a positive agreement of 90.9% and a negative agreement of 100%. CONCLUSIONS: The PelvoCheck CT/NG assay is a suitable test method for the detection of CT and NG in female FVU samples, with sensitivity and specificity comparable with other Food and Drug Administration approved CT/NG nucleic acid amplification tests. To the best of our knowledge, this is the first commercial test system validated for the analysis of pooled urine specimens. No false-positive or invalid result was observed in 55 analysed pools. Nevertheless, 5 samples were false negative due to a target concentration below the limit of detection of the PelvoCheck CT/NG test as a consequence of pooling-associated dilution.