Gating Input to Visual Cortex by Feedback to LGN

Anatomical studies have documented massive back-projections from higher to lower visual cortices and to the lateral geniculate nucleus (LGN). The large number of synapses from these sources suggest that they should have a profound influence on the information carried by feed-forward inputs to these cells. However, the functional role of these connections is unclear. In order to explore the role of the feedback connections, we have recorded spike trains from electrodes placed in LGN in the macaque monkey under sufenta anesthesia, and have compared LGN cells' activity with and without suppression by cooling of feedback from primary visual cortex (V1). Normally, magno and parvo LGN cells show a wide range over which their responses are proportional to stimulus contrast. Inactivation of V1 feedback causes LGN cells to become more nonlinear and less sensitive to high contrast than during normal conditions. Responses during V1 inactivation have a similar shape to those of retinal ganglion cells. We have also tested the properties of the so-called extended surround as they relate to cortical activity and to influences on responses to LGN stimulation. A model of this data suggests an interpretation in terms of two fnuctional components of feedback: a contrast-dependent component which dominates at high input contrast, and a constant baseline level of inhibitory feedback. We also show that the influence of the extended surround on the classical center mechanism is more complicated than a simple integration model.National Institutes of Health (EY-05156); Office of Naval Research (N00014-95-1-409

On the complex dynamics of intracellular ganglion cell light responses in the cat retina

We recorded intracellular responses from cat retinal ganglion cells to sinusoidal flickering lights and compared the response dynamics to a theoretical model based on coupled nonlinear oscillators. Flicker responses for several different spot sizes were separated in a 'smooth' generator (G) potential and eorresponding spike trains. We have previously shown that the G-potential reveals complex, stimulus dependent, oscillatory behavior in response to sinusoidally flickering lights. Such behavior could be simulated by a modified van der Pol oscillator. In this paper, we extend the model to account for spike generation as well, by including extended Hodgkin-Huxley equations describing local membrane properties. We quantified spike responses by several parameters describing the mean and standard deviation of spike burst duration, timing (phase shift) of bursts, and the number of spikes in a burst. The dependence of these response parameters on stimulus frequency and spot size could be reproduced in great detail by coupling the van der Pol oscillator, and Hodgkin-Huxley equations. The model mimics many experimentally observed response patterns, including non-phase-locked irregular oscillations. Our findings suggest that the information in the ganglion cell spike train reflects both intraretinal processing, simulated by the van der Pol oscillator) and local membrane properties described by Hodgkin-Huxley equations. The interplay between these complex processes can be simulated by changing the coupling coefficients between the two oscillators. Our simulations therefore show that irregularities in spike trains, which normally are considered to be noise, may be interpreted as complex oscillations that might earry information.Whitehall Foundation (S93-24

Data mining and neural network simulations can help to improve deep brain stimulation effects in parkinson’s disease

Parkinson’s Disease (PD) is primary related to substantia nigra degeneration and, thus, dopamine insufficiency. L-DOPA as a precursor of dopamine is the standard medication in PD. However, disease progression causes L-DOPA therapy efficiency decay (on-off symptom fluctuation), and neurologists often decide to classify patients for DBS (Deep Brain Stimulation) surgery. DBS treatment is based on stimulating the specific subthalamic structure: subthalamic nucleus (STN) in our case. As STN consists of parts with different physiological functions, finding the appropriate placement of the DBS electrode contacts is challenging. In order to predict the neurological effects related to different electrodecontact stimulations, we have tracked connections between the stimulated part of STN and the cortex with the help of diffusion tensor imaging (DTI). By changing a contacts number and amplitude of stimulus (proportional in size to stimulated area), we have determined connections to cortical areas and related neurological effects. We have applied data mining methods to predict which contact (and at what amplitude) should be stimulated in order to improve a particular symptom. We have compared different data mining methods: Wekas Random Forest classifier and Rough Set Exploration System (RSES). We have demonstrated that the Weka classifier was more accurate when predicting the effects of stimulations on general neurological improvements, while RSES was more accurate when using specific neurological symptoms. We have simulated other effects of stimulation related to the interruption of pathological oscillation in the basal ganglia found in PD. Our model represents possible STN neural population with inhibitory and excitatory connections that have pathologically synchronized oscillations. High-frequency electrical stimulation has interrupted synchronization. something that is also observed in PD patients

Non-irradiated Bystander Fibroblasts Attenuate Damage to Irradiated Cancer Cells

Introduction and aim: Radiation-induced bystander effect is described as a different type of responses, displayed by non-irradiated neighbouring cells, induced by signals transmitted from irradiated cells. These responses in bystander cells include genetic damage (SCE, micronuclei, genomic instability), apoptosis induction and other non-necessarily detrimental effects. Bystander effect might bear some implications for coexisting normal cells non-targeted by cancer radiotherapy. However, it is possible, that bystander effect can act in opposite direction, and non-irradiated cells can in some way influence the response of targeted cells. Our experiments in vitro were aimed at evaluating this concept. Materials and methods: Lung Lewis carcinoma cells (LLC), growing in monolayer in 6-well plates, were irradiated with 2 or 4 Gy dose of X-rays (using a 6 MeV accelerator suited for therapeutic purposes). After irradiation, the cells were co-cultured with non-irradiated NIH3T3 mice fibroblasts, the latter growing (in monolayer) in special inserts. Such system allowed separation of the two kinds of cells, with medium freely circulating through the separation membrane (pore size 0.4 µm). Thus, species released by irradiated cells could be transmitted to non-irradiated neighbours and vice versa. Results and discussion: The bystander effects, caused by irradiated cancer cells, which were observed in non-targeted fibroblasts included dose-dependent elevation of micronucleus and apoptosis frequency indicating that irradiated cancer cells can induce damage in normal fibroblast cells. However, irradiated LLC cancer cells co-incubated with fibroblasts presented lower levels of this type of cytogenetic damage and apoptosis in comparison with LLC cells incubated after irradiation without fibroblasts on inserts or without inserts at all. Our current studies attempt to search for agents and signalling responsible for observed bilateral bystander effects. Supported by the Polish Ministry of Science and High Education, grant no N406 101/31/387

Spatial receptive field organization of macaque V4 neurons

Subfield analysis of the receptive fields (RFs) of parafoveal V4 complex cells demonstrates directly that most RFs are tiled by overlapping second-order excitatory inputs that for any given V4 cell are predominantly selective to the same preferred values of spatial frequency and orientation. These results extend hierarchical principles of RF organization in the spatial, orientation and spatial frequency domains, first recognized in V1, to an intermediate extrastriate cortex. Spatial interaction studies across subfields demonstrate that the responses of V4 neurons to paired stimuli may either decrease or increase as a function of inter-stimulus distance across the width axis. These intra-RF suppressions and facilitations vary independently in magnitude and spatial extent from cell to cell. These results taken together with the relatively large RF sizes of V4 neurons - as compared with RF sizes of their afferent inputs - lead us to hypothesize a novel property, namely that classes of stimulus configurations that enhance areal summation while reducing suppressive interactions between excitatory inputs will evoke especially robust responses. We tested, and found support for, this hypothesis by presenting stimuli consisting of optimally tuned sine-wave gratings visible only within an annular region and found that such stimuli vigorously activate V4 neurons at firing rates far higher than those evoked by comparable stimuli to either the full-field or central core. On the basis of these results we propose a framework for a new class of neural network models for the spatial RF organizations of prototypic V4 neurons

WiMAX Cell Level Simulation Platform Based on ns-2 and DSP Integration

The WiMAX (Worldwide Interoperability for Microwave Access) system based on the IEEE 802.16 family of standards is a promising technology for last-mile access. Both IEEE 802.16 and 3GPP-LTE systems candidate for becoming the 4G network of choice. The need to evaluate multiple performance enhancing techniques like MIMO, OFDM(A), novel channel coding schemes like non-binary LDPC codes, together with development of standards like IEEE 802.21, that aims at enabling handover and interoperability between heterogeneous network types, make rapid prototyping-based simulations an important issue. This paper presents a novel approach to 4G-oriented simulation environment that integrates popular network simulator (ns-2) and a Digital Signal Processing (DSP) to enable comprehensive link layer and cell level simulations. Proposed simulation environment is intended as an evaluation platform for assessing QoS/QoE and Connection Admission Control (CAC) algorithms designed for WiMAX systems. Moreover we study ways to improve simulation time (with focus on AWGN channel simulation) by using CUDA parallel processing technology for NVIDIA graphic cards