Dissociated Hippocampal Neurons Implicate Rate--论文代写范文

这篇生物paper代写范文得出这样的结论，编码不能靠长时间的分层网络传播。分层的网络一直是关注的焦点,是信息传递的主要框架,尽管这些系统结构简单,显示各种复杂现象和感兴趣的主要是皮质组织。下面的paper代写范文进行论述。

Abstract
  The ability of synchronous population activity in layered networkstotransmit a rate code is a focus of recent debate. We investigatethese issues using a patterned unidimensional hippocampal culture. The network exhibits population bursts that travel its full length, with the advantage that signals propagate along a clearly defined path. The amplitudes of activity are measured using calcium imaging, a good approximate of population rate code, and the distortion of the signal as it travels is analyzed. We demonstrate that propagation along the line is precisely described by information theory as a chain of Gaussian communication channels. The balance of excitatory and inhibitory synapses is crucial for this transmission. However, amplitude information carried along this layered neuronal structure fails within 3 mm,  10 mean axon lengths, and is limited by noise in the synaptic transmission. We conclude that rate codes cannot be reliably transmitted through long layered networks. 
  Key words: layered network; hippocampal culture; rate coding; Shannon information; Gaussian chain; inhibition; one-dimensional network

 Introduction 
  Layered networks have been the focus of much attention and constitute a major framework for discussion of information transfer (Wilson and Cowan, 1973; Miles et al., 1988; Abeles, 1991; Ermentrout and McLeod, 1993; Idiart and Abbott, 1993; Traub et al., 1993; Shadlen and Newsome, 1994, 1998; Diesmann et al., 1999; van Rossum et al., 2002; Litvak et al., 2003; Reyes, 2003). Despite their simple structure, these systems display a variety of complex phenomena and are primarily of interest because they give a reasonable model of cortical organization (Mountcastle, 1957; Wilson and Cowan, 1973; Christian and Dudek, 1988; Miles et al., 1988; Abeles, 1991; Traub et al., 1993; Mountcastle, 1997). 

  Because it is impractical to follow information transfer directly in the cortex, computer simulation and theoretical modeling were a natural substitute. Using layered networks for analysis of coding was suggested by Abeles (1991), and shortly thereafter such a system was directly applied for an in depth discussion of coding strategies (Shadlen and Newsome, 1994, 1998). They investigated whether rate (Adrian and Zotterman, 1926; Stein, 1967; Barlow, 1972) or temporal (Abeles and Gerstein, 1988; Abeles et al., 1993; Bair and Koch, 1996; van Steveninck et al., 1997; Hatsopoulos et al., 1998) codes are the preferred mode for reliable information transfer. 

  They showed that, in the presence of an excitatory/inhibitory balance, the accumulating transmission errors could be significantly reduced by averaging over the activity of large neuronal populations (Mountcastle, 1957, 1997) and concluded that rate coding is the preferred cortical coding strategy. Diesmann et al. (1999) realized that, in such systems, activity tends to synchronize and is characterized by large-amplitude population bursts termed “synfire” (Abeles, 1991). Although this leads to large-scale correlations that destroy the advantages of rate-code averages, it may still allow exact temporal patterns to reliably pass through the layers (Diesmann et al., 1999; Litvak et al., 2003). 

  Their conclusion is that rate code cannot be used in layered cortical structures. although temporal schemes cannot be ruled out. The first attempt to build a fully accessible layered network that incorporates experimental data were performed by Reyes (2003). He incorporated a single recording neuron from a hippocampal slice into a simulation of a layered network, using simulated data to control current injected to the recording neuron. 

  The output response was used in an original bootstrap scheme to calculate the input for the next layer. Using this system, he showed that, despite the evolution of synchrony, information on spike rate was sustained and could be propagated onto as far as 10 layers. In this paper, we study coding efficiency using a fully experimental, in vitro system in which dissociated hippocampal neurons are patterned to form a layered structure (Feinerman et al., 2005). This culture may be regarded as a simplified yet biological model of layered structures that yields a direct measurement of neural activity and of information transfer. We identify fluorescence intensities with population spiking rates to allow the exten sion of the experimental results to the stability of rate-coded information progressing along the culture.

 Signal transmission 
  Bursts propagate stably at an average velocity of 55 4.5 mm/s (Feinerman et al., 2005) and can travel distances of 8 cm. We measured the transmission of these bursts in both the stimulated and spontaneous cases. Almost all population bursts traveled the full length of the line to excite activity on its distant edge. The arrival of a signal without regard to its amplitude is a yes/no event that carries information only about its timing. For this reason, we monitor the amplitudes of activity in each burst. More specifically, in stimulated cultures, 88% (157 of 178) of bursts propagated at least 8.5 mm from the point of stimulation, as measured on five cultures. A total of 84% (96 of 114) of locally stimulated population bursts propagated the full 17 mm of the line as measured on four cultures. 

  To identify propagation characteristics in spontaneous activity, three distant areas on a 17 mm line were visualized: its two ends and its central part (Feinerman et al., 2005). A total of 94% (299 of 319 bursts) of bursts propagated to reach two of these three areas (the central part and one of the ends). In this case, bursts had propagated a total of at least 8.5 mm. In 88% of bursts (144 of 164), activity was evident in all three areas. In this case, activity had initiated in a hotspot situated along the line (which we did not attempt to localize), from which it propagated to excite the full 17 mm of the culture. 

  The gain in amplitude when the signal traverses from one ROI to its neighbor must, on average, be close to 1. Any deviation from unity gain at the transfer between ROIs would be exponentiated by 17⁄0.3 50 mm (the line length over the mean axonal length in millimeters), leading to either a blow up or decrease in amplitude to 0, neither of which were observed. The fact that the amplitude does not saturate at a maximal value as it progresses along the line can also be seen by the removal of inhibition. Shutting down inhibition by bath application of bicuculline leads to a global rise in amplitude up to approximately a factor of 4 and to an increase of propagation velocities to an average value of 97 10 mm/s (Feinerman et al., 2005). Addition of bicuculline furthermore increases the stability of propagation and allowed 96% of spontaneously induced bursts to reach 8.5 mm (268 of 279), 93% to reach 17 mm (153 of 164), and 83% to reach 45 mm (30 of 36) of the culture.

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