Max Planck Institute for Brain Research, Frankfurt/M., Germany,
Cogn Neurodyn. 2009 Sep;3(3):189-96. doi: 10.1007/s11571-009-9087-z. Epub 2009 Jun 28.
The cerebral cortex presents itself as a distributed dynamical system with the characteristics of a small world network. The neuronal correlates of cognitive and executive processes often appear to consist of the coordinated activity of large assemblies of widely distributed neurons. These features require mechanisms for the selective routing of signals across densely interconnected networks, the flexible and context dependent binding of neuronal groups into functionally coherent assemblies and the task and attention dependent integration of subsystems. In order to implement these mechanisms, it is proposed that neuronal responses should convey two orthogonal messages in parallel. They should indicate (1) the presence of the feature to which they are tuned and (2) with which other neurons (specific target cells or members of a coherent assembly) they are communicating. The first message is encoded in the discharge frequency of the neurons (rate code) and it is proposed that the second message is contained in the precise timing relationships between individual spikes of distributed neurons (temporal code). It is further proposed that these precise timing relations are established either by the timing of external events (stimulus locking) or by internal timing mechanisms. The latter are assumed to consist of an oscillatory modulation of neuronal responses in different frequency bands that cover a broad frequency range from <2 Hz (delta) to >40 Hz (gamma) and ripples. These oscillations limit the communication of cells to short temporal windows whereby the duration of these windows decreases with oscillation frequency. Thus, by varying the phase relationship between oscillating groups, networks of functionally cooperating neurons can be flexibly configurated within hard wired networks. Moreover, by synchronizing the spikes emitted by neuronal populations, the saliency of their responses can be enhanced due to the coincidence sensitivity of receiving neurons in very much the same way as can be achieved by increasing the discharge rate. Experimental evidence will be reviewed in support of the coexistence of rate and temporal codes. Evidence will also be provided that disturbances of temporal coding mechanisms are likely to be one of the pathophysiological mechanisms in schizophrenia.
大脑皮层呈现出一种分布式动力系统的特征,具有小世界网络的特点。认知和执行过程的神经元相关性似乎常常由广泛分布的神经元的协调活动组成。这些特征需要机制来选择性地在密集互联的网络中路由信号,灵活地将神经元群体绑定到功能上连贯的集合中,并根据任务和注意力将子系统整合在一起。为了实现这些机制,有人提出神经元反应应该并行地传递两个正交的信息。它们应该表明(1)它们调谐的特征的存在,以及(2)它们与哪些其他神经元(特定的靶细胞或连贯集合的成员)进行通信。第一个信息是由神经元的放电频率(速率码)编码的,有人提出第二个信息包含在分布式神经元的单个尖峰之间的精确时间关系中(时间码)。进一步提出,这些精确的时间关系要么由外部事件的时间(刺激锁定)建立,要么由内部时间机制建立。后者被认为由不同频率带中的神经元反应的振荡调制组成,这些频率带覆盖了从<2 Hz(δ)到>40 Hz(γ)和涟漪的广泛频率范围。这些振荡将细胞的通信限制在短的时间窗口内,这些窗口的持续时间随着振荡频率的降低而减小。因此,通过改变振荡群之间的相位关系,可以在硬连线网络内灵活配置功能协作的神经元网络。此外,通过同步神经元群体发出的尖峰,可以增强它们的响应的显著性,因为接收神经元的巧合敏感性与增加放电率非常相似。将回顾实验证据来支持速率和时间码的共存。还将提供证据表明,时间编码机制的干扰可能是精神分裂症的病理生理机制之一。