Degeneracy, the ability of elements that are structurally different to perform the same function or yield the same output, is a well known characteristic of the genetic code and immune systems. Here, we point out that degeneracy is a ubiquitous biological property and argue that it is a feature of complexity at genetic, cellular, system, and population levels. Furthermore, it is both necessary for, and an inevitable outcome of, natural selection.

A neuronal network inspired by the anatomy of the cerebral cortex was simulated to study the self-organization of spiking neurons into neuronal groups. The network consisted of 100 000 reentrantly interconnected neurons exhibiting known types of cortical firing patterns, receptor kinetics, short-term plasticity and long-term spike-timing-dependent plasticity (STDP), as well as a distribution of axonal conduction delays. The dynamics of the network allowed us to study the fine temporal structure of emerging firing patterns with millisecond resolution. We found that the interplay between STDP and conduction delays gave rise to the spontaneous formation of neuronal groups--sets of strongly connected neurons capable of firing time-locked, although not necessarily synchronous, spikes. Despite the noise present in the model, such groups repeatedly generated patterns of activity with millisecond spike-timing precision. Exploration of the model allowed us to characterize various group properties, including spatial distribution, size, growth, rate of birth, lifespan, and persistence in the presence of synaptic turnover. Localized coherent input resulted in shifts of receptive and projective fields in the model similar to those observed in vivo.

Several observations suggest that the Ca2 -dependent postsynaptic release of nitric oxide (NO) may be important in the formation and function of the vertebrate nervous system. We explore here the hypothesis that the release of NO and its subsequent diffusion may be critically related to three aspects of nervous system function: (i) synaptic plasticity and long-term potentiation in certain regions of the adult nervous system, (i) the control of cerebral blood flow in such regions, and (iii) the establishment and activity-dependent refinement of axonal projections during the later stages of development. In this paper, we detail and analyze the basic assumptions underlying this NO hypothesis and describe a computer simulation of a minimal version of the hypothesis. In the simulation, a 3-dimensional volume of neuropil is presented with patterned afferent input; NO is produced, diffuses, and is destroyed; and synaptic strengths are determined by a set of synaptic rules based on the correlation of synaptic depolarization and NO levels. According to the hypothesis, voltagedependent postsynaptic release of this rapidly diffusing substance links the activities of neurons in a local volume of tissue, regardless of whether the neurons are directly connected by synapses. This property is demonstrated in the simulation, and it is this property that is exploited in the hypothesis to account for certain aspects of long-term potentiation and activitydependent sharpening of axonal arbors.Recent experiments indicate that nitric oxide (NO) is produced in the granule cells of the cerebellum in response to glutamate application (1,2). Other work demonstrates that NO is the endothelial-derived relaxing factor responsible for relaxation of vascular smooth muscle (3). The Ca2+-dependent enzyme nitric oxide synthetase is found in the vertebrate forebrain (4) as well as in the cerebellum (5). Cortical activity is well known to be accompanied by regionally specific changes in blood flow, and therefore we suggest that NO production in a cortical region is one cause of these local vascular responses.There are reasons to suggest an even more fundamental role for NO in altering synaptic efficacy within various brain regions in the adult as well as in synaptogenesis during development. For example, a body of data has accumulated demonstrating that the synaptic changes of long-term potentiation (LTP) result from the temporal correlation of presynaptic activity and postsynaptic depolarization (6). A number of studies support the notion that LTP is the result of separate presynaptic and postsynaptic components, both of which can contribute to its induction and maintenance (6-11). In development, the segregation and refinement of afferent projections also critically depends upon correlated electrical activity and synaptic changes both within afferent projections and between the afferents and their targets (12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22). The Nmethyl-D-aspartate receptor and excitatory transmission in general have been implica...

The Dynamic Core and Global Workspace hypotheses were independently put forward to provide mechanistic and biologically plausible accounts of how brains generate conscious mental content. The Dynamic Core proposes that reentrant neural activity in the thalamocortical system gives rise to conscious experience. Global Workspace reconciles the limited capacity of momentary conscious content with the vast repertoire of long-term memory. In this paper we show the close relationship between the two hypotheses. This relationship allows for a strictly biological account of phenomenal experience and subjectivity that is consistent with mounting experimental evidence. We examine the constraints on causal analyses of consciousness and suggest that there is now sufficient evidence to consider the design and construction of a conscious artifact.