PDF《Enlarging炉he爏cope：爂rasping?》

s brain (and body). Collective power of neuronal synchrony We will come back to information transmission later, but let us now explore the matter of spatial scales. As humans tend to agree, increased size makes up for smarter brains (disclosure: both authors are human), and those bigger brains have room to organize themselves at multiple levels, coalescing into functional ensembles at several steps along the way up from neurons to functional areas and to the entire brain [9,10,11]. At larger and more integrated levels of description, other ordering phenomena were discovered that brain scientists conceive in terms of information exchange as well. In the late nineteen eighties, two groups of scientists, one with Reinhard Eckhorn [12] and another with Charles Gray and Wolf Singer [13], discovered that perceptual integration (or Gestalt) elicited transiently synchronous action potentials amongst neurons that had shared- stakes in the sensory object being viewed. Those neurons dealt with separate parts of the visual field, and they generally disagreed on when to elicit their action potentials in the regular course of their participation in visual function. Somehow however, through the complex labyrinth of the visual cortex and despite the fact that some finite amount of time was required to get from any one to any other of them (delays and frustrations manifested in their usual asynchrony), they managed to coincide when they responded to the same object. What we knew from those neurons is that they responded strongly to orientation, fragments of contours with sharp luminance gradients. Their synchrony it seems, was a trace of their joint participation in the construction of something bigger (the object) than what each of them was about (pieces of contour). These discoveries resonated with earlier theorizing regarding the collective behavior of neurons such as Donald Hebb'

s cell assemblies [14] or Walter Freeman'

s mass action [15]. The findings by Eckhorn, Singer and Gray launched a relentless quest for synchrony in all parts of the brain and for numerous functions [16], and took the form of several variants (the most basic being coincidence of action potentials and phase-locking of neural oscillations). Irreconcilables Theories and dedicated experimental paradigms were built upon both discoveries of synaptic transmission and neural synchronization. And from each side, supporting evidence abounded. In spite of their prominence and ubiquity though, the theories carefully avoided confrontation with each other, remaining mostly in the separate territories of distinct research groups. One may note already some difficulties in reconciling them. Let us follow the two extreme views: perfect synchronization and perfect transfer. If all neurons were completely synchronized, they would remain in a changeless state of simultaneity. It is unclear how this system could have flows of information from one place to another. On the other end, if each neuron relayed information in a strict sense, the system would lack basic simultaneity through which synchronous phenomena could emerge. In their radical form it seems, the theories of information exchange qua synaptic transfer or neural synchrony are mutually exclusive. Can we find directions in the brain? The tension is also visible in some empirical facts. Although directed flows of information in the Shannonian spirit do most certainly occur in neural networks, it is indeed quite challenging to track information otherwise than in local or statistical sense (by tracking, we mean to follow the path of information on a brain map as one would follow any object in motion on a symbolic representation of its spatial domain C see figure 1). The brain 3? ? network after all, is a web, as Francisco Varela emphasized [17], and one gets quickly lost with all the branchings, loops and loops within loops [18,19];