> > Brad Wyble wrote: > > > Heck, even the underlying PC hardware is more complex in a number of > > > ways than the brain, it seems... > > > > The brain is very RISCy... using a relatively simple processing > >> pattern and then repeating it millions of times. > > > > Alan, I strongly suggest you increase your familiarity with > > neuroscience before making such claims in the future. I'm not sure > > what simplified model of the neuron you are using, but be assured that > > there are many layers of complexity of function within even a simple > > neuron, let alone in networks. > > I havn't looked at the neuron in quite a while. =P > But I don't consider myself [completely] insane in this context either. >
The situation for understanding a single neuron is somewhat disastrous. A few weeks of reading peer reviewed papers will leave you feeling awash in a sea of chemical processes, all of which are relevant to the computational characterstics of that neuron. For example, calcium is held in intracellular stores in the post synaptic side. These stores are released at certain voltage thresholds because of certain channel dynamics. Released calcium has a number of effects on channels and internal mechanisms within the synapse. Also, it may surprise you to know that the current dogma regarding synaptic modification involves structural changes. That, synapses grow and die off dynamically throughout your life, on the scale of *hours*. The idea of a synaptic tree that is pruned is obsolete. I'm just trying to give you a taste of the sophistications that are relevant to brain function and cannot be glossed over. > > Dendrites are not simple summators, they have a variety of nonlinear > > processes including recursive, catalytic chemical reactions and complex > > econd-messenger systems. That's just the tip of the iceberg once you > > get into pharmacological subsystems, the complexity becomes a bit > > staggering. > > Yeah, the dendrite _trees_ are quite complex. My interest, however, lies > in the *forest*. ;) > > So the question is: what program is necessary to generate a system with > the same computational charactoristics as the brain? (completely > ignoring the implementation details, most of which are irrelevant or > artifacts of the general implementation strategy). > The "implementation details" are what tells you how the brain functions. We don't know the computational characteristics yet because they are so extraordinarily complex. We don't yet completely understand how a *single synapse* functions. There's been good progress, but it's turning out to be much more complicated than we thought. Many of those metabolic doo-dads that we thought were life support happen to be computationally relevant. > My current understanding draws heavily on the Cerebral Code by William > H. Calvin (assuming I don't have to go all the way over to the shelf to > check the name). Calvin proposes what ammounts to a sophisticated, > optomized Celular Automata. > He's a fine author of pop neuroscience, but in order to be accessible he necessarily glosses over many layers of complexity. It is a mistake to take his simplified representations at face value. He needs to simplify to get his good ideas across. Use the ideas, but don't extrapolate brain functions from his simplistic depictions. > > This system is still too dynamic, we want to ground it in a more stable > system. We create two classes of state, a persistant structural state > and a dynamic state that expresses the present activation of the > persistant state. In almost all higher animals, a sleep period is > required to clear the chaotic dynamic state of the matrix and > re-initialize it from the persistant state. The reset process occours > during delta wave sleep and the re-init process occours during beta wave > sleep. Also during this time, the almost totally unbiased computational > matrix which is the cortex is programmed through a program running on a > small subset of the cortex loaded from what is essentially a ROM being > the Amigdalya and hypothalamus as well as certain structures in the > reticular formation. Incorrect. The cortex has genetically pre-programmed systems. It cannot be said that is a matrix loaded with software from subcortical structures.. > > The neocortex, as far as I know, is fairly uniform in general algorithm. > We only need to "wire" it up slightly differently for each region. I > don't know wheather this applies to the older cortical regions such as > the hypocampus as well. I do know that the latter structures use a > different and moderately less complex algorithm... It is not, in fact, fairly uniform. It varies in architecture (the type & percentage of various cell types as well as layer thickness) as well as by connectivity with other structures. The variations are on the scale of millimeters, so there will be quite alot of them. I've spent 8 years studying hippocampal anatomy. It is fascinating and highly structured in a way the cortex isn't (or its simplicity allows us to perceive the structure). Vast volumes of data about its anatomy are available and I have read most of it. I( and the rest of the hippocampal community) am at a loss to tell you how it functions. Even this one tiny brain piece, much simpler in architecture than the cortical mantle, is beyond our ability to describe at present. My aim here is trying to give you a sense of perspective on where we stand in understanding the brain. When I began studying neuroscience(as a compsci graduate) over 10 years ago, my initial estimation was as optimistic as yours is and I would tell people that complete understanding of the brain was at most 30 years away. On being hit with the facts, day after day, page after page, I came away with one indisputable fact: The brain is H-A-R-D to understand. There are not overarching properties that you can use to simplify its function. Each system is carefully and skillyfully constructed and has to be picked apart piecemeal. I'm trying to give you the benefit of my perspective without costing you 10 years to get it. > That's not a proof at all. The evident fact that nobody has yet tried > the right approach has no relationship to the nature of that correct > approach. > No, it's not proof, but it is evidence. > > > The cortex is highly differentiated containing probably dozens if not > > hundreds of systems, not to mention the enormous variety of specialized > > systems at the subcortical level. > > But how many of these require special code? How many of those are > simply programs that were acquired through learning? (and hence *should* > not be coded by the AI designer.) We don't know. But the fact that everyone tends to pick up more or less the same set of systems indicates that they are likely pre-programmed to emerge through life, even if not present from birth. Such things would have to be programmed by the designer. > > > The complex soup of the reticular formation is sufficient to turn a > > sane anatomist into a sobbing wreck with its dozens of specific nerve > > clusters. > > The reticular formation is currently my favorite part of the brain. =P It was described as a "pile of manure" or something to that effect, by one of the first anatomists to really get into it (Schweibel maybe?) -Brad ------- To unsubscribe, change your address, or temporarily deactivate your subscription, please go to http://v2.listbox.com/member/?[EMAIL PROTECTED]
