The Great Gray Ravelled Knot: Overview It is hard not to wax eloquent when describing the construction and the capabilities of the human brain. From a purely computational point of view, the human brain appears to be about 100 times faster and more complex then the upcoming Cray T3E supercomputer or about 1,000,000 times more elaborate than a top-of-the-line PC. Complexity: The human brain contains about 50 billion to 200 billion neurons (nobody knows how many for sure), each of which interfaces with 1,000 to 100,000 other neurons through 100 trillion (1014) to 10 quadrillion (1016) synaptic junctions. Each synapse possesses a variable firing threshold which is reduced as the neuron is repeatedly activated. If we assume that the firing threshold at each synapse can assume 256 distinguishable levels, and if we suppose that there are 20,000 shared synapses per neuron (10,000 per neuron), then the total information storage capacity of the synapses in the cortex would be of the order of 500 to 1,000 terabytes. (Of course, if the brain's storage of information takes place at a molecular level, then I would be afraid to hazard a guess regarding how many bytes can be stored in the brain. One estimate has placed it at about 3.6 X 1019 bytes.) Speed: Neurons require about a millisecond to discharge, followed by a 4 millisecond refractory period That could amount to as many as 2 X 1018 connection updates per second. In practice, the firing rate and the synaptic count probably isn't that high. There are 40-Hertz firing waves that sweep the entire brain from back to front. The 6,000,000,000 neurons in the visual cortex also fire about 40 times a second to give us our 20-frame-per-second visual update rate, so we might be looking at perhaps 240 billion firings per second in the visual cortex. Each neuron connects to a number of other neurons through dendrites and an axon (an average of 15,000 interconnections per neuron in… the visual cortex), so we might be dealing with about 50 trillion (5 X 1013) synaptic junctions in the visual cortex. At 40 firings a second, the visual cortex should be able to perform about 2 quadrillion synaptic activations per second—2 X 1015 connection updates per second or 2,000,000 Gcups (giga-connection-updates-per-second)—compared to 10 Gcups for current neural networks. For the brain as a whole, assuming 10,000 interconnections per neuron, the number might be about 10 times this amount, or 20,000,000 Gcups. It has been estimated that computational speeds of 109 calculations per second (1 Gigops) would be required to match the edge and motion detection capabilities of the first four layers of the human retina, and 1013 operations per second (10,000 Gigops) to 1016 operations per second (10,000,000 Gigops) would be necessary to emulate what is done in the brain overall. Accuracy and Redundancy: Another computational parameter which may be of interest in assessing the brain's capabilities is its information storage capacity. However, my preliminary estimates of the storage we would actually need to store what we remember suggests numbers much lower than those above.) Some redundancy in cerebral memory storage may be needed to accommodate for the quantum unreliability of the brain's nanocircuitry. (The synaptic junctions measure only a few atomic diameters across.) Power: &n Ukázat celý příspěvek
