New PDF release: From the Retina to the Neocortex: Selected Papers of David

By Lucia M. Vaina (auth.), Lucia Vaina (eds.)

ISBN-10: 1468467751

ISBN-13: 9781468467758

ISBN-10: 1468467778

ISBN-13: 9781468467772

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Extra info for From the Retina to the Neocortex: Selected Papers of David Marr

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Each Purkinje cell receives inhibition from about 40 stellate cells: the inhibition to the Purkinje cell is therefore driven by a dendritic field about 40 times as large as that of a single stellate cell. If these 40 were distributed randomly just next to the Purkinje cell, a good sample (approximately 1 - l/e 2 = 0·86) would be obtained if each tree had even 1/20 of the synapses that a Purkinje cell has. If the mossy fibre distribution alters slowly (which it has to do anyway for the system to work), the saving in dendrite could therefore be a factor of up to 20; and, in practice, the sampling is certainly not random.

Their effect through the basket eells is to release Purkinje cells from inhihition, hut their influence through the Golgi eells is more eomplieated. It is likely that this influence will ultimately be excitatory at a given Purkinje cell P only if most of the granule eells therehy released from Golgi inhihition have modified synapses at P: and this will he true only if P has already learned a numher of mossy fibre inputs all quite similar to the eurrent input. The only ohviously reasonahle interpretation of the effect of thcse eollaterals is that they tend to exeite the Purkinje eells in the cortex to whieh they distrihute; and in eertain eircumstances can loosen the discrimination exereised hy those cells.

This assumption can be justified on the grounds first that any estimation of capacity arrived at by using it is likely to be too low; and, secondly, that 30 A THEORY OF CEREBELLAR CORTEX 457 almost any other assumption would involve a computational effort out of all proportion either to the probable truth or to the value of any results thereby achieved. '! are unlikely to have a random structure over the space of possible mossy fibre inputs. Some types of strueturc on the learned inputs will positively confuse the system into giving false responses.

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From the Retina to the Neocortex: Selected Papers of David Marr by Lucia M. Vaina (auth.), Lucia Vaina (eds.)

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