integer on Sun, 23 Apr 2000 16:35:45 +0200 (CEST)

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hallo + hou do u do. [man!ere art!f!z!el +? pas de tout]
je su!s fat!gue. egal zalut.
= kop! pazte + 01 zleep rout!n.
= nato.0+55 eku!v ov belou 
= kl!k + kl!k !n progresz. nn

how is the development of the processing capabilities and organization
of the brain's cerebral cortex controlled? intrinsic mechanisms (such as
genetically encoded developmental programmes) and extrinsic inputs (such
as the things we see and hear, and the ways that this information is
encoded by specific discharges within particular sensory systems) both
have a say. but to what extent can the developmental pathways be
overruled by inputs from the outside world? two fascinating papers by
sur and colleagues, on pages 841 and 871of this issue1, 2, provide some
of the most compelling evidence yet for the exquisite sensitivity of
cortical development to external cues. 

how does one even start to determine the relative contributions of
external and internal factors to cortical development? ferrets have
proved a useful model animal, in part because they are born before their
development has progressed too far. over the past decade, sur and
co-workers have been perfecting an experimental approach that consists
of surgically manipulating the nerves that feed into different parts of
the cortex of very young ferrets. specifically, the nerves from the
retina (which normally lead to a subcortical region, the visual
thalamus, which in turn feeds into the primary visual cortex, or v1) are
redirected to grow into the auditory thalamus (which feeds into the
primary auditory cortex, or a1). the auditory thalamus itself is
deprived of its normal auditory inputs in this model.

in early experiments3, 4, sur and colleagues showed that this 'rewiring'
procedure results in the emergence of a functional v1 in a cerebral
cortex zone that was otherwise destined to develop into primary auditory
cortex. the new visual cortex has a topographic organization that
parallels that in normal v1. moreover, different neurons in this rewired
cortical zone -- like those in normal v1 -- are selective for
differently orientated visual stimuli. the normal organization of a1, in
contrast, goes awry: the a1 territory is taken over by visual inputs.
such experiments have provided important evidence that the organization
and responsiveness of different cortical regions can be shaped by the
particular patterns of neuronal discharge that result from neuronal
stimulation by different inputs -- in this case, by retinal versus
cochlear (auditory) inputs.

sur and colleagues' latest papers1, 2 advance this theory by several
crucial steps. first, sharma, angelucci and sur1 show that particular
higher-order features seen in normal v1 emerge in the rewired visual
cortex (fig. 1). these features are called 'visual orientation columns':
each consists of a group of neurons that share a preference for visual
stimuli with a particular orientation. the layout of these columns
provides a basis for representation of important spatial characteristics
of visual stimuli. the 'pinwheel' organization of these columns (fig. 1)
in the rewired animals resembles that in v1. the authors go on to show
that horizontal connections -- links between separate columns that
represent corresponding stimulus orientations -- emerge in the rewired
auditory cortex, just as in normal v1. these horizontal connections and
organizational structure have no equivalents in the normal a1. all of
these studies1, 3, 4 convincingly show that much of what typifies the
nal organization of v1 can be generated within a1 by delivering retinal
inputs to a1 through the auditory thalamus. 

but the story does not end there. von melchner, pallas and sur2
demonstrate that rewired animals show behavioural responses to visual
stimuli that are presented only to the neurons feeding into the rewired
cortex. in other words, the animals 'see' with what was their auditory
cortex. the ferrets were given the option of receiving a reward from a
spout to their right following a light stimulus, or to their left after
a sound stimulus. after vi

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