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[integer at www.god-emil.dk]

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 functio!
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