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In many mammals (cats and humans included), layer IV of the visual cortex initially receives input from both eyes. During the critical period, which lasts up to the first six years of life in humans, inputs from the two eyes become segregated such that a given layer IV neuron responds predominantly to activity in just one eye. In this manner, ocular dominance columns are established, which are stripes or columns of cells in the layer IV visual cortex with each cell in a column responding to activity in the same eye.

In the lateral geniculate nucleus (LGN) of the thalamus, inputs from the two eyes are initially intermingled as well. Later on, just as in the visual cortex, synapses representing one eye are eliminated, while synapses representing the other eye grow in number. Ultimately, inputs from the two eyes are segregated into layers, where each neuron from the same layer responds predominantly to inputs from the same eye.

There is a profound difference in how inputs are segregated into columns and layers in the visual cortex and thalamus. Establishment of ocular dominance columns critically depends on the stimulus-triggered activity of the retinal ganglion cells. While in the thalamus, segregation into layers is driven by the spontaneous activity of retinal ganglion cells.

Which observations support the existence of these two distinct mechanisms?

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Monocular deprivation during the critical period results in the expansion of ocular dominance columns representing the open eye, while the ones representing closed eye shrink.


Segregation of inputs into eye-specific layers in the thalamus is complete well before retina is sensitive to light.


During the critical period for eye-specific segregation of inputs into layers in the thalamus, retinal ganglion cells display waves of spontaneous activity. Pharmacological inhibition of these waves prevents input segregation in LGN.


Monocular deprivation after the critical period in the visual cortex is complete does not alter the size of the ocular dominance columns.

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