Thalamocortical afferents are thought to play a major role in the organization of visual cortical maps, however, we still have a poor understanding of how they are horizontally arranged within the cortex. To address this question, we measured horizontal changes in ON and OFF cortical retinotopy within cat visual cortex by using multielectrode arrays. We then performed computer simulations to estimate the horizontal pattern of thalamic axons that best reproduced the ON-OFF cortical changes. After giving random values of receptive field position/size/polarity, synaptic strength per cortical point and axon-terminal size, thalamic afferents went through two different stages of pruning. In the first stage (retinotopic pruning), the synaptic weights and axonal branches changed to match the ON-OFF cortical retinotopy. In the second stage (fine pruning), we tested three different approaches

Multielectrode arrays of 32 channels were horizontally introduced through cat visual cortex Simulation approaches : 1) minimize number of thalamic inputs , 2) minimize size of axon terminals, 3) both.

Among the three approaches, the first method provided the best fits to the cortical data. It reproduced more closely the ON/OFF receptive field structures, the physiological correlation between the ratios of ON/OFF axon number and synaptic strength at each cortical point and the larger horizontal distances covered by OFF afferents

1_Horizontal changes in cortical retinatopy can be accurately reproduced with a linear combination of thalamocortical inputs. 2_Limiting number of thalamic inputs made OFF afferents to cover larger horizontal distance than ON afferents and generated correlations between ON per OFF number of afferents and ON per OFF synaptic strength that resembled those measured in physiological experiments.