Non technical summary  Neurones of the mammalian primary visual cortex have the remarkable property of being selective for the orientation of visual contours. It has been controversial whether this selectivity arises purely from mechanisms within the cortex itself, from the special way afferents from the thalamus project to the cortex or from the sharpening of a bias for orientation that already exists in the retina and the thalamus. Our experiments support the last of these three hypotheses. We used a special protocol to study both inhibitory and excitatory interactions within the cortex as well as in the thalamus and we find that the orientation selectivity may depend upon multiple mechanisms – including the thalamic biases for orientation and intracortical inhibition and excitation.