Neuroscientists describe vision in terms of the processes that unfold when light falls onto the retina. Between the retina and the visual cortex, there are many transformations that the original (retinal) stimulus undergoes. In humans (as in all mammalian species), there are the photoreceptors in the retina, ganglion cells, ganglion cell axons (optic nerve), and synaptic transitions. At higher levels following the optical tract, neural activation is set in motion by the original stimulus that passes through the superior colliculus, lateral geniculate nucleus, and optic radiations before reaching the visual cortex.