Visual pathways

• The outputs of the retina (ganglion cell axons) target two main structures in the brain: the lateral geniculate nucleus of the thalamus (or LGN), and the superior colliculus. The LGN is the main conduit to the visual cortex and conscious visual perception. The superior colliculus is involved in guiding eye movements and other automatic visuo-motor responses.

 
• The outputs of the LGN terminate in the visual cortex (area V1). There are about as many output fibers exiting the LGN as there are input fibers from the retina. However, there are about ten times as many fibers descending from the visual cortex to the LGN. The function of these feedback pathways is largely unknown.

 
• The receptive field properties of neurons in the LGN are largely similar to those of ganglion cells in the retina. For this reason, the LGN has been dubbed a "relay nucleus," although this is probably a vast understatement of its actual function.

 

Primary visual cortex (area V1)

• The visual cortex is just one part of the cerebral cortex, which is a two-dimensional sheet of neurons about 2-3 mm thick that is highly convoluted. In macaque monkeys, the visual cortex constitutes about 50% of the surface area of the entire cerebral cortex. In humans, the fraction is about 20%. Structurally, the cortex can be subdivided into six layers.

 
• The primary visual cortex, or area V1 (also known as striate cortex), is the primary target of fibers (axons) coming from the LGN. These fibers terminate in layer IV.

 
• Area V1 contains a topographic map of visual space, meaning that neighboring neurons have receptive fields in neighboring parts of visual space. If each neuron in V1 were to light up as it became active, the image of activity you would see on the cortex would look something like a warped television image of the world currently being viewed by a person.

 
• Neurons in the primary visual cortex come in two major varieties: simple-cells and complex cells. Both of these cells exhibit a property known as orientation selectivity, meaning that they respond selectivity to luminance edges of a particular orientation. Simple-cells are quite choosy about the position of the edge, whereas complex-cells will respond to the edge as long as it is in some general region of the image.

 
• Neurons in the primary visual cortex also exhibit spatial-frequency selectivity, meaning that they respond preferentially to luminance changes at different spatial scales. One way of measuring this is to use a sinewave grating as the stimulus and plot the cellÕs activity as a function of the spatial-frequency of the grating.

 
• The visual cortex contains a columnar organization, meaning that neurons within a column perpendicular to the surface of the cortex have similar response properties - i.e., similar position, orientation, and spatial-frequency selectivities.