Even though most of the attention in cognitive neuroscience has been focused on studying the role of gray matter brain areas / structures in perceptual and cognitive functions, it is increasingly recognized that brain connectivity (i.e., the tracts that run in the white matter) plays an important role in cognition. This is due to the fact that even though specific functions can be associated with single brain areas, a closer inspection reveals that it is usually a network of brain areas that, through intricate interactions and modulation of other brain areas, underlies cognitive processes. There are different types of connections in the brain. From the sensory organs, there are ascending fibers that traverse through specific nuclei in brain stem and thalamus to cortex via multiple synapses. On the other hand, there are an order of magnitude more descending fibers from the cortex to thalamus and the other subcortical structures than there are ascending fibers, making it possible for cortex to modulate processing in the lower order areas that is relevant for selective attention