Brain activation and the locus of visual awareness.

A major problem in visual neuroscience is to distinguish neuronal activity which is directly related to the conscious percept. The word "directly" is used here as opposed to an indirect relationship, as is for example the case with activity in the retina, which is produced by a stimulus in the outside world and will eventually lead to the perception of this stimulus. As for the word "related", it is used to mean activity which creates the perceptual experience or, even more extremely, activity that is the perceptual experience. The distinction between the two (is vs. creates) is not straightforward, although there might be some differences between them. Philosophers would argue that they have a different phenomenology, the percept existing only for the perceiving person, whereas the underlying neuronal activation exists for all to observe. One could go on to argue that it is actually not the neuronal activation that "everybody" observes, but each one observes his own percept of it, which is also unique and subjective. Still, the content of this percept and the one of the original stimulus are quite different. The purpose of the present review is not to dig deep into such philosophical issues, but rather to give an overview of neuroscientific approaches trying to locate the neural correlate of conscious perception.