To investigate consciousness empirically, it is necessary that we have an empirical or operational definition of what we mean by the term. The quest for neural correlates of consciousness relies primarily on our ability to meaningfully identify the range of subjective states that constitute consciousness. But is there only one kind of phenomenon that can be called conscious? What criteria do we use to determine if something is conscious? There has been a range of suggested answers to these questions. Some, like philosopher Daniel Dennett, propose that consciousness is indeed a single phenomenon. Others, such as philosopher Ned Block, make critical distinctions between different types of conscious states, with important relationships between them. Block’s framework, which has become popular in the scientific literature, makes a distinction between two different kinds of consciousness: phenomenal (P) and access (A) consciousness.
Types of consciousness
P-consciousness is defined as the “minimal neural basis of the content of an experience, that which differs between the experience as of red and the experience as of green” (Block, 2005). The reason these phenomenal contents of consciousness are described as the experience as of something is because there is no necessary relationship between the external world and what is being experienced. Afterimages, a phenomenon that occurs when one stares at an object for long enough and a visual trace is left afterwards, are an example of the possible discordance between experience and reality. P-consciousness refers to the experience of subjective discriminations made by the brain. These experiences are not necessarily verbally reportable. This is evident in people who suffer from a condition called blindsight in which the primary visual area of the brain is damaged and patients report that they cannot see, even though they perform better than chance on some visual tasks.
A-consciousness refers to the “contents information about which is made available to the brain’s ‘consumer’ systems” (Block, 2005). Examples of consumer systems are memory and language. Therefore under this framework, A-consciousness is the aspect of consciousness that can be reported by subjects. It consists of the contents of P-consciousness that have been selected by a kind of “winner-take-all” competition for access to a global workspace of higher cognitive processes. Subjects claim to have awareness of this kind of consciousness, which includes contents that are completely illusory. An example of this is the phenomenon known as Anton-Babinsky syndrome in which patients claim to have visual experience, despite being cortically blind and being unable to use their alleged visual information to navigate. Another pathology that distinguishes A-conscoiusness from P-consciousness is that of split-brain patients who the two hemispheres of their brain have severed from each other by surgically cutting the connecting structure called the corpus callosum. These patients can report sensory information provided to the left hemisphere where most language functions are located, but claim to have no experience when the same information is provided to the right hemisphere. In these cases it can be inferred that the phenomenal contents of the right hemisphere cannot be accessed by the necessary consumer systems of the left hemisphere, although winner-take-all phenomenal selection may still be occurring in both.
Works Cited/ Suggestions for further reading:
Block, N. (2005). Two neural correlates of consciousness. TRENDS in Cognitive Science. (9)2:46-52.