True progress in understanding how experience arises from the brain has been relatively slow when viewed from a historical perspective. Recently, several technologies to study and stimulate the brain have been applied to this field of inquiry. Such progress was made only 2,500 years after the ancient Greek philosopher Parmenides first adopted a technical procedure involving the application of formal logic instruments to explore the perception of experiences.

At the phenomenological level, consciousness has been referred to as “what vanishes every night when we fall into dreamless sleep and reappears when we wake up or when we dream. It is also all we are and all we have: lose consciousness and, as far as you are concerned, your own self, and the entire world dissolves into nothingness”. According to the integrated information theory, consciousness is integrated information.

The term “consciousness” therefore has two key senses: wakefulness and awareness. Wakefulness is a state of consciousness distinguished from coma or sleep. Having one's eyes open is generally an indication of wakefulness and we usually assume that anyone who is awake will also be aware. Awareness implies not merely being conscious but also being conscious of something. The broad definition of consciousness includes a large range of processes that we normally regard as unconscious (eg, blindsight or priming by neglected or masked stimuli).

Both sleep and anesthesia are reversible states of eyes-closed unresponsiveness to environmental stimuli in which the individual lacks both wakefulness and awareness. In contrast to sleep, where sufficient stimulation will return the individual to wakefulness, even the most vigorous exogenous stimulation cannot produce awakening in a patient under an adequate level of general anesthesia.