“Consciousness and the Brain”的意思、由来-开放百科全书

Consciousness and the Brain: Deciphering How the Brain Codes Our Thoughts is a 2014 book by Stanislas Dehaene. It summarizes research on the neuroscience of consciousness, particularly from recent decades.

Book outline

Introduction: The Stuff of Thought

Dehaene reviews historical intuitions that consciousness must be separate from matter. He explains how consciousness was not even mentioned in neuroscientific circles until the late 1980s, when a revolution in consciousness research began. Dehaene believes that "access consciousness" (being aware of and able to report on information) is the right definition to start with for scientific investigation. While some philosophers insist that access consciousness differs from "phenomenal consciousness" (e.g., the way qualia feel), Dehaene considers the access/phenomenal distinction "highly misleading" and feels it "leads down a slippery slope to dualism" (p. 10).

Ch. 1: Consciousness Enters the Lab

Dehaene distinguishes conscious access from related but not identical ideas: "attention, wakefulness, vigilance, self-consciousness, and metacognition" (p. 25).

He introduces the project of measuring neural correlates of consciousness using paradigms like minimal contrasts of images, masking (subliminal stimuli), binocular rivalry, and attentional blink. The attentional blink relates to the psychological refractory period, inattentional blindness, and change blindness. Olaf Blanke's studies on out-of-body experiences explore an example where conscious experience changes while external stimuli stay the same.

In Ch. 4, Dehaene notes that correlates of consciousness are actually insufficient, because many things can correlate with conscious perception, including even brain states prior to presentation of a stimulus. Dehaene is most interested in neural signatures of consciousness that represent the consciousness brain processing itself. (p. 142)

Ch. 2: Fathoming Unconscious Depths

Dehaene reviews unconscious brain processing of various forms: subliminal perception, Édouard Claparède's pinprick experiment, blindsight, hemispatial neglect, subliminal priming, unconscious binding (including across sensory modalities, as in the McGurk effect), etc. Dehaene discusses a debate over whether meaning can be processed unconsciously and concludes based on his own research that it can be. An N400 meaning-based wave occurs for unexpected words even when masked or not attended to. Unconscious processing is not just bottom-up but can be enhanced when top-down attention is directed toward a target, even if the target never becomes conscious. Brains can even do some mathematical operations unconsciously, and sitting on a problem to let the unconscious mind work out an answer has proved helpful in several experiments.

Ch. 3: What Is Consciousness Good For?

While some view consciousness as an epiphenomenon of brains, Dehaene sees it as playing functional roles, such as

Ch. 4: The Signatures of a Conscious Thought

Consciousness seems to have a "tipping point" or "phase transition" of sorts, an all-or-nothing cutoff. Dehaene uses the phrase "global ignition" to describe the process of neurons bursting into widespread activation, similar to the way an audience begins with a few claps and then erupts into synchronous applause (p. 131).

Consciousness is slower than events in the external world. The flash lag illusion illustrates this because we can predict future positions of moving objects but not those of objects that suddenly appear.

Conscious percepts have properties of "stability over time, reproducibility across trials, and invariance over superficial changes that leave the content intact" (p. 149).

To prove causation between brain states and conscious experiences, neuroscientists have used transcranial magnetic stimulation and intracranial electrodes for patients undergoing surgery to directly create perceptions. An example is phosphene.

Ch. 5: Theorizing Consciousness

Dehaene discusses his version of the Global Workspace Theory of consciousness. Dehaene proposes that "When we say that we are aware of a certain piece of information, what we mean is just this: the information has entered into a specific storage area that makes it available to the rest of the brain" (p. 163). He adds: "The flexible dissemination of information, I argue, is a characteristic property of the conscious state" (p. 165).

Dehaene and colleagues have developed computer simulations of neural dynamics that successfully replicate the way in which distributed processing at the brain's periphery gives way to a stable, serial "thought" at higher levels due to feedback amplification of one signal and inhibition of others. The simulation showed the four signatures of consciousness described in Ch. 4 (p. 184). Consciousness seemed to behave like a "phase transition" between one unconscious stable state of low-level activity and another conscious state consisting of snowballing self-amplification and reverberation (p. 184). Subliminal stimuli fail to become conscious because by the time the higher layers try to amplify the signal, the original input stimulation has vanished (p. 193).

Dehaene suggests that noise fluctuations in neural activity can be amplified and give rise to randomness in our streams of thought (p. 190).

Ch. 6: The Ultimate Test

Dehaene discusses coma, vegetative states, minimally conscious states, and locked-in syndrome.

Recent findings have shown that a few patients without any ability to move (not even to move their eyes) still show intact consciousness as seen by their ability to answer questions in an fMRI. The trick is to instruct the patients to think about their apartments if they want to say "no" and about playing tennis if they want to say "yes", and the corresponding differences in brain activity can be observed.

Different tests can give different answers regarding whether a clinical patient is conscious, and responses may depend on time of day or other factors. Hence, Dehaene suggests "to develop a whole battery" of tests that can be applied in many contexts (pp. 214-215). fMRI tests are expensive and burdensome, so researchers are exploring easier EEG communication methods (p. 215) and other brain-computer interfaces (p. 216). Dehaene and colleagues also developed a simple test for consciousness based on novelty of patterns in sounds.

Ch. 7: The Future of Consciousness

Dehaene explores consciousness in human babies, non-human animals, and machines.

Dehaene reviews evidence that young infants are indeed conscious, although their global workspaces may run 3-4 times slower than in adults, perhaps because their myelin is not well established.

Monkeys can be trained to "report" on their conscious experiences via actions rather than speech, and monkeys show the same sorts of brain and behavioral patterns as humans in response to consciousness tests. Dehaene adds that some animals, like monkeys and dolphins, show evidence not just of consciousness but also of metacognition. He speculates that maybe what makes human cognition unique is "the peculiar way we explicitly formulate our ideas using nested or recursive structures of symbols" (p. 250).

Dehaene suggests that computers could become more like animal brains if they had greater communication between processes, more learning plasticity, and more autonomy over decisions. Of these design changes, he suggests that "at least in principle, I see no reason why they would not lead to an artificial consciousness" (p. 261). Dehaene suggests that the hard problem of consciousness "just seems hard because it engages ill-defined intuitions", and it "will evaporate" as people better understand "cognitive neuroscience and computer simulations" (p. 262). Dehaene also defends a compatibilist notion of free will and suggests even that free will "can be implemented in a standard computer" (p. 264).

Reactions

James W. Kalat thinks "Consciousness and the Brain is beautifully written, erudite, thoughtful, and likely to provoke discussion for years to come." Kalat explains how Dehaene believes that consciousness is important for performing certain calculations that can't be done unconsciously. However, Kalat suggests that this leaves us with a puzzle: "Unless we assume that computers are conscious, the question remains why we are conscious when we perform certain functions, whereas computers can perform virtually the same functions without consciousness." Kalat also finds inadequate Dehaene's dismissal of the hard problem of consciousness "in barely over a page of text" without further exploration of the subject.^[1]

Alun Anderson agrees with the critics of Dehaene who think that consciousness as "brain-wide information sharing" isn't enough to resolve the hard problem. That said, he appreciates Dehaene's book and recommends to "read a chapter at a time because it is jam-packed with intuition-altering experiments."^[2]

Matthew Hutson calls Dehaene's book "smart, thorough and lucid, though a terrible choice for beach reading." Hutson admires Dehaene's success with neural correlates of consciousness but feels that the hard problem remains unresolved. Like Kalat, Hutson finds Dehaene's dismissal of the hard problem unjustified because consciousness is "unique" in being "inherently private, subjective", unlike other phenomena that can be reductively explained.^[3]