Consciousness curriculum

Call a person conscious and you describe a whole creature: awake, responsive, able to register the world. Call a perception conscious and you describe one state inside that creature. An alert person can still have perceptions that never become conscious, so the two uses split. Rule: ask what the word is attached to.

You can be conscious of a smell, which takes an object, or just conscious, as opposed to knocked out, which takes none. Philosophers call the first transitive and the second intransitive. Whether a fish is awake and whether it notices a predator are therefore separate questions.

Nagel's 1974 essay asks you to imagine being a bat that navigates by echo. If there is a point of view from which things seem some way, that creature is conscious in this sense. A thermostat responds to heat, yet few suppose it has a point of view. The phrase marks experience, not intelligence.

Block's pair separates two things the word can mean. The taste of coffee as felt is phenomenal; the same taste as something you can think about, mention or act on is access. They usually arrive together, so critics ask whether they truly come apart, and Block argues that they can.

Being drowsy, alert or under anesthesia varies how conscious you are overall. Noticing a smell rather than a song varies what you are conscious of. Sleep and coma research tends to study the first, and perception experiments the second. Keep them apart, because a finding about one need not apply to the other.

A person with an injured brain can open their eyes each morning and sleep each night, yet show no response to their surroundings. Arousal systems are running while nothing indicates experience of the world. Dreaming is the opposite case: little wakefulness, vivid experience. Rule: never infer one from the other.

A dog may feel pain without being aware that it is aware. Self-consciousness is that further capacity, and philosophers debate how much thought it takes. Treating all consciousness as self-consciousness would wrongly rule out creatures and moments with experience but no self-reflection.

Noticing that you are annoyed adds something to being annoyed: a state directed at your own mind. Some theories make this inner awareness central, and others treat it as a special, later addition. Much everyday experience seems to happen with no such monitoring, which is why the label is useful.

What is the nature of consciousness? How does it arise from physical processes? What does it do for a creature? An answer to one can leave the others open: a perfect map of where it occurs in the brain would not by itself say what it is for.

Telling red from green, reporting a memory and focusing attention are all jobs a mechanism can do, so explaining them means finding the mechanism. Chalmers's word easy is relative: the work may take decades. Its contrast is with a question that a mechanism might not answer at all.

Suppose science maps every step from light on the retina to the words “that is red.” You could still ask why all this comes with a felt redness rather than happening in the dark. Chalmers argues that no account of function closes this question; critics reply that the question itself is confused.

Saying water is hydrogen and oxygen explains why water boils and freezes as it does. Saying pain is a certain brain process seems not to explain why it hurts. Levine treats this as a gap in our understanding, and leaves open whether it also shows that experience is something beyond the physical.

The pattern has two steps: if you can coherently imagine all the physical facts without experience, that scenario is possible; if it is possible, experience is more than physical. Many physicalists grant the imagining and deny that it shows what is really possible.

McGinn does not say consciousness is supernatural. His claim concerns us: the concepts that would make the answer intelligible may be ones human minds cannot form. The answer may exist and stay out of reach. Rule: a limit on understanding is not a claim about what minds are made of.

Daniel Dennett treats qualia as something like an illusion. On his view, explain the discriminations, reactions and judgments, and the sense of an extra ingredient is explained too. Chalmers would say this changes the subject; Dennett would say the leftover was never there. The disagreement remains open.

Could a creature do everything you do with no experience? If so, experience seems to add nothing, which makes its evolution a puzzle. If not, it must do some work, perhaps in flexible control or learning. Views that give experience a job differ on what the job is.

The ache of a bruise and the smell of coffee differ in how they feel, not only in what they cause. Philosophers call these felt qualities qualia. The word is contested: some use it for any felt character and others for properties beyond the physical. Check which sense a writer means.

The setup stipulates complete physical knowledge gained in a black-and-white room. When Mary first sees a ripe tomato, it seems she learns what red looks like. Jackson concluded that some facts are not physical, and he later gave up that conclusion himself, so the argument now stands apart from its author.

Learning to ride a bike gives you know-how, not a new fact about bikes. David Lewis and Laurence Nemirow say Mary's change is of this kind: a new skill and not a new item of knowledge. A critic can ask whether skill covers everything, since she also seems to learn that red looks this way.

Picture your exact physical twin, talking about sunsets and wincing at burns, with nothing going on inside. Nobody claims such beings exist. The question is whether they are possible even in principle, since physicalism seems to say they are not. Do not confuse them with horror-film zombies, which behave oddly.

Your friend's experience of ripe tomatoes might be like yours of grass, while you both call tomatoes red and stop at the same lights. No color test could reveal it. If that is possible, experience is not fixed by behavior or role. Critics argue that the structure of color makes a perfect swap impossible.

Here nothing is swapped: the system does all the right things and feels nothing. The target is functionalism, which makes mental states a matter of causal organization. Functionalists reply that the case rests on an intuition we have no reason to trust, especially for systems unlike us.

Try attending to your experience of a blue mug, not the mug. Many people report finding only the blueness of the mug, as if looking through a clear pane. Representationalists lean on this observation. Others reply that you can attend to the experience itself, as when your vision blurs.

On this view, seeing red differs from seeing green because the two experiences represent different colors, not because of some extra inner paint. That would fit experience more easily into a physical world, since representation has a physical story. Inverted-spectrum cases are the standard challenge.

Physicalism does not say every mental word can be swapped for a word from physics. It says that once the physical facts are fixed, the mental facts come along with them. Its challenge is to explain experience without adding anything further, which is what the hard-problem arguments deny it can do.

U. T. Place compared mind and brain to lightning and electrical discharge: one thing found to be another. J. J. C. Smart took up the view. The claim concerns kinds, so every pain of one sort is the same sort of brain state. That strength is also its weakness, since it ties the mental to one kind of hardware.

Functionalists argue that pain is unlikely to be one neural state across humans, octopuses and whatever else can hurt. A single psychological kind could run on many physical kinds, as one program runs on different computers. Rule: a kind defined by what it does is not tied to what it is made of.

Think of a carburetor: what makes something one is the job it does, not the metal. Functionalism treats belief, desire and pain the same way, defining each by inputs, outputs and links to other states. It explains thought well. Its contested point is whether role alone captures how pain feels.

In your brain, some neural state does the work of pain; in another creature, a different state may do it. That state is the realizer, and the role is what the two share. Confusing them produces muddles about whether pain is in the neurons or is something more abstract.

Hand everyone a radio and a set of instructions so the group plays the role of neurons for an hour. Would the group feel anything? Most people say no, and that pull is the whole argument. A functionalist can answer that intuition was never trained on anything this large or slow.

Silicon, in principle, could do what neurons do if the organization were right. This is why functionalism matters for the question of conscious machines. It is a conditional, not a forecast: it says nothing about whether any existing system has that organization, and rival views reject the premise.

Descartes held that a thinking thing and an extended thing are distinct kinds, so you could in principle exist apart from your body. Few philosophers or scientists hold this today, partly because of the problem of how the two connect. Do not assume every dualist holds it; the next view keeps one substance.

A property dualist can agree that you are a physical organism with no soul inside. The claim is that the organism has some properties, such as how things feel, that physics does not capture. That is a modest dualism compatible with neuroscience, though it still owes an account of what those properties do.

Elisabeth of Bohemia questioned Descartes on this: how can a mind with no physical extension move a body? Picturing it as one ball striking another is too crude, so the modern form of the worry asks whether physics leaves any room for such a cause. Any view with a non-physical mind that moves limbs has to answer it.

Common sense is interactionist: a burn causes pain, and the pain makes you pull away. Treated as a theory with a non-physical mind, this requires the mind to make a difference in the brain's physical events. That commitment is what puts it at odds with the idea that physics has no gaps.

T. H. Huxley's image was a locomotive's whistle: the engine makes the sound, and the sound moves nothing. The view sounds odd because your pain seems to make you flinch, and here it does not. A further puzzle is how you could then come to talk about experience.

Leibniz's pre-established harmony is the classic version: mind and body run in step because God arranged them so, with no causal link between them. It dodges the interaction problem, but the coordination then needs an explanation of its own. Few defend the view today.

Trace a hand movement back through muscles and neurons and, so far, you find physical causes at each step. If that holds everywhere, a non-physical mind has no gap to act through. An interactionist must deny closure or accept that effects can have two sufficient causes.

The claim is not that rocks think. A rock is an aggregate; the idea is that fundamental things such as particles have some very simple form of experience, and that complex minds are built on this. It looks extravagant, and its defenders answer that every other option has costs of its own.

Put many tiny experiencers together and why would you get one mind rather than many tiny ones? William James raised the worry long before the current revival. It is widely seen as the hardest objection panpsychism faces, and it is why some defenders look to the whole universe or to non-experiential ingredients.

Physics describes matter through what it does: charge and mass are spelled out by their effects. Bertrand Russell asked what does all that doing. One suggestion is that the missing nature is experience or something close to it, which places mind inside matter with no second kind of stuff.

It shares panpsychism's starting point that matter has an intrinsic nature tied to experience, but it denies that the basic things feel anything. This avoids tiny conscious particles. It then faces its own version of the puzzle: how can non-experiential ingredients add up to experience at all?

Turn the combination problem upside down: start with one conscious universe and ask how smaller minds come from it. Defenders think this handles the problem better. The puzzle does not vanish, though, since you must then explain how individual points of view could split off.

On the constitutive version, your experience is made of tiny experiences, as a wall is made of bricks. A non-constitutive version says micro-level experience produces yours as something new. The first meets the combination problem head on; the second accepts a kind of emergence panpsychism hoped to avoid.

The wetness of water can be explained by how its molecules behave. Experience arising from matter with no trace of experience looks different to some philosophers, more like magic than chemistry. If you share that doubt, putting experience at the base is attractive; if not, this motive for panpsychism weakens.

Introspection can mislead, much as a visual illusion does. Illusionists apply this to experience itself: the brain models its own states in simplified form, and you believe what the model tells you. Ask of any version how far it takes the claim.

Illusionism is less a theory than a kind of theory. Any member must say what real process lies underneath and how the brain's picture of it goes astray. Refuting one member leaves the others standing, so ask which illusionist claim is on the table.

Told that consciousness is an illusion, many people hear that nobody feels anything. That is one end of a range. At the other end, experience merely differs somewhat from how it seems, which is far easier to accept. Graziano says the view tends to be judged by its most startling form.

A model earns its keep by leaving things out, so no self-model copies its target exactly. If you know your own awareness only through such a model, expect some mismatch between how it seems and how it is. On Graziano's argument, any theory of consciousness inherits at least that much illusionism.

The brain keeps models of the body, of other people and of its own attention. A simplified attention model might depict a kind of inner grasp of an object, and that depiction is what you describe when you report awareness. Graziano counts it as the one theory in his field that openly takes this line.

A body schema exists to help you move, not to teach anatomy. Attention schema theory assigns the attention model a similar working job, and talk of awareness draws on that model. Being useful for control does not make a model accurate.

Ask why people say they have inner experience, and you can answer without assuming they are right about it: a model depicts it, and the rest of the system reads the model. The illusionist move is to make that explanation do all the work. Opponents reply that it leaves out experience itself.

Your whole brain is enough for any experience you have, but saying so explains nothing. That is why the definition asks for the minimal activity: the smallest part that suffices for one specific experience, given background conditions such as arousal. Rule: a correlate has to narrow things down.

Show a faint image many times. On some trials it is seen and on others it is not, with the same physical input. Subtract the two, and what remains is a candidate for the difference that awareness makes. The weakness is that the subtraction can also pick up what precedes or follows awareness, such as reporting.

Put a face before one eye and a house before the other. You see face, then house, then face, though nothing on the screen changes. Recordings in monkeys and imaging in humans have tracked which brain activity follows the switch, separating responses to the input from responses tied to perception.

Flash a word, then follow it at once with a pattern, and the word often goes unseen. Processing of the word can still be measured, which gives researchers seen and unseen conditions with nearly identical input. How much unseen material gets processed is disputed and depends on how awareness was tested.

To know what someone saw, you normally ask them, and answering involves attention, decision and movement. Those processes engage frontal areas too. So a frontal signal may show the report rather than the experience. This is one of the main reasons the map of correlates is still contested.

Involuntary eye behavior can reveal what a person sees with no button press. The gain is removing report-related activity. The cost is relying on an indirect sign of what was seen, and critics say the sign might not track experience as closely as hoped.

One camp points to prefrontal cortex, tied to access, thought and report. Another points to a posterior zone of sensory cortex, arguing that frontal activity reflects what comes after experience. Lesion, stimulation and no-report studies are cited on both sides. Treat any headline that settles this with caution.

Two events can occur together because one causes the other, because a third thing causes both, or because one follows from the other. Imaging alone cannot separate those. Stimulating or silencing a region gets closer to cause, although even that leaves the question of what the activity amounts to.

Block argues that phenomenal consciousness can outrun access, so you may have experiences you cannot report. Others doubt this, holding that attention and access are needed for experience. No direct test is agreed, since the disputed experience is by definition not reported.

Think of a newsroom where one story goes out to every desk at once. Most work in the brain stays inside specialized desks. A content becomes conscious when it reaches the shared workspace, and every other process can then use it. This explains why one conscious content can feed speech, memory and planning alike.

Seeing, speaking and moving draw on many systems working at once, mostly out of awareness. The workspace is the narrow channel they compete to enter. That competition is how the theory squares a slow, limited stream of experience with so much parallel work.

Once a content is broadcast, you can describe it, remember it and plan around it. Without broadcast, a stimulus can still affect behavior in narrow ways but cannot be used flexibly. The theory therefore predicts a link between conscious access and flexible use, which is one of its main tests.

Dehaene and his colleagues gave the idea an anatomy: neurons with long-range connections link prefrontal and parietal cortex to the rest of the brain and can hold a content active. This turned a cognitive model into one that makes predictions about timing and location, which is what made it testable.

The theory's signature is a sudden, widespread surge of activity when a content enters the workspace. The COGITATE paper puts the predicted burst at about 0.3 to 0.5 seconds, at a stimulus's onset and offset, with content held silently in between. So do not read ignition as activity lasting as long as the experience.

Because the theory is framed around availability for use, critics argue that it explains reporting and reasoning and not experience as felt. Its supporters treat broadcast as what conscious experience corresponds to. That dispute mirrors the debate between access and phenomenal consciousness.

If a content is conscious, the theory says prefrontal neurons should carry it. Cases of prefrontal damage with preserved experience, and studies that remove report, are pressed against this. Defenders point to findings that prefrontal cortex still carries content without report. Here the theory can be tested and can lose.

Most theories begin with the brain and ask which activity is conscious. Tononi's theory begins with experience, asks what its essential features are, and then asks what kind of physical system could have them. Its answer is a quantity of integrated information, and it applies to any physical system, not only brains.

The axioms claim that every experience exists for itself, is structured, is specific, is unified and is definite. Each is turned into a demand on causal structure. Critics question whether these are self-evident and whether the step from axiom to postulate is justified.

Cut a system into parts and ask how much of what it does is lost. If the parts do just as well alone, phi is zero. Exact phi cannot be calculated for anything close to a brain, because the calculation grows explosively with size, so empirical work relies on rough proxies.

Phi gives the amount; the structure gives the character. The theory says the quality of experience, such as seeing blue rather than hearing a bell, corresponds to the shape of the system's causal relations. This is one of its boldest claims and one of the hardest to test.

Information passing one way through layers has no loops and, on this theory, no integration. A feedforward network could in principle match a looped one's behavior. So the theory separates consciousness from intelligence and function, which some find insightful and others take as a sign it cannot be tested.

A coded message means something only to a reader who holds the key. The theory asks instead for information that matters from the system's own perspective. So what counts is physical causal structure, not what an outside observer takes the system to compute, and a strictly feedforward design is ruled out whatever it does.

Your neurons, your brain and you plus your phone all overlap. The theory says only one of these overlapping candidates is conscious: whichever has the most integrated information. This keeps many overlapping minds out of your head, at the price of a rule that sounds odd at first.

Because any integrated system has some phi, a simple circuit arranged the right way can count as slightly conscious, more so than a very complex feedforward one. Supporters accept this; critics treat it as a reductio. Tononi has even allowed that a grid of inactive logic gates might score high.

D. M. Armstrong's example is absent-minded driving: you come to after miles on autopilot, having plainly seen the road. Higher-order theories say what was missing is a state directed at the perception. Consciousness becomes a relation between mental states rather than a feature of a perception by itself.

A higher-order thought is conceptual, like thinking “I see red.” Inner-sense views compare it to perceiving, with a monitor scanning your own mind. The first demands concepts of your own mental states, and the second faces the objection that there is no obvious inner sense organ.

A constant stream of thoughts about every experience would be a lot for a mind to produce. Peter Carruthers's version needs only that a state be poised for such thoughts. Critics ask how a merely possible thought could make any actual difference to how something feels.

Suppose a higher-order state says you see red, though no such perception occurs. Is there an experience of red? Yes suggests the first-order state was never needed. No suggests the higher-order state is not enough. Either answer changes what the theory claims, so it is a central test for it.

If consciousness needs thoughts about your own mental states, a newborn or a mouse may not qualify. Some defenders accept this, while others weaken what the higher-order state requires. Anyone who finds animal pain obvious will treat this as a serious cost of the view.

Because forming a state about your own perception sounds like a job for prefrontal areas, these theories tend to predict that frontal activity tracks awareness even when performance is held constant. The link is an empirical bet added to the philosophy, so a finding against it need not refute every version.

Blindsight patients use visual information while denying they see. First-order processing seems to be working; awareness of it is missing. Higher-order views read this as what their theory predicts. Others explain the same pattern by damage to first-order processing itself, so the case does not settle things alone.

Scientists usually test their own ideas and interpret results in their favor. In this design, opponents write down beforehand what each expects and what would count as a failure, and a separate team runs the work. Nobody can easily claim afterward that the test was unfair, though people can still dispute what the results mean.

Fixing analyses in advance stops researchers from picking, after seeing the data, whichever analysis looks best. It separates testing a prediction from exploring data. Preregistered does not mean correct, because a test can still be poorly aimed, but it makes failed predictions harder to hide.

Participants viewed pictures of faces, objects, letters and meaningless symbols shown for different durations, while separate labs recorded brain activity with several methods. Using more than one method and site was meant to make the results harder to dismiss as an artifact of one lab or tool.

If the posterior network's causal structure specifies what you see, that network should stay synchronized for as long as the experience lasts. A claim about structure thus became a test about timing. The synchrony did not hold up as predicted, and the theory's proponents recorded their own reading of what that means.

Since a content enters the workspace when it appears and must be replaced when it ends, the theory expected a brief burst of frontal activity at both points. It also expected prefrontal cortex to carry what was seen. Offset bursts mostly failed to appear, and frontal coding covered some conscious features and not others.

Results matched parts of each theory: content could be read from the back of the brain, and some frontal activity appeared. Yet central claims of both ran into trouble, so a headline saying one theory won misreads the paper. Any rival making the same predictions inherits the same trouble.

Train a classifier on brain recordings, then test whether it can tell faces from objects on new trials. Success shows a region carries that information. It does not by itself show the region is where you experience it. Treat decoding as evidence of content, not of awareness.

In a famous video, viewers count passes and many miss a person in a gorilla suit. The figure was in plain view and their eyes worked. Some argue they did see it and quickly forgot, so the effect may concern memory as well as experience. The lesson holds either way: looking does not guarantee noticing.

Swap something large in a photo during a brief flicker and people often search for many seconds before spotting it. Without the interruption, the change would pop out. Results like these challenge the sense of holding a detailed picture of the scene, though how much is seen and then lost is still argued.

You cannot process everything at once, so the brain gives priority to some signals, as when you follow one voice at a crowded party. Philosophers debate whether attention is one process or many. Either way, attending is not the same as being conscious, a distinction this chapter turns on.

An image hidden from awareness can still speed responses at its location, which suggests attention can operate on material that never reaches experience. Critics reply that the attention may go to the location and not the unseen item, so the case is not conclusive. Either way, it concerns sufficiency and not necessity.

When you are busy with a central task, you can often still tell that a flashed scene had an animal in it. Some researchers read this as experience with little attention. Others say some attention must have been left over. The debate turns on how to show that attention was truly absent.

The idea is that attention decides what gets into experience, which explains why unattended things often go unnoticed. A single clear case of experience with no attention would refute it. The difficulty is that any report of the experience seems to require attending to it, so clean tests are hard to build.

Hemispatial neglect is a common consequence of stroke. A patient may ignore things on one side of space although the eyes work. The deficit lies in attending to that side, which suggests that attention helps decide what reaches experience. It differs from being blind on that side.

Attention can be directed at a location where a masked item goes unseen, so attending is no guarantee of awareness. Even a gatekeeper view therefore treats attention as one condition for consciousness and not the whole of it. Rule: keep “needed for” apart from “enough for.”

Patients with this damage deny seeing things in the affected field, yet when pressed to guess they point to them far above chance. Researchers think routes that bypass primary visual cortex carry the information. The finding is famous because behavior and report come apart, although how complete the lack of awareness is remains argued.

Ask “did you see it?” and the patient says no. Ask “was it on the left or right, even if you must guess?” and performance is far above chance. The gap between these two questions is the whole evidence. It stays invisible in ordinary testing, where people who say they cannot see are not asked to guess.

If a person's accuracy does not rise with their confidence, they may have no access to how well they are doing. If it does rise, some awareness may be present. Researchers therefore look at how well confidence tracks accuracy, not only at accuracy itself.

Imagine two listeners who detect a faint tone equally well. One says yes whenever unsure, and the other says yes only when certain. Counting yeses makes them look different, while the comparison with false alarms reveals the same ability under different thresholds. Here it separates seeing less from reporting less.

You can perform well and not know it. Metacognition is how well your sense of confidence tracks your real accuracy. Some researchers treat it as a measure of awareness, since blindsight subjects feel they are guessing even when accurate. Others warn that it may measure a separate ability, monitoring, and not experience.

An objective test asks whether a person can perform above chance, and a subjective one asks whether they say they saw the item. A stimulus can pass one and fail the other. Claims of unconscious perception therefore depend on which test was used, so read the method before trusting the headline.

That felt guessing is the report the whole phenomenon rests on. Critics using signal detection note that a patient with weak experience and a strict standard for saying “I see” would report the same thing. So the report alone does not show an absence of experience, and researchers test it further.

The corpus callosum is a thick band of fibers connecting the two hemispheres. Cutting it limits the spread of seizures across the brain. In daily life, patients seem surprisingly ordinary, so the interesting effects appeared only in careful laboratory tests that send information to one side.

While a patient fixes on a central point, an image briefly appears off to one side. The left visual field projects to the right hemisphere and the reverse. Keeping the image brief stops the eyes from moving to it. That simple control lets the experimenter address one hemisphere at a time.

Flash a word to the left visual field and a patient may say they saw nothing, yet find the matching object with the left hand. Because language usually lives on the left, the speaking side did not get the word. That gap between speech and hand is what made these studies famous.

Sperry's experiments, later recognized with a Nobel Prize, suggested each hemisphere could perceive, learn and remember on its own. Hence the striking idea of two separate streams of experience in one person. The reading fit the early data well, but it went beyond what reports alone could show.

Ask whether two shapes shown on opposite sides match, and patients often cannot say. Similar failures show up in touch and in directing attention. Each hemisphere holds what it received, and the comparison is what is lost. This is the strongest evidence that the cut has real effects on perception.

One striking exception is acting: patients do not usually work at cross purposes, and some studies report coordinated responses across visual fields. Routes below the cortex and cross-cueing may help explain this. This is why a simple picture of two complete minds now looks too strong.

The first-person question is difficult because you cannot ask a hemisphere directly, and the speaking side reports only for itself. Reviewers judge the present evidence insufficient and suggest the answer may not be a simple one or two. Treat confident claims either way with care.

When REM sleep was discovered in the 1950s, dreaming seemed to have found its home. Later awakenings showed that non-REM reports occur as well, so REM cannot be equated with dreaming, and studies of the link must take care about which reports they count.

You usually cannot ask a dreamer what is happening without waking them. Reports arrive afterward, shaped by memory and by the effort of putting things into words. So dream science works with a filtered record, the same general problem that besets all report-based consciousness research.

Usually a dream's oddities pass without question. In a lucid dream you realize that it is a dream, and you may even steer it. Reflection and dreaming are therefore not fully exclusive. Philosophers use this to question whether dreams wholly lack the kind of insight found in waking.

Most of the body's muscle tone drops away in REM sleep while the eyes keep moving, which leaves a channel open. LaBerge and colleagues had dreamers agree on an eye pattern beforehand, then make it once lucid. Recordings showed the pattern during REM sleep, a rare marker of something happening in a dream as it happened.

Norman Malcolm doubted that dreams are experiences at all, and Dennett asked whether they might be composed on waking. If so, no one would be conscious during them. Signals sent from within lucid dreams count against this for those dreams, though whether that generalizes to ordinary dreams is still open.

Wake people at random in deep sleep and they often report nothing. That contrast with REM awakenings makes deep sleep a natural test case for what consciousness needs. But nothing remembered is not quite nothing experienced, and some researchers argue for a faint awareness in that state.

Dreams can feel fully real at the time. Descartes used this to ask whether any experience can prove that you are awake. The argument is about knowledge rather than sleep science, and critics ask whether dreams really are as convincing as waking life, or only seem so afterward.

The same brain, on the same night, shifts between rich experience, thin experience and perhaps none. That offers a within-person comparison with no drugs or injury. Researchers compare activity before awakenings with and without dream reports to look for what marks experience, whatever the sleeper's behavior.

A patient may have no experience, may dream with no link to the room, or may be aware of the surroundings. All three can look identical from the outside. The distinction gives researchers a finer question than whether someone is out or not, and each state calls for different evidence.

You usually cannot tell which state applies just by looking, because the patient does not move. Later reports and specialized tests show that some unresponsive patients were having experiences the surgical team could not see. Rule: treat unresponsiveness as a fact about behavior, not about experience.

Drugs differ. Propofol and ketamine act on different receptors and leave different patterns of brain activity, and a low dose can sedate without removing experience. So findings about anesthesia in general often turn out to be findings about one drug at one dose.

Ketamine can leave a person unresponsive while they later report vivid, strange experiences. That makes it a useful contrast for research: by outward signs the person is gone, yet experience continues. It is an anesthetic given under medical care, and nothing here is guidance on its use.

Anesthetics can disrupt forming memories, so a patient who recalls nothing may have had no experience or simply may not have stored it. Recall after surgery is therefore only a partial measure, which is a serious problem for anyone wanting to study the state.

With many agents, brain regions seem to stop exchanging information well, even where each keeps responding locally. Researchers link this to loss of experience. Drugs differ, though, and a correlate of unresponsiveness is not automatically a correlate of unconsciousness.

An older idea pictured anesthesia as simply switching the brain off. Research now points to changed communication, including disconnection between thalamus and cortex. Because the thalamus relays signals to and from the cortex, it is a natural focus, yet it is only one part of the story.

You cannot ethically switch off a person's consciousness for an experiment, except that anesthesia does this daily for medical reasons. Researchers can compare the same person before, during and after, at controlled doses. It is one of the cleanest contrasts available, though a drug's side effects muddy the comparison.

A person in a coma cannot be roused and does not open their eyes even when stimulated. It is usually a temporary stage after severe brain injury: people may recover, or may move into a state with eye opening. Do not confuse it with deep sleep, from which you can be woken.

The renaming reflects worry about the word vegetative and what it suggests about a person. Eyes open and close, and reflexes remain, but no purposeful response appears. The diagnosis describes behavior; it does not prove that nothing is experienced, which is why careful repeat testing matters.

A patient may follow a command on one visit and not on the next. Inconsistent is not the same as absent, so repeat exams matter. This state differs from recovery, in which communication or the use of objects is reliable.

Examiners test hearing, sight, movement, speech, communication and arousal using set procedures. Because it is standardized, the scale catches signs a routine exam might miss. Even so, it can only find what the patient's body can show, and fatigue or sedation on the day can hide signs.

A person with damaged motor pathways might understand everything and still produce nothing to observe. Some patients judged unaware by behavior show other signs of capacity. This is why brain-based tests are under study, alongside bedside exams rather than in place of them.

In the cited study, 4 of 11 patients with the unresponsive wakefulness syndrome gave high-complexity responses, which the authors say might suggest a hidden capacity. That is a cautious claim: capacity is not proof of ongoing experience. Rule: report such a finding at the strength the authors give it.

Stimulate the cortex with a magnetic pulse, record the echo with EEG (a recording of the brain's electrical activity), then compress the resulting pattern as you would a file. A pattern that compresses poorly scores high. Because it needs no response from the patient, the method can be used with anyone who cannot communicate.

An awake brain answers the pulse with activity that spreads and varies by region. In dreamless sleep or deep anesthesia, the response stays local or is a simple wave. A cutoff set in people whose state is known was then applied to patients whose state was uncertain.

Compare a dream, deep meditation and a psychedelic session with ordinary waking. In each, the overall quality of experience has shifted, often in perception, thought and sense of self together. The term is descriptive: it says nothing on whether such a state reveals truth or is good or bad for you.

Drugs in this family share this receptor as a main target, which is part of why their effects resemble one another. That shows where the effect starts; how activity at one receptor changes experience is a further question. Nothing here is advice on using any drug.

Some people report that the boundary between themselves and the world faded. Because this appears to alter the sense of self while other faculties continue, researchers find it interesting for theories of selfhood. Reports are hard to interpret, and questionnaires and expectations can shape them.

Questionnaires ask about a felt unity, a sense of insight, and difficulty putting things into words. Scoring high describes what an experience was like. It does not show that any metaphysical claim made during it is true, a distinction worth keeping when you read this research.

Entropy here measures how disordered brain signals are. Two small studies suggest psychedelics raise it, though similar changes follow other drugs. The hypothesis connects this with subjective variety. It is an early proposal about what accompanies rich experience, not an account of why there is any experience.

On the predictive view, the brain interprets input through prior beliefs. REBUS proposes that psychedelics weaken high-level priors, which gives lower-level signals more influence. It builds on the free energy principle and the entropic brain idea. It is a model under test, not an established mechanism.

Psychedelic findings may bear on how contents or the sense of self change. They do not obviously address why there is experience at all, so the authors call for humility about what the research can show. If a headline says a drug study explains consciousness, ask which sense of the word it means.

Close your eyes and touch your nose: receptors in muscles and joints tell you where your limbs are. You rarely notice this sense until it fails. Rare patients who have lost it must watch their limbs in order to move them, which shows how much everyday action relies on it.

You feel your hand as yours without any reasoning. Philosophers ask whether this is a distinct feeling or a by-product of other sensations. Disorders offer clues: after some strokes, a patient may deny that a limb belongs to them, a condition called somatoparaphrenia.

Botvinick and Cohen reported this in 1998. After minutes of matched stroking, people report feeling touch on the fake hand and misjudge where their real hand is. Ownership turns out to depend on how the senses are combined, even against what you know to be true.

The brain tends to bind signals that arrive together. Synchronous sight and touch suggest one event on one hand, and out-of-step strokes break the match. Control conditions like these are what make the result credible, and they help separate real effects from suggestion.

You walk through a narrow door without thinking about your shoulders: that is the schema. Judging your own height or size is the image. Some patients show one intact and the other impaired, which is the reason for the distinction. The terms are used loosely, so check how a writer defines them.

If someone moves your arm, it is still yours, yet you did not do it. Theories link agency to a match between the movement you predicted and what happened. When it breaks down, as reported in some cases of schizophrenia, people may feel that their actions come from elsewhere.

People who have lost a limb often still feel it, sometimes as painfully cramped. Nothing is there to send the signal, so the sense must be generated centrally. Phantom cases are key evidence that the felt body is a construction, one that normally keeps close track of the real one.

Participants flexed a wrist whenever they felt like it and noted a dot's position on a fast clock face at the moment they first felt the urge. Scalp recordings measured brain activity over the same period. The design's simplicity is its appeal, and its arbitrary choice is one of its main limits.

Electrodes on the scalp reveal this slow drift only once many trials are averaged, lined up on the moment of movement. That matters: a smooth ramp in the average need not appear on each trial, a fact central to later arguments about what the signal means.

The order ran: first the signal, then the reported urge, then the movement a fraction of a second later. This appeared to put brain preparation ahead of awareness. Keep the claim as narrow as the finding: simple, arbitrary movements, timed by report.

The finding concerns timing, and the conclusion concerns freedom, so an argument has to link them. That argument assumes the early signal is the decision, and that a free act must begin in awareness. Each step can be disputed. Separate what was measured from what it is taken to show.

Noting a clock position while deciding to move is a difficult dual task, and the reported moment can shift with attention and with events after the movement. So the gap may be smaller or larger than reported. It does not vanish easily, but it is not a precise measurement either.

Libet thought the last moment before movement left time for conscious cancellation, which he offered as a place for free will. Critics ask what prepares the veto, since it too might have an earlier cause. Experiments on last-moment canceling have tested the limits of this window.

Soon and colleagues used brain imaging to predict which of two options a person would choose, ahead of their reported decision. Fried and others recorded related signals from single neurons. Replications make the timing pattern harder to dismiss, but they do not change the question of what these signals mean.

One proposal, from Schurger and colleagues, is that spontaneous activity drifts up and down, and a movement occurs when it happens to cross a threshold. Averaging back from those moments would produce a ramp. On that reading, the signal is not a decision made early, which is what keeps the debate open.

Keep two questions apart. Whether a bee has any experience can be investigated by looking for markers. What that experience would be like is harder still, since a bee's senses differ so much from yours. Answering the first does not answer the second.

Bats perceive by echo, a sense no human has. Nagel argues that knowing all the physics of echolocation would not tell you what it is like for the bat. That essay gave the field its famous phrase. Its point is a limit on an outside view, not a claim that bats lack experience.

You infer that other humans feel pain because they are built and behave like you. Extending this to animals works best for close relatives and weakens with distance, since behavior may have other causes. Critics warn of two errors: attributing experience too freely and denying it too readily.

Injured fish may change their behavior, and pain-relief drugs can reduce those changes. Critics point out that fish lack a neocortex and that such responses could be reflexes. Because each side interprets the same data differently, this case shows how theory shapes what the evidence seems to say.

Put a mark where an animal can see it only in a mirror. Chimpanzees pass while other primate species fail, with claims for dolphins and some elephants. The test targets one capacity, recognizing your own reflection, so failing it says little about experience in general.

Octopuses solve problems and bees learn rules, yet their brains evolved separately from ours, so similarity of anatomy will not settle anything. This makes them hard cases and valuable ones, since a conclusion about them tests which features a theory takes as essential.

If consciousness needs a layered cortex, then only mammals have it. Birds, with no such layers, show flexible cognition using other structures, and this weakens that requirement. A theory's anatomical claims thus determine its verdicts on animals, which is why they are argued over.

Any one response could in principle run without experience, as a reflex does. Researchers therefore look at clusters of evidence, such as flexible learning and responses to injury. This raises confidence without ever giving certainty. Rule: weigh a pattern of evidence, not one clever experiment.

Alan Turing proposed the imitation game to replace a vague question about thinking with a test of conversation. He did not offer it as a test of experience. A system may pass by imitating human talk, which tells you about its performance. Keep intelligence and consciousness apart.

A person who knows no Chinese follows a rulebook to answer notes in Chinese, and from outside the room it looks like understanding. Searle concludes that symbol manipulation is not sufficient. A common reply says the whole system understands, even if the person does not.

A model trained on text full of human talk about feelings will produce such talk as a matter of course. The statement therefore cannot be taken at face value the way a person's might be. That does not show the system lacks experience; it shows this evidence is weak either way.

List what leading theories say consciousness requires, such as global broadcast or self-monitoring, and check whether a system has it. The method is only as good as those theories, and they disagree. A system could score well on one theory and poorly on another, so results come as conditional verdicts.

Some researchers tie experience to having a body to regulate and a life to sustain, not to computation alone. If they are right, a disembodied model lacks something essential, however fluent. Others treat the body as one way of getting the relevant organization. The question stays open.

If role is all that counts, the right software organization could in principle be enough. If specific biology or physical causal structure matters, it will not be. Much disagreement over machines is really disagreement about this one premise, so ask which view a claim assumes.

The same system may count as a candidate on a workspace view and be ruled out on another. Integrated information theory, for one, rules out strictly feedforward designs whatever they do. There is no agreed test to break the tie, so ask which theory a flat claim about a given AI relies on.