Neural correlate
Encyclopedia
A neural correlate of a content of experience
Experience
Experience as a general concept comprises knowledge of or skill in or observation of some thing or some event gained through involvement in or exposure to that thing or event....

 is any bodily component, such as an electro-neuro-biological state or the state assumed by some biophysical
Biophysics
Biophysics is an interdisciplinary science that uses the methods of physical science to study biological systems. Studies included under the branches of biophysics span all levels of biological organization, from the molecular scale to whole organisms and ecosystems...

 subsystem of the brain, whose presence necessarily and regularly correlates with such a specific content of experience
Experience
Experience as a general concept comprises knowledge of or skill in or observation of some thing or some event gained through involvement in or exposure to that thing or event....

.

When the full ontological
Ontology
Ontology is the philosophical study of the nature of being, existence or reality as such, as well as the basic categories of being and their relations...

 consistence or build-up of the reality variably called mind
Mind
The concept of mind is understood in many different ways by many different traditions, ranging from panpsychism and animism to traditional and organized religious views, as well as secular and materialist philosophies. Most agree that minds are constituted by conscious experience and intelligent...

, soul
Soul
A soul in certain spiritual, philosophical, and psychological traditions is the incorporeal essence of a person or living thing or object. Many philosophical and spiritual systems teach that humans have souls, and others teach that all living things and even inanimate objects have souls. The...

, psyche
Psyche (psychology)
The word psyche has a long history of use in psychology and philosophy, dating back to ancient times, and has been one of the fundamental concepts for understanding human nature from a scientific point of view. The English word soul is sometimes used synonymously, especially in older...

, or existentiality is called "consciousness
Consciousness
Consciousness is a term that refers to the relationship between the mind and the world with which it interacts. It has been defined as: subjectivity, awareness, the ability to experience or to feel, wakefulness, having a sense of selfhood, and the executive control system of the mind...

" and deemed to exclusively consist in mental contents associated with and at least partly generated by the brain
Human brain
The human brain has the same general structure as the brains of other mammals, but is over three times larger than the brain of a typical mammal with an equivalent body size. Estimates for the number of neurons in the human brain range from 80 to 120 billion...

 organ, the notion of neural correlate of consciousness
Neural correlates of consciousness
The neural correlates of consciousness constitute the minimal set of neuronal events and mechanisms sufficient for a specific conscious percept. Neuroscientists use empirical approaches to discover neural correlates of subjective phenomena...

is commonly employed. When it is only the sensations that are held to be produced by brain states, whether exclusively or not (e.g., when sensations are also deemed capable of being generated by the mind reacting against itself), then the notion of neural correlate of a content of experience is commonly utilized. A mid-way concept, not always clarified, is that of a neural correlate encompassing the production of every mental content but not of consciousness itself.

Conceptual frameworks using the notion

The notion of a neural correlate of a mental state is an important concept for materialists, those philosophers and researchers who believe that all mental states are equivalent to brain states. According to strict materialists, all properties credited to the mind
Mind
The concept of mind is understood in many different ways by many different traditions, ranging from panpsychism and animism to traditional and organized religious views, as well as secular and materialist philosophies. Most agree that minds are constituted by conscious experience and intelligent...

, including consciousness
Consciousness
Consciousness is a term that refers to the relationship between the mind and the world with which it interacts. It has been defined as: subjectivity, awareness, the ability to experience or to feel, wakefulness, having a sense of selfhood, and the executive control system of the mind...

, emotion
Emotion
Emotion is a complex psychophysiological experience of an individual's state of mind as interacting with biochemical and environmental influences. In humans, emotion fundamentally involves "physiological arousal, expressive behaviors, and conscious experience." Emotion is associated with mood,...

, beliefs, and desires
Motivation
Motivation is the driving force by which humans achieve their goals. Motivation is said to be intrinsic or extrinsic. The term is generally used for humans but it can also be used to describe the causes for animal behavior as well. This article refers to human motivation...

 have direct neural correlates. This is also a pragmatic view adopted by a number of scholars. This view frequently depends on considering mind
Mind
The concept of mind is understood in many different ways by many different traditions, ranging from panpsychism and animism to traditional and organized religious views, as well as secular and materialist philosophies. Most agree that minds are constituted by conscious experience and intelligent...

s exclusively as sentient knots in nature
Nature
Nature, in the broadest sense, is equivalent to the natural world, physical world, or material world. "Nature" refers to the phenomena of the physical world, and also to life in general...

's causal net.

Instead, other neuroscientists find minds inaugurating causal actions
Causality
Causality is the relationship between an event and a second event , where the second event is understood as a consequence of the first....

 in nature, rather than solely being able of merely continuing previously initiated causal sequences. These neuroscientists thus describe minds as percipient agencies, the mind's very agency or causal aptitude being deemed extraneous to any neural correlate.

Still, some neurobiologists consider reasons to further assume that rememberings are not engraved in brain tissue but retained by the very mind, which thereby transforms itself in a way different from the time changes underwent by non-minds. These later neuroscientists consider that there is no neural correlate for episodal and some other memories, but that the mind moves the brain state (like as it may also inaugurate a finger movement) in such a way as to produce a brain state which the mind then reacts to by fleshing its selected memory again (Husserl's Einfüllung), re-imagining it. This view develops a notion by Aristotle
Aristotle
Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...

 and, again, makes no use of the notion of neural correlate for certain experiences, namely in regard to complex mental contents. It only admits neural correlates for the mind's elementary sensational reactions, which are called "intonations" (following neurobiologist Christfried Jakob, 1866-1956).

The concept of a neural correlate only encompassing the production of intonations occurs in a variety of researchers on different neurobiological traditions, including Scholasticism
Scholasticism
Scholasticism is a method of critical thought which dominated teaching by the academics of medieval universities in Europe from about 1100–1500, and a program of employing that method in articulating and defending orthodoxy in an increasingly pluralistic context...

 (see References). Rather, the concept of a neural correlate encompassing the production of every mental content occurs only in a specific conceptual line of theoretical neuroscience
Neuroscience
Neuroscience is the scientific study of the nervous system. Traditionally, neuroscience has been seen as a branch of biology. However, it is currently an interdisciplinary science that collaborates with other fields such as chemistry, computer science, engineering, linguistics, mathematics,...

, characterized by the following features:
  • Minds are sometimes viewed as incapable of positing causal acts upon self-initiated, internal modifications. This posits observer
    Observation
    Observation is either an activity of a living being, such as a human, consisting of receiving knowledge of the outside world through the senses, or the recording of data using scientific instruments. The term may also refer to any data collected during this activity...

    s in nature as epiphenomenal
    Epiphenomenon
    An epiphenomenon is a secondary phenomenon that occurs alongside or in parallel to a primary phenomenon.-Medicine:...

    , unable to introduce perturbations, even less to absolutely inaugurate causal series.

  • Mental contents are considered stand-alone realities, similar to any other physical reality as apples, rocks, or molecules. Such a line of opinion does not admit a difference in level between non-mental things and mental contents, namely that while things as apples and rocks do not belong with any other natural reality and thus are capable of standing alone, mental contents rather belong with a particular mind or, as sometimes called, a particular existentiality: e.g., this red is in Jane's experience whereas that red (which might be sensationally identical to Jane's) is John's experience and not Jane's at all. This distinctive feature of the contents of experience is dismissed (see Szirko's article cited in References), so that the physical condition of mental contents is deemed identical to the stand-alone condition of apples and rocks.

  • The relationship of a mind with the body in which it finds itself is plato
    Plato
    Plato , was a Classical Greek philosopher, mathematician, student of Socrates, writer of philosophical dialogues, and founder of the Academy in Athens, the first institution of higher learning in the Western world. Along with his mentor, Socrates, and his student, Aristotle, Plato helped to lay the...

    nistically portrayed, so that any a mind might have found itself in any a brain-body system, like as a steerman may find himself in any ship that he chances to steer. Therefore this line of theoretical neuroscience
    Neuroscience
    Neuroscience is the scientific study of the nervous system. Traditionally, neuroscience has been seen as a branch of biology. However, it is currently an interdisciplinary science that collaborates with other fields such as chemistry, computer science, engineering, linguistics, mathematics,...

     does not admit that every mind and its particular body could intrinsically make a unity besides and apart from their causal interactions, which interactions, inasmuch as the mind is also deemed epiphenomenal
    Epiphenomenon
    An epiphenomenon is a secondary phenomenon that occurs alongside or in parallel to a primary phenomenon.-Medicine:...

    , reduce to bodily influences upon the mind's experiences or states.

  • As a consequence, the connections of a mind and the body in which it finds itself are deemed to exclusively be of causal-efficient
    Causality
    Causality is the relationship between an event and a second event , where the second event is understood as a consequence of the first....

     nature, similar to the energy supplied for a domestic appliance to function. This view entails that minds are no more than the mental contents which may be causally generated in them by their respective brains. Sentience (minds' intonability), semovience (minds' capability of inaugurating efficient causal actions
    Causality
    Causality is the relationship between an event and a second event , where the second event is understood as a consequence of the first....

     on internal forces) and circumstantiation (every mind's finding itself in not another body) are thus viewed as highly problematic and their research is usually relegated.


In the conceptual line characterized by these four features, and in recent years, papers have been published on the neural correlates of awareness
Awareness
Awareness is the state or ability to perceive, to feel, or to be conscious of events, objects or sensory patterns. In this level of consciousness, sense data can be confirmed by an observer without necessarily implying understanding. More broadly, it is the state or quality of being aware of...

, emotions, and decision making
Decision making
Decision making can be regarded as the mental processes resulting in the selection of a course of action among several alternative scenarios. Every decision making process produces a final choice. The output can be an action or an opinion of choice.- Overview :Human performance in decision terms...

. Francis Crick
Francis Crick
Francis Harry Compton Crick OM FRS was an English molecular biologist, biophysicist, and neuroscientist, and most noted for being one of two co-discoverers of the structure of the DNA molecule in 1953, together with James D. Watson...

 wrote a popular book "The Astonishing Hypothesis" whose thesis is that the neural correlate for consciousness lies in our nerve cells and their associated molecules. Crick and his collaborator Christof Koch
Christof Koch
Christof Koch is an American neuroscientist working on the neural basis of consciousness. He is the Lois and Victor Troendle Professor of Cognitive and Behavioral Biology at California Institute of Technology, where he has been since 1986...

 have sought to avoid philosophical debates that are associated with the study of consciousness, by emphasizing the search for "correlation" and not "causation".

There is much room for disagreement about the nature of this correlation (e.g., does it require synchronous spikes of neurons in different regions of the brain? Is the co-activation of frontal or parietal areas necessary?). The philosopher David Chalmers
David Chalmers
David John Chalmers is an Australian philosopher specializing in the area of philosophy of mind and philosophy of language, whose recent work concerns verbal disputes. He is Professor of Philosophy and Director of the Centre for Consciousness at the Australian National University...

 maintains that a neural correlate of consciousness, unlike other correlates such as for memory, will fail to offer a satisfactory explanation of the phenomenon.

Laboratory studies

Neurophysiological studies in animals provided some insights on the neural correlates of conscious behavior. Vernon Mountcastle
Vernon Mountcastle
Vernon Benjamin Mountcastle is Professor Emeritus of Neuroscience at Johns Hopkins University.He discovered and characterized the columnar organization of the cerebral cortex in the 1950s...

, in the early 1960s, set up to study this set of problems, which he termed "the Mind/Brain problem", by studying the neural basis of perception in the somatic sensory system. His labs at Johns Hopkins were among the first, along with Edward V.Evarts at NIH, to record neural activity from behaving monkeys. Struck with the elegance of SS Stevens approach of magnitude estimation, Mountcastle's group discovered three different modalities of somatic sensation shared one cognitive attribute: in all cases the firing rate of peripheral neurons was linearly related to the strength of the percept elicited. More recently, Ken H. Britten, William T. Newsome, and C. Daniel Salzman have shown that in area MT of monkeys, neurons respond with variability that suggests they are the basis of decision making about direction of motion. They first showed that neuronal rates are predictive of decisions using signal detection theory, and then that stimulation of these neurons could predictably bias the decision. Such studies were followed by Ranulfo Romo in the somatic sensory system, to confirm, using a different percept and brain area, that a small number of neurons in one brain area underlie perceptual decisions.

Other lab groups have followed Mountcastle's seminal work relating cognitive variables to neuronal activity with more complex cognitive tasks. Although monkeys cannot talk about their perceptions, behavioral tasks have been created in which animals made nonverbal reports, for example by producing hand movements. Many of these studies employ perceptual illusions as a way to dissociate sensations (i.e., the sensory information that the brain receives) from perceptions (i.e., how the consciousness interprets them). Neuronal patterns that represent perceptions rather than merely sensory input are interpreted as reflecting the neuronal correlate of consciousness.

Using such design, Nikos Logothetis
Nikos Logothetis
Nikos Logothetis is Greek a biologist and neuroscientist.Logothetis is director of the department "Physiology of Cognitive Processes" at the Max Planck Institute for Biological Cybernetics in Tübingen...

 and colleagues discovered perception-reflecting neurons in the temporal lobe. They created an experimental situation in which conflicting images were presented to different eyes (i.e., binocular rivalry
Binocular rivalry
Binocular rivalry is a phenomenon of visual perception in which perception alternates between different images presented to each eye.When one image is presented to one eye and a very different image is presented to the other, instead of the two images being seen superimposed, one image is seen for...

). Under such conditions, human subjects report bistable percepts: they perceive alternatively one or the other image. Logothetis and colleagues trained the monkeys to report with their arm movements which image they perceived. Interestingly, temporal lobe neurons in Logothetis experiments often reflected what the monkeys' perceived. Neurons with such properties were less frequently observed in the primary visual cortex that corresponds to relatively early stages of visual processing. Another set of experiments using binocular rivalry in humans showed that certain layers of the cortex can be excluded as candidates of the neural correlate of consciousness. Logothetis and colleagues switched the images between eyes during the percept of one of the images. Surprisingly the percept stayed stable. This means that the conscious percept stayed stable and at the same time the primary input to layer 4, which is the input layer, in the visual cortex changed. Therefore layer 4 can not be a part of the neural correlate of consciousness. Mikhail Lebedev
Attention versus memory in prefrontal cortex
A widely accepted theory regarding the function of the brain's prefrontal cortex is that it serves as a store of short-term memory. This idea was first formulated by Jacobsen, who reported in 1935 that damage to the primate prefrontal cortex caused short-term memory deficits...

 and their colleagues observed a similar phenomenon in monkey prefrontal cortex. In their experiments monkeys reported the perceived direction of visual stimulus movement (which could be an illusion) by making eye movements. Some prefrontal cortex neurons represented actual and some — perceived displacements of the stimulus. Observation of perception related neurons in prefrontal cortex is consistent with the theory of Christof Koch
Christof Koch
Christof Koch is an American neuroscientist working on the neural basis of consciousness. He is the Lois and Victor Troendle Professor of Cognitive and Behavioral Biology at California Institute of Technology, where he has been since 1986...

 and Francis Crick
Francis Crick
Francis Harry Compton Crick OM FRS was an English molecular biologist, biophysicist, and neuroscientist, and most noted for being one of two co-discoverers of the structure of the DNA molecule in 1953, together with James D. Watson...

 who postulated that neural correlate of consciousness resides in prefrontal cortex. Proponents of distributed neuronal processing may likely dispute the view that consciousness has a precise localization in the brain.

Neural correlates of cognitive variables

In addition to neural correlate of consciousness in general, much progress has been made in elucidating neural correlates of specific cognitive variables. Thus, Earl Miller and colleagues discovered prefrontal cortex neurons that represent perceptual categories (cats versus dogs in their experiments). The work of Richard Andersen, Steven Wise, Carl Olson, Jun Tanji, Apostolos Georgopoulos and other neuroscientists has illuminated neuronal correlates of motor planning, selective visual attention, motor sequences and spatial reference frames in which these entities are represented by brain cells. The progress in understanding neuronal correlates of motor planning has led to creation of brain-machine interfaces, the devices that translate neuronal activity into purposeful commands to artificial actuators.

A large number of studies have addressed the problem of neural correlates of mental representations in human subjects. For example, functional neuroimaging have shown that parts of the cortex are still active in vegetative patients that are presumed to be unconscious (Laureys, Trends Cogn Sci, 2005, 9:556-559). However, these areas appear to be functionally disconnected from associative cortical areas whose activity is needed for awareness.
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