A neural correlate of a content of experience is any bodily component, such as an electro-neuro-biological state or the state assumed by some biophysical subsystem of the brain, whose presence necessarily and regularly correlates with such a specific content of experience.
When the full ontological consistence or build-up of the reality variably called mind, soul, psyche, or existentiality is called "consciousness" and deemed to exclusively consist in mental contents associated with and at least partly generated by the brain organ, the notion of neural correlate of consciousness 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.
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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, including consciousness, emotion, beliefs, and desires have direct neural correlates. This is also a pragmatic view adopted by a number of scholars. This view frequently depends on considering minds exclusively as sentient knots in nature's causal net.
Instead, other neuroscientists find minds inaugurating causal actions 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 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 (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, characterized by the following features:
In the conceptual line characterized by these four features, and in recent years, papers have been published on the neural correlates of awareness, emotions, and decision making. Francis Crick 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 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 maintains that a neural correlate of consciousness, unlike other correlates such as for memory, will fail to offer a satisfactory explanation of the phenomenon.
Neurophysiological studies in animals provided some insights on the neural correlates of conscious behavior. Vernon Mountcastle, 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 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). 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 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 and Francis Crick 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.
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.