Neural correlate of consciousness

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Neural correlates of conscious experience (Engl. Neural correlates of consciousness ) are brain activities associated with consciousness associated processes. The search for neural correlates is a central project in neuroscientific research into conscious experience.

Theories on neural correlates of conscious experience are basically still in a preliminary stage. This is partly due to technical problems, such as the lack of temporal and spatial resolution of imaging processes that record activities in the brain. In addition, the way in which the brain stores information has not yet been satisfactorily clarified.

Already in phrenology , mental abilities were assigned to individual regions of the skull and thus indirectly to certain brain structures.

History and overview

The search for neuronal correlates of conscious experience is still a relatively new branch of neuroscience. Many discoveries have been made in the past 30 years. Nevertheless, the idea of ​​a correspondence between mental and neural structures is quite old. Such a research program did not make sense in the context of Cartesian metaphysics , since Descartes assumed an immaterial spirit that should only interact with the brain at one point - the epiphysis . However, at the end of the 18th century , Franz Josef Gall tried to relate mental abilities to certain areas of the brain. Gall's phrenology , however, could not prevail because it lacked precise data that would have been necessary for a successful description of correspondences.

The first empirical advances came through neuropsychological research in the 19th century . Scientists such as Paul Broca and Carl Wernicke succeeded in locating regions of the brain that were related to certain cognitive abilities. The examination of patients with cognitive deficits due to precisely localized damage to the brain played a major role. However, even the methods of 19th century neuroscience were still too crude to be able to describe correlates of certain consciousness processes. A significant improvement was only achieved with electroencephalography (EEG) and imaging procedures . These new methods made it possible to record neural activity in certain areas of the brain. Furthermore, a method was developed to temporarily change processes in selected brain regions from the outside using transcranial magnetic stimulation (TMS) in a non-invasive way.

Neuroscientific research on consciousness shows various connections to the philosophy of the mind : On the one hand, research seems for the first time to predict the description of the biological processes that are related to the phenomenon of conscious experience. On the other hand, the search for neural correlates does not involve any direct partisanship for a particular philosophical position: reductionists can assume that the search for correlates is the first step in returning consciousness to biological processes. Other theorists do not see the search for neural correlates as reductionist: they can be used to describe the relationship between mind and brain without tracing one back to the other. Even some dualistic positions - such as that of David Chalmers - are compatible with the existence of neural correlates.

What is a neural correlate?

Neural correlates of consciousness are structures and processes in the brain that are related to conscious experience. On closer inspection, however, the term turns out to be ambiguous.

First of all, it must be asked whether one is looking for a correlate of conscious experience in general or for correlates of specific conscious experiences , such as a certain perception or memory. Both goals can lead to meaningful research projects. A correlate in the first sense would be sufficient for assigning conscious experience to a living being. In the neurosciences , however, correlates in the second sense are mostly searched for.

Methods and research results

Conscious and unconscious processing

An important aspect is the distinction between conscious and unconscious processing. Standard cognitive psychological techniques such as priming show that perception and memory remain partially unconscious. This has obvious consequences for neuroscience: when an object is visually presented to a person, the brain processes a great deal of information about this object, but the person only becomes aware of some of this information. For the search for neural correlates of conscious experience, the challenge arises to separate those neural processing processes that are linked to conscious processes from those that occur unconsciously.

From the visual cortex (red) the dorsal current leads to the posterior parietal lobe (yellow), the ventral current to the inferior temporal cortex (green).

An example of this is the distinction between a dorsal and a ventral stream of visual perception. Supported by lesion studies, Leslie G. Ungerleider and Mortimer Mishkin introduced the distinction between two processing pathways. From the visual cortex go two main streams: 1) Ventral current : Here signals are directed into the inferior temporal cortex (IT), where an analysis of characteristics - such as color, pattern and shape - takes place. 2) Dorsal current : Here signals are passed on to the posterior parietal lobe , where the object is spatially localized. In more recent works, Melvyn Goodale and David Milner have adopted the idea of ​​the two processing streams of visual perception and linked it with a thesis on conscious experience. According to Goodale and Milner, processing in the ventral stream is associated with conscious, phenomenal perception, while dorsal processing is largely unconscious. According to this thesis, neural correlates of conscious visual perception would be located in the inferior temporal cortex.

Binocular rivalry

A classic method in the search for neural correlates of conscious experience is based on the phenomenon of binocular rivalry . Binocular rivalry is the spontaneous change in the consciously perceived object. A binocular rivalry occurs when the two eyes are presented with two different images that cannot be integrated into a single image. An example: You can present a red, vertical bar to the left eye and a green, horizontal bar to the right eye. When confronted with this, the test person perceives a red or a green bar in alternating sequence, but never both images at the same time. In addition, the person cannot willfully control the internal image changes that occur in them.

In the search for neural correlates of conscious experience, one can make use of this phenomenon by examining which changes in the neural process take place at the moment of the change in the perceptual image. Important experiments on this topic were carried out in particular by Nikos Logothetis . The results showed that many elements of neural processing remained unchanged during the perceptual shift. Not only did the stimulation of the retina remain the same in the various perceptual states, there was even no change in large parts of the visual cortex during the change in perception. Rather, the information about the two images presented seemed to be available there. The situation was different in parts of the temporal lobe, however, where a change in neural activity actually correlated with the change in the subjectively perceived image.

Lesions

Of particular importance for consciousness research are cases in which conscious experience is disturbed, but the associated information processing remains intact in parts. A well-known case is the cortical blindness (blindsight) researched in particular by Lawrence Weiskrantz . Patients with cortical blindness perceive themselves as completely blind people. You cannot subjectively experience any visual input. However, when patients are asked to “guess” where a given visual stimulus is, these patients perform well beyond random guessing. The explanation is that in cortical blindness it is not the retina but the visual cortex that is damaged. However, processing paths that enable an unconscious flow of information remain intact. Similar phenomena can also be identified for other cognitive functions . Patients with aphasia , amnesia or agnosia subjectively experience a loss of speech understanding, memory or object recognition. Experiments show, however, that some of these patients can unconsciously achieve parts of the performance that they subjectively seem no longer capable of. Such findings make important contributions to the question of which structures and processes are necessary for conscious experience.

Theories

There are many hypotheses on the question of which neural structures and processes are correlates of conscious experience. Some researchers suspect that the neurons involved fire in a very specific way. Other theories attempt to anatomically narrow down the neural correlates. The different hypotheses do not always have to be understood as opposites. In some cases they even complement each other quite well.

Binding problem and 40 Hz oscillations

An influential theory is represented by Francis Crick and Christof Koch , for example , and is based on ideas that have been formulated in the context of the attachment problem. The attachment problem arises from the question of how the brain succeeds in combining a wide range of sensory information into uniform perceptions. Experiments and theoretical considerations led to the realization that the brain has to access information in the form of distributed representations . It may sound plausible at first that the brain represents an object by means of a specific neuron : If such a “ grandmother neuron ” were active, the corresponding object (such as the grandmother) would be represented by the brain - otherwise not. However, such a form of representation cannot be implemented consistently. The multitude of possible combinations of features means that people can perceive an almost unlimited number of different objects. With such a combinatorial explosion , a special neuron cannot be kept ready for each object.

Through distributed representations, the brain itself would create a combinatorial explosion. Different neurons would no longer represent objects, but features. These features could be combined with one another and thus a visually perceived object represented. However, the question now arises as to how the brain registers which features are linked to one another in an object. This problem is particularly pressing when - as is common in everyday life - several objects are perceived at the same time. Assume that the brain stores the different characteristics of the various objects through distributed representations. How does a correct connection of the characteristics and a uniform perception come about? This question is known as the “attachment problem”. The neuroinformatics Christoph von der Malsburg developed in the early 1980s a proposed solution for the binding problem: The brain can combine the features by the representative of neurons by synchronous formed fires a temporary alliance. This hypothesis attracted international attention a few years later after the research group around Wolf Singer was able to support it experimentally.

Since the phenomenon of synchronous fire was supposed to explain the emergence of uniform perception, it was natural to consider it for conscious experience as well. The essay by Crick and Koch from 1990 has become particularly influential here. The two researchers assumed that oscillating activity in the 40 Hz range was the neural correlate of conscious experience. Even today, synchronous neuron activities play a central role in many theories on neuronal correlates. However, it is mostly assumed that such a synchronous firing alone is not sufficient for a conscious perception. Even Crick and Koch explained this in one of their last joint essays: "We no longer believe that synchronized firing - such as the so-called 40 Hz oscillation - is sufficient for the neural correlate of consciousness." combine conscious experience with the idea of ​​the neural correlate, so one could limit the thesis to certain compound formations - such as those that occur in a defined brain region.

Neuroanatomical Theories

An fMRI scan, the thalamus is marked by the arrow.

Which neuronal structures now have a prominent position? The above results on the dorsal and ventral flow and on the binocular rivalry suggest a central role of the inferior temporalis cortex, at least in terms of visual perception.

Another important region is the thalamus , a partial structure of the diencephalon . As early as 1937 Wilder Penfield stated: "All parts of the brain can be involved in the normal conscious process, but the indispensable substrate of consciousness is probably outside the cerebral cortex - in the diencephalon." The diencephalon - and in particular the thalamus - continues to play a major role in the search for neural correlates of conscious experience. Joseph Bogen, for example, claims that consciousness is correlated with activity in and around the non-specific thalamic nuclei. Even Gerald Edelman and Giulio Tononi emphasize the role of the thalamus in their theory. However, in their opinion, specific activity in the thalamus alone is not sufficient for conscious experience. Rather, it has two central properties: 1) The various characteristics of what is experienced are understood as a unit. The quality of the consciousness of the experience cannot be split up into sub-components. 2) Conscious experience is differentiated in the sense that it is possible to experience extremely many, very different elements in a short period of time. According to Edelman and Tononi, an adequate neuroscientific theory must take these properties into account. Neurophysiological processes should be identified that are characterized by both unifying integration and differentiation. Edelman and Tononi therefore assume recursive neural processes , the activation of which runs through reentrant loops. The thalamus plays a central role in these loops. The authors take a more holistic approach. Accordingly, they expect that there will be no narrowly defined groups of neurons that can be regarded as a neural correlate of conscious experience. Rather, it always depends on a very extensive activity in large parts of the brain.

Success in the search for neural correlates is particularly due to the modular way in which the brain works: regions in the brain can be identified that are selectively active in certain conscious experiences. Examples are the Fusiform Face Area , which becomes active with facial perception, and the Parahippocampal Place Area , which responds to houses and visual scenes. Activities in these regions therefore allow conclusions to be drawn about the content of the perception. Even more detailed knowledge of the current state of perception can be achieved by measuring the activity of individual neurons. So neurons were found that only respond to the faces of a certain celebrity. In one patient, for example, a neuron was found that only became more active than average in images from Bill Clinton. For 50 images presented, the neuron only responded to a cartoon of Bill Clinton, his official portrait, and a group picture with Clinton. Similar results were obtained with pictures of other celebrities.

Philosophical problems

Some philosophers consider the reductionist possibility presented above to be unrealistic. They argue that even a detailed knowledge of neural processes will not close the explanatory gap between conscious experience and biological processes. The explanation gap arises from the fact that conscious experience is characterized by special properties - especially by qualia . “Qualia” is understood to mean the subjective experience content, such as pain or joy. It is now argued that no description of neural events, no matter how detailed, can explain why something is experienced subjectively. One can only show that certain activities occur in the brain in the case of headaches, for example. But this does not explain why this activity leads to the experience of pain. This means that a neuroscientific theory cannot explain experience.

A similar argument against reductionism is based on the concept of intentionality . By "intentionality" it is meant that some mental processes relate to objects or facts in the world. It is this reference that makes thoughts, like sentences, true or false . An example: the idea that Salamanca has the oldest university in Spain can be related to the historical fact that Salamanca has the oldest university in Spain. It is this reference that makes the thought come true. Now some philosophers argue that neural processes do not relate to Salamanca or anything about Salamanca and therefore cannot be true or false. Since thoughts have the property of intentionality, but neural processes are non-intentional, thoughts cannot be reduced to neural processes. This is not possible even if one has found and researched the neural correlates of thoughts.

Other philosophers and scientists reject these arguments. Researchers consider neuroscientific explanations to be possible, particularly with regard to the phenomenon of intentionality. Such attempts at explanation often refer to the concept of representation : some neural processes are representations of facts. Such representations establish an intentional reference and the processes can be called true or false. It is pointed out that machines also have true and false representations, in some cases even recognize and correct them, and also have programmed triggers for actions.

Some philosophers, such as Daniel Dennett and Paul Churchland, dispute the legitimacy of the acceptance of qualia. Other researchers do not want to go as far as Dennett and the Churchlands but still hold on to the possibility of a reductive explanation. They often argue with the analogy of the scientific explanation of life : the phenomenon of life was also long considered (scientifically) inexplicable, which led to seemingly mysterious conceptions like that of vitalism . However, with the advances in modern biology , a large part of the explanatory gap has disappeared, so that today life is no longer considered a fundamentally mysterious and inexplicable phenomenon. While some researchers believe they can expect a similar development in terms of conscious experience, others reject this analogy. The differences between the problems of that time in explaining life and today's problems in explaining conscious experience are too great. In addition, however, there are still philosophers (for example representatives of neo-scholasticism ) who consider not only conscious experience , but also life to be sufficiently explainable only through an immaterial substance such as the spirit or the soul.

See also

literature

Web links

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  1. ↑ Franz Josef Gall: Philosophical-Medicinical Investigations on Nature and Art in Sick and Healthy Man , Grässer and Comp, Vienna, 1791
  2. ^ David Chalmers: The conscious mind: in search of a fundamental theory , Oxford University Press, Oxford, 1996, ISBN 0-19-511789-1
  3. ↑ One exception is for example: Joseph Bogen: On the neurophysiology of consciousness. Part 1: Overview. In: Consciousness and Cognition , 1995
  4. ^ Leslie Ungerleider, Mortimer Mishkin: Two cortical visual systems . In: Analysis of visual behavior , 1982
  5. ^ Melvyn Goodale, David Milner: Separate visual pathways for perception and action . In: Trends in Neuroscience , 1992
  6. ↑ D. Alais, R. Blake (Eds.): Binocular Rivalry , MIT Press, 2005, ISBN 0-262-01212-X
  7. ^ NK Logothetis, JD Schall: Neuronal correlates of subjective visual perception . In: Science , 1989
  8. ^ NK Logothetis, DA Leopold, DL Sheinberg: Neural mechanisms of perceptual organization in: Cognitive Studies: Bulletins of the Japanese Cognitive Science Society 4 , 1997
  9. ↑ Lawrence Weiskrantz: Blindsight: A case study and its implications Oxford University Press, Oxford, 1989, ISBN 0-19-852192-8
  10. ↑ Lawrence Weiskrantz: Disconnected awareness for detecting, processing, and remembering in neurological patients . In: Journal of the Royal Society of Medicine , 1991
  11. ^ A b Francis Crick, Christof Koch: Towards a neurobiological theory of consciousness . In: Seminars in the Neurosciences , 1990
  12. ↑ a b Joseph Bogen : On the neurophysiology of consciousness. Part 1: Overview. In: Consciousness and Cognition , 1995
  13. ^ Christof von der Malsburg: The Correlation Theory of Brain Function . In: Technical Report 81-2, Biophysical Chemistry, MPI , 1981
  14. ^ Charles Gray, Wolf Singer: Stimulus-specific neuronal oscillations in the cat visual cortex: A cortical functional unit . In: Society of Neuroscience Abstracts , 1987
  15. ^ Francis Crick, Christof Koch : A framework for consciousness. In: Nature Neuroscience , 2003, p. 1139.
  16. ↑ Wilder Penfield: The cerebral Cortex and consciousness , 1937
  17. ^ Giulio Tononi and Gerald Edelmann: Consciousness and Complexity in Science , 1998
  18. ↑ Nancy Kanwisher , J. McDermott, MM Chun: The fusiform face area: a module in human extrastriate cortex specialized for face perception . J. Neurosci. 17, 4302-4311 (1997).
  19. ↑ R. Epstein, N. Kanwisher: A cortical representation of the local visual environment . Nature 392: 598-601 (1999).
  20. ↑ G. Kreiman, C. Koch, I. Fried: Category-specific visual responses of single neurons in the human median temporal lobe . Nature Neurosci. 3: 946-953 (2000).
  21. ^ Joseph Levine: Materialism and Qualia: The Explanatory Gap . In: Pacific Philosophical Quarterly , 1983
  22. ↑ Ulrike Haas-Spohn (Ed.): Intentionality between subjectivity and relation to the world , Paderborn, Mentis, 2003, ISBN 3-89785-065-6
  23. ↑ Daniel Dennett: Quining Qualia . In: Marcel, Bisach: Consciousness in Contemporary Science. Calrandon Press, Oxford, 1993, pp. 42-77, ISBN 0-19-852237-1
  24. ^ Paul Churchland: Matter and Consciousness , MIT Press, Cambridge, Mass., 1988, ISBN 0-262-03135-3
This version was added to the list of articles worth reading on July 20, 2006 .