Association cortex

The association cortex is the part of the cerebrum that cannot be assigned to the primary projection fields. Contrary to previous definitions, the association cortex serves as a so-called unspecific cortex not only as a connection between primary projection centers (the so-called specific cortex ), as an apparatus with cortico-cortical fiber connections, but also maintains feedback connections to deeper nuclei of the thalamus or the limbic system .

Topographical classification

The association cortex is divided into:

frontal association cortex - Brodmann areas 9-12, 46-47
limbic association cortex - Brodmann areas 28 (?), 34 (?)
parieto-temporo-occipital or only briefly parietal association cortex - Brodmann areas 5, 7, 37 (?), 39, 40
- somatosensitive association cortex - Brodmann area 5 and 7 as a subdivision of the parieto-temporo-occipital association cortex.

Functional classification

Fig. 1. Myelination stages of the brain according to Paul Flechsig . The dark areas become early, the light gray later and the white z. Sometimes myelinated only during puberty.

A distinction is also made between secondary and tertiary association areas. In the secondary association areas the integration of the respective individual sensory performances takes place , in the tertiary association centers , however, the integration between all sensory performances takes place in general. "Individual sensory services" are provided by the respective centers of the individual sensory modalities , i. H. of the specific, primarily sensory bark fields in connection with the secondary association centers. “Tertiary Association Centers” process the results of all surrounding secondary centers. Such a tertiary center can be found e.g. B. in the transition region between secondary visual, auditory, tactile or kinaesthetic association areas of Brodmann areas 39 and 40, probably also area 37. The activity of this tertiary center can be assumed to be the highest forms of human perception and knowledge. - The feedback to the nuclei of the thalamus is used to compare it with previous experiences. - Electrophysiologically, the secondary association areas can be differentiated with the help of so-called secondary potentials. Electrophysiologically, however, they cannot be assigned to a specific sense organ.

The topographical and functional classification of the primary, secondary and tertiary centers roughly corresponds to the various stages of myelination , see also Fig. 1. The dark areas are the primary (sensory or motor) centers, the light gray areas the secondary and the white fields around the tertiary centers. The development of the neocortex is largely due to the unfolding of associative areas, which are most strongly developed in humans compared to vertebrates.

In the area of the frontal lobe, instead of “tertiary association areas”, a series of mental deficits have been described as local brain psychosyndromes as a result of frontal localizable destruction, cf. Chapter 5.1 Frontal Association Cortex . The neural structure and the fiber connections of the frontal association field correspond to the double-barreled connections of the parieto-temporo-occipital transition regions to the thalamus. The premotor area ( areas 6 and 8 ) is still counted as part of the projection areas in cytoarchitectonic terms, although there are clear differences to the primary cortex, but contains - as already shown as typical for the association areas - double-ended connections to the thalamus. In cytoarchitectural terms, Betz's giant cells are also absent compared to Area 4 of the primary motor cortex. Disturbances of the parieto-temporo-occipital association fields lead to the various neuropsychological syndromes such as B. Disorders of the body schema .

histology

The histological structure has six layers. Compared to the primarily motor cortex (enlarged layer V , many efferents ) and the primarily sensory cortex (enlarged layer IV , many afferents ), the association cortex has a pronounced layer III with many association fibers, i.e. H. Fibers that connect the cortical centers to one another.

Basic concept of functional neuroanatomy

In addition to the basic concepts of ascending (afferent) and descending (efferent) nerve tracts (cf. specifically sensory cortex and specifically motor cortex or sensorimotor cortex ), the concept of the association cortex has proven to be heuristic or groundbreaking for research in neurophysiology . Due to this classification, not all areas of the cerebral cortex can be classified into specifically sensory or specifically motor areas. The brain areas that cannot be classified as specific are therefore referred to as unspecific cortex or as an intermediate layer or as a transition region (e.g. Brodmann area 39, 40). These names are based on the idea of the reflex arc . Since the function of a reflex arc runs more or less automatically, the attentive and conscious processing of stimuli in higher living beings is linked to the existence of a third group of nerve cells.

A prime example of this type of neuronal processing is the question of the decision, for example between sauerkraut (S) and vanilla sauce (V). As is well known, the two do not taste good together. This is known to be a logical exclusive OR link (XOR). Such decisions can not be resolved by a two- tier network if both variants are equally appetizing . A so-called three-layer network is required here. In addition to the representation of both variants, i.e. of (S) and (V), a third affinity is required in the sense of a further input for decision-making. A two-layer network , on the other hand, consists of a fixed and direct, always uniform interconnection between input and output. There are only possible variations in the sense of a logical yes / no link. Either the incoming stimulus is passed on or not, as z. B. happens in a monosynaptic reflex via the interconnection of the reflex arc. Input and output take place here via sensitive and motor neurons . Practical example: When touching a hot stove top, only a two-layer input / output system is sufficient. At a certain level of heat, pulling your hand away is the only sensible alternative.

In humans, however, the unspecific cortex also takes up significantly more space than the specific one in great apes. This underscores the importance of the association cortex. The name goes back to traditional association experiments in psychology. According to an older definition, projection trajectories connect the primary centers of a hemisphere as specific performers with afferent or efferent - not necessarily efferent-motoric - fibers. Association tracks, on the other hand, as a non-specific system, provide assistance as mediation between these specific centers. However, this would be too simplistic, as new types of services arise with each level of integration. So there are certain levels or degrees of integration here. The extrapyramidal cortex not only relieves the pyramidal tract, but also makes its activity superfluous as soon as z. B. a certain sequence of movements is learned. This can also be seen as antagonism. The services of the specific cortex cannot only be understood in terms of a specific input / output system - as described above. Rather, it is a question of often integrated functional units based on the concept of the module in communications technology .

topography

Frontal association cortex

The frontal association cortex is formed by the frontal lobe and lies in front of the supplementary motor cortex . Simply put, it can be called the “seat of personality” . The surgical removal of the frontal association cortex is called a lobectomy . The model patient is the American demolition master Phineas Gage , who got a bolt through the frontal cortex in an explosion. He had neither a reduced intelligence nor learning problems (compared to the amnesia patient with removal of the temporal lobes ). However, his personality had changed. He was then unreliable, vulgar, and emotionally unstable.

Limbic Association Cortex

The limbic association cortex is ascribed a major role in the process of learning and recognition, mainly of faces, but also characteristic properties. The entorhinal region is assumed to be an important area of association here. It is located lateral to the hippocampus in the area of the parahippocampal gyrus . This corresponds to area 28 ( area entorhinalis ventralis ) and area 34 ( area entorhinalis dorsalis ) according to Brodmann. There is a multitude of convergent fiber courses of an olfactory, somatosensory, visual, auditory and motor type, which are transmitted from here to the hippocampus and from there to the limbic system. According to Heiko Braak and E. Braak et al. a. the entorhinal region fulfills the function of a multimodal association center. This is an extremely important link for memory. Only the collaboration between Allocortex and Isocortex guarantees that it will be remembered. For example, it is the task of the limbic association cortex to recognize handwriting and assign it to a person, but not to read what is written.

Parietal association cortex

The parietal association cortex is the best studied of all. It has the greatest degree of side asymmetry. He is responsible for neglect syndrome, in which patients ignore the left side of their world. Functionally, the parietal cortex can be divided as follows:

left association cortex	right association cortex
Representation of the right visual field	Representation of the left visual field
lexical language	emotional tone of speech
Write	spatial orientation
Speak	abstract spatial thinking
logical abstract thinking	Neglect
lens	subjective

It should always be noted that these classifications are very academic and should not be viewed strictly. Much more, preferential phenomena are described that result from the observations made on patients with certain functional failures.

Clinical relevance

There are many diseases associated with the association cortex. An important example are the so-called split-brain patients: because of generalized epilepsy, the corpus callosum was severed (callosotomy) so that the two hemispheres could no longer communicate with each other. Due to the functional asymmetry, they could no longer verbally name objects that were not seen and only felt with the left hand (this is represented in the right hemisphere), since the language center is in the left hemisphere. The objects were recognized and could be used correctly. This is known as disconnection syndrome .

Biological relevance

Animation of the central field theory

The differentiation between fields of association and fields of projection is to be regarded as biologically meaningful. Projection fields represent the locations of the primary cortex and association fields the locations of the secondary and tertiary cortex. Association fields and projection fields were also distinguished from one another by terms such as unspecific cortex and specific cortex . Why it makes sense that not the entire cortex as a "uniform control center" is also consequently to be viewed as "uniformly highly specialized" results from field theory . The validity of this theory for living organisms results from the applicability of physiological principles to the organization of nerve cells, which is a principle of neurophysiology and psychophysiology . For these spatial principles speaks u. a. also the confirmation of the vectorial functioning of nerve cells of the cerebral cortex. This organizational principle of combining specialized and less specialized nerve cells becomes more understandable through technical applications of this principle, such as the system of central locations .

Web links

Tertiary fields of association : Spectrum Academic Publishing House

Individual evidence

↑ Norbert Boss (ed.): Roche Lexikon Medizin , Hoffmann-La Roche AG and Urban & Schwarzenberg, Munich, ² 1987, ISBN 3-541-13191-8 , head. “Association fields”, page 125.
↑ ^a ^b ^c Peter Duus : Neurological-topical diagnostics. Anatomy, physiology, clinic. Georg Thieme Verlag, Stuttgart, ⁵ 1990, ISBN 3-13-535805-4 , page 387 ff. (A), 383 f. (b), 275 (c).
↑ ^a ^b ^c Manfred Spitzer : Spirit in the net , models for learning, thinking and acting. Spektrum Akademischer Verlag Heidelberg 1996, ISBN 3-8274-0109-7 . Pages 195 f. (a), 125 ff. (b), 92 ff.
↑ Karl Kleist : Brain Pathology . In: Handbook of medical experiences in the World War 1914/18. Vol. IV. Barth, Leipzig 1922–1934; See there the syndrome of convexity of the prefrontal region described by Kleist .
↑ Leonore Welt : About changes in character in humans as a result of lesions of the frontal lobe. Dtsch.Arch.klin.Med. (1888) 42 , 339-390; This organic pattern of damage includes syndromes with bilateral damage to the orbital cortex, e.g. B. as a result of contusions .
↑ ^a ^b Robert F. Schmidt (Ed.): Outline of Neurophysiology . Springer Berlin 3rd edition 1979, ISBN 3-540-07827-4 , pages 281 (a), 282 (b).
^ Walter Siegenthaler : Clinical Pathophysiology. Georg Thieme Verlag, Stuttgart 1970, page 922 f. (Chapter agnosia and apraxia).
↑ Marvin Minsky and Seymour Papert : Perceptions (1969/1988) In: JA Anderson and E. Rosenfeld (eds.): Neurocomputing . MIT-Press, Cambridge MA, pp. 157-169.
^ Phil Hubbard, Gill Valentine et al .: Key Texts in Human Geography. A reader guide . TJ International, Padstow (Cornwall / England), ISBN 978-1-4129-2260-9 .

[1] Norbert Boss (ed.): Roche Lexikon Medizin , Hoffmann-La Roche AG and Urban & Schwarzenberg, Munich, ² 1987, ISBN 3-541-13191-8 , head. “Association fields”, page 125.

[NTD-2] Peter Duus : Neurological-topical diagnostics. Anatomy, physiology, clinic. Georg Thieme Verlag, Stuttgart, ⁵ 1990, ISBN 3-13-535805-4 , page 387 ff. (A), 383 f. (b), 275 (c).

[GIN-3] Manfred Spitzer : Spirit in the net , models for learning, thinking and acting. Spektrum Akademischer Verlag Heidelberg 1996, ISBN 3-8274-0109-7 . Pages 195 f. (a), 125 ff. (b), 92 ff.

[4] Karl Kleist : Brain Pathology . In: Handbook of medical experiences in the World War 1914/18. Vol. IV. Barth, Leipzig 1922–1934; See there the syndrome of convexity of the prefrontal region described by Kleist .

[5] Leonore Welt : About changes in character in humans as a result of lesions of the frontal lobe. Dtsch.Arch.klin.Med. (1888) 42 , 339-390; This organic pattern of damage includes syndromes with bilateral damage to the orbital cortex, e.g. B. as a result of contusions .

[GDN-6] Robert F. Schmidt (Ed.): Outline of Neurophysiology . Springer Berlin 3rd edition 1979, ISBN 3-540-07827-4 , pages 281 (a), 282 (b).

[7] Walter Siegenthaler : Clinical Pathophysiology. Georg Thieme Verlag, Stuttgart 1970, page 922 f. (Chapter agnosia and apraxia).

[8] Marvin Minsky and Seymour Papert : Perceptions (1969/1988) In: JA Anderson and E. Rosenfeld (eds.): Neurocomputing . MIT-Press, Cambridge MA, pp. 157-169.

[9] Phil Hubbard, Gill Valentine et al .: Key Texts in Human Geography. A reader guide . TJ International, Padstow (Cornwall / England), ISBN 978-1-4129-2260-9 .