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BEYİNDE ASOSİASYON ALANLARI Integrative Processing of Internal and External Information Introduction People usually perceive the external world as a seamless whole. Our perception of the external world depends on the integration of information from different senses (Driver & Spence, 1998). When and where in the human brain the integration of such multisensory information occurs is not yet known (Giard & Peronnet, 1999). The human brain cannot be considered a passive, stimulus-driven device or a passive transformer, but rather as an extraordinary integrative organ, which not only perceives but also creates new realities (Erdi, 2000; Nunez, 2000). Main Subdivisions of The Cerebral Cortex Functionally the cerebral cortex can be divided into 5 main subtypes (Mesulam-kitap): 1. Idiotypic (primary sensory-motor) cortices 2. Unimodal association cortices 3. Heteromodal association cortices 4. Paralimbic cortices 5. Limbic cortices 1. Idiotypic Cortex (The Primary Sensory-Motor Zones) The primary visual, somatosensory, and auditory cortices display a “koniocortical” architecture representing the highest level of development with respect to granulization and lamination. On the other hand, primary motor cortex displays a unique “macropyramidal” architecture charactherized by highly specialized giant pyramidal neurons known as Betz cells. The visual, auditory, and somatosensory systems provide the major channels of communication with the extrapersonal world. The vestibular, gustatory, and olfactory sensations do not have the same type of prominence in the primate brain (cytoarchitectonically less differentated). 2-3. Homotypical Association Isocortex (Neocortex) Homotypical Association Isocortex can be subdivided into two major zones: 1. Unimodal (modality-specific) 2. Heteromodal (High-order) Unimodal sensory association areas are further divided into “upstream” and “downstream” components. Upstream areas are only one synapse away from the relevant primary sensory area; whereas downstream areas are at a distanca of two or more synapses from the corresponding primary area. Unimodal: 1. The constituent neurons respond predominantly, if not exclusively, to stimulation in only a single sensory modality. 2. The predominant sensory information comes from the primary cortex and the other unimodal regions of the same modality. 3. Lesions yield deficits only in tasks guided by that modality. Heteromodal: 1. Neuronal responses are not confined to any single sensory modality. 2. The predominant sensory inputs come from unimodal areas in multiple modalities and from other heteromodal areas. 3. Deficits resulting from lesions in these areas are always multimodal, and never confined to tasks under the guidance of a single modality. Some neurons in heteromodal association areas respond to stimulation in more than one modality, indicating the presence of direct multimodal convergence (eg. sensory or motor). Major heteromodal cortices in human brain are in: 1. Prefrontal cortex 2. Posterior parietal cortex 3. Lateral temporal cortex 4. Some portions of Parahippocampal gyrus 4. Paralimbic Zone (Mesocortex) These areas are intercalated between allocortex and isocortex, it provides a gradual transition from one to the other. There are five major paralimbic formations: 1. Orbitofrontal cortex (BA11-13) 2. Insula (BA14-16) 3. The Temporal pole (BA38) 4. Parahippocampal cortices (BA27-28 and 35) 5. The Cingulate complex (BA23-26 and 29-33) 5. Limbic Zone Limbic zone is divided into two: (1) Corticoids, (2) Allocortex Corticoids 1. Septal region 2. Substantia innominata 3. Amygdaloid complex 4. Anterior olfactory nucleus Allocortex 1. Piriform cortex 2. Hippocampus Basal forebrain is usually considered a subcortical structure. But, constituents are directly on ventral and medial surface of cerebral hemispheres. Allocortex has more differentiated layers. Allocortex have mainly two parts: 1. Hippocampal formation 2. Primary olfactory cortex (Piriform cortex= paleocortex) 
Gate of the Internal World : Hypothalamus Through neural and humoral mechanisms, hypothalamus control: 1. electrolyte balance, 2. glucose levels, 3. basal temperature, 4. metabolic rate, 5. autonomic tone, 6. hormonal state, 7. sexual phase, 8. circadian ossilations, and immunoregulation, 9. experience and expression of hunger, aggression, fear, flight, thirst, and libido. Although all types of cortical areas, including association isocortex, receive direct hypothalamic projections, such connections reach their highest intensity within components of the limbic zone. Cortical areas of the limbic zone assume pivotal roles in the: 1. regulation of memory, 2. emotion, 3. motivation, 4. hormonal balance, 5. autonomic function. These specializations of limbic structures are related to the upkeep of the internal milieu (homeostasis) and the associated operations necessary for the preservation of the self and the species. Gate to the External World : Eyes, Ears and Skin Primary visual, auditory and sensory cortices provide obligatory portal for the entry of information from the envinonment into cortical circuitry. On the other hand, primary motor cortex provides a final common pathway for coordinating the motor acts which allow us to manipulate the environment and alter our position within it. Unimodal, heteromodal and paralimbic cortices are intercalated between two poles: 1. Limbic cortices 2. Primary Sensory-motor cortices Between External and Internal World Unimodal, heteromodal and paralimbic cortices: 1. Enable the associative elaboration of sensory information, its linkage to motor strategies, and the integration of experience with drive, emotion, and autonomic states. 2. Unimodal and heteromodal are most closely involved in perceptual elaboration and motor planning; 3. Paralimbic zone plays a critical role in channeling emotion and motivation to behaviorally relevant motor acts, mental content, and extrapersonal events. CORTICAL ORGANIZATION Component of each zone (all five) have extramural connections with components of the other functional zones and intramural connections within the same zone. Intermediary Processing and Transmodal Areas A fundamental charactheristic of the primate brain is the insertion of the obligatory synaptic relays between stimulus and response, and also between the representation of the internal milieu (at the level of the hypothalamus) and that of the external world (at the level of the primary sensory-motor cortex). These intercalated synaptic relays collectively provide the substrates for “intermediary” or “integrative” processing. The physiological outcomes of intermediary processing are known as “cognition”, “consciousness”, and “comportment” and include the diverse manifestations of memory, emotion, attention, language, planning, judgement, insight, and thought. Intermediary processing has a dual purpose. First, it protects channels of sensory input and motor output from the motivationally-driven influence of the internal milieu. Secondly, it enables identical stimuli to trigger different responses depending on situational context, past experience, present needs, and contemplated consequences. The neurons that support intermediary processing are located within the unimodal, heteromodal, paralimbic and limbic zones of the cerebral cortex. Unimodal areas contain the initial synaptic relays for intermediary processing. The absence of interconnections linking a unimodal area in one sensory modality with another in a different modality protects the sensory fidelity of experience and delays cross-modal contamination until further encoding has been accomplished. Unimodal areas are thus in a position to register the most accurate representation of sensory experience. However, in the absence of acsess to information in other modalities, unimodal areas do not have the ability to lead from word to meaning, from physiognomy to facial recognition, or from isolated sensory events to coherent experiences. Such integration of sensation into cognition necessiates the participation of “transmodal” areas. The defining feature of a transmodal area is the ability to support cross-modal integration. All components of heteromodal, paralimbic, and limbic zones are therefore also transmodal. Transmodal areas are not centers for storing convergent knowledge but rather critical gateways for integrating and accessing the relevant distributed information. They also provide “neural bottlenecks” in the sense that they constitute regions of maximum vulnerability for lesion-induced deficits in the pertinent cognitive domain. Heteromodal Association Cortex may be divided into three units: 1. Posterior AC: Integration of Senses essential for Language, Attention, Consciousness; Visuospatial Localization 2. Limbic AC: Mood, Affect and Memory 3. Anterior AC: Cognitive and Intellectuel Functioning, Motor Planning, Language Formation and Judgement |
