Historically, there have been two mayor views on the localization of higher cognitive functions. One view, in its extreme form, is phrenology, which postulated that all cognitive abilities and personality traits had specific areas of the brain that controlled each one separately. This view was discarded in the first decades of this century, mainly because most of the proponents of phrenology did not follow rigorous scientific methods. The second view, hence derived from opposition to the phrenologists, postulated that higher cognitive functions and traits could only be localized in the cerebral cortex, but that the cortex functioned as a whole, with any part of it could substitute for the functions of another. It also postulated that the effects on cognitive functions due to cortical lesions was due to the extent of damaged tissue, but not on the localization of it. This holistic view gained much acceptance, and nearly neutralized all of the phrenologists postulates.
From early on, some clinical observations contradicted the holistic view, particularly the consistent observation that aphasias (language distinctions) occurred when either of two specific brain regions were lesioned. These areas are the Brocca and Wernicke areas of the cortex. These areas are localized only in the left hemisphere of the brain in over 80% of the population, and can be anatomically localized. Broca's area is localized just beneath the motor cortex that controls face movement, and is specialized in the expression of language.
Patients with lesion of this area can understand language and conversation, even complex abstract concepts, but cannot talk coherently. They can pronounce words with perfect intonation, so the motor aspects of word utterance are intact, but the words seem to be randomly emitted. Typical speech could go like: "... house ran duck ink ink, resolve..."
In the case of Wenicke's area, which is above the left temporal auditory cortex, lesions seem to alter understanding, but not hearing, since these patients can understand simple requests and attend only when called by their names, but cannot understand a simple conversation. In opposition to lesions to the Broca area where talk is incoherent, Wernicke area lesions produce incongruent talk. Patients can form complete understandable phrases, but will emit them out of any context. An answer to the question "how did you wake up this morning?" could be "cold water that runs from the spring".
Today, most scientist accept a midway version between holism and phrenology, specially with the discovery of association cortices. Even in the holistic views, it was accepted that some specific cortical areas were necessary for sensory input, but the high order processing was carried out by the cortex as a whole.
The study of sensory systems, particularly vision, led to the discovery that a lot of processing was carried out by the system itself, still dependent of cortical localization in what is known as higher-order sensory cortices; such as detection of direction, intensity, contrast, speed, and several other combined attributes of visual stimuli. Furthermore, some areas were described as responding to exclusively a combination of two sensory inputs, these were called association cortices. The great discrepancy between the modern view and phrenology is that in the former the localization is not of functions, but of systems, and it is the interconnections and interactions between systems that give rise to specific cognitive functions.
In this context, cortical systems can be classified on the basis of the functions they participate in. As a first level, we can find the primary sensory cortices, where the information first arrives at the cortical level. Second, there are the higher order sensory cortices mentioned earlier. Third, there are association cortices.
The cortical association areas are supposedly the anatomical basis for thought and perception, since stimulation of these areas produce little if not no obvert behavioral changes, but receive sensory input from high-order sensory systems and project to motor cortex. There are three identified association areas of the cortex. One is the parieto-temporal-occipital cortex. This cortex has regions that receive somatosensory, auditory and visual projections, and also receives the high-order input from their respective cortices, and is therefore thought to integrate information from these sensory modalities and is necessary for language. The second cortex that is identified, is the prefrontal association area, which is believed to control several cognitive behaviors such as propositive behavior, and also to control motor planning
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