Localization Of Function

The term functional localization is used to indicate that certain functions can be localized to particular areas of the cerebral cortex. The mapping of cortical function began with inferences made from the deficits produced by cortical lesions in humans. Subsequently, techniques such as single-cell recording and electrical stimulation of cells in the cerebral cortex have been used in animals, nonhuman primates, as well as humans undergoing surgery for diseases such as epilepsy and Parkinson's disease to map out functional areas of the brain. This research has generated important findings, such as the somatotopic organization of both sensory and motor systems. Recently, research into functional localization in the cerebral cortex has been aided by the introduction of cerebral metabolism and blood flow imaging methods.

Functional neuroimaging techniques such as positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) have been used to confirm previous knowledge about localization of function within the cerebral cortex as well as to conduct studies in healthy human subjects that were previously not possible. PET has been available for nearly 20 years. It involves monitoring the changes in metabolism or blood flow of a radiolabeled substance in response to a cognitive or sensorimotor task. PET studies have furthered our understanding of the organization of the somatomotor cortex, enabled us to examine visual perception, and begun to shed light on higher cognitive processes such as learning and attention. Moreover, we can localize receptors and determine the sites of action of different drugs in both healthy individuals and those inflicted with disorders such as epilepsy and schizophrenia.

fMRI is a relatively new (approximately 10 years) method that monitors local changes in blood oxygen levels that accompany the performance of a task. With its greater spatial resolution, fMRI has enabled more detailed studies of cortical function. For instance, a recent fMRI study demonstrated that the face representation in the somatosensory cortex, as described by Penfield, is in fact incorrect and is actually inverted along the central sulcus. In addition, it has been demonstrated that the topography in the motor planning areas (Brodmann area 6) is based on function (i.e., learned behaviors localized topographically) rather than on somatotopy. Recently, fMRI has been used to assess complex processes such as pain perception and to assess the effect of analgesic drugs on the areas of the cerebral cortex that are activated (Fig. 4).

Thus, both PET and fMRI have been used to study a wide range of processes, including sensation, perception, and attention in healthy human subjects, as well as changes in cerebral cortex function in patients suffering from neurological disorders. Numerous studies employing these techniques have contributed greatly to our knowledge of functional localization in the cerebral cortex.

Figure 4 Images obtained using fMRI in response to a painful stimulus induced by stimulation of the ulnar nerve in the right arm. The images, taken at three different levels in the brain, illustrate sites of activation in the cortex (white) to the painful stimulus before (left) and after (right) administration of an analgesic (remifentanil). The top of each image represents the left side of the brain. Note the typical contralateral activation of the somatosensory cortex in response to the peripheral stimulus.

Figure 4 Images obtained using fMRI in response to a painful stimulus induced by stimulation of the ulnar nerve in the right arm. The images, taken at three different levels in the brain, illustrate sites of activation in the cortex (white) to the painful stimulus before (left) and after (right) administration of an analgesic (remifentanil). The top of each image represents the left side of the brain. Note the typical contralateral activation of the somatosensory cortex in response to the peripheral stimulus.

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