Ojemanns Localization Mapping

Ojemann compiled data from 117 subjects in an electrical stimulation mapping study of language areas of the left hemisphere, and he concluded that language areas are variable mosaics of 1-cm2 sites ranging across and beyond classically defined areas such as Broca's and Wernicke's. As in Penfield and Roberts' study, patients were asked to name line drawings of common objects while electrical current was delivered to random sites on the cortex in the area surrounding the proposed excision. Anatomical markers were used to relate the sites of stimulation in each different brain to an arbitrary grid created using the Sylvian fissure and the rolandic cortex as landmarks. Not all patients had stimulation sites in all regions, but a map of the variability of language localization across all patients was created taking this into account. The most common finding was that sites where stimulation caused errors were separated by less than 1 cm in all directions from sites with no errors. The boundaries of these language areas were sharp in some cases, and in others they were surrounded by areas in which stimulation evoked single speech errors, suggesting a gradation from areas with no role in naming to areas essential for it. In an individual, these language areas were arranged in a mosaic pattern extending 1 or 2 cm2. Ojemann's study shows the high degree of individual variability that exists for essential language areas coupled with discrete localization in a given individual. The combined area of the inferior frontal mosaics in a given individual is much less than that of traditionally defined Broca's area and less than the corresponding area identified by Penfield and Roberts as essential. Ojemann did not attempt to differentiate among BA 44 and BA 46/47, but he did demonstrate that the most posterior portion of the inferior frontal gyrus was one of the least variable areas. This would correspond to BA 44. His work was limited in the specificity of what part of language function was being deactivated by the stimulation; for example, failures of perception, motor control, naming, or consciousness could all potentially have caused patients to fail at the naming task. In addition, buried sites, such as the insula, could not be directly stimulated and thus may also be essential to language and were not detected by this method. A final limitation is that the technique is necessarily performed on patients with lesions or tumors, which adds the possibility that their language function may be altered from that of normal individuals as a result oftheir medical condition. Despite the limitations, electrical stimulation does identify areas of the cortex essential for language, and in this case Ojemann demonstrated the great individual variability of those essential language regions. This challenges the assumption that Broca's seat of articulate language is a clearly defined region easily generalized among individuals.

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