Penfields Cortical Stimulation Studies

Penfield and Roberts used excision studies coupled with electrical stimulation studies to map language areas, and they concluded that Broca's area plays a role in ideational (as opposed to motor) speech but its excision did not produce permanent symptoms of aphasia; rather, speech is recovered at least to some extent. They used preoperative electrical mapping throughout the mid-20th century to identify areas of the cortex involved in language in order to avoid them when performing cortical excisions to treat epilepsy. When electric current was applied to the cortex at points potentially involved in language, two speech effects occurred that were termed positive and negative. A positive effect was stimulation of speech when motor areas for face and mouth were stimulated, and negative effects included interference of speech when ideational speech areas, such as Broca's, Wernicke's, and supplementary motor areas, were stimulated. Patients were awake, and they were asked either to perform a picture-naming task or to count aloud. The interference was only effective approximately one-half of the time, and movement induced in one spot at one time might not occur upon restimulation of the same spot in the same patient; thus, results were difficult to replicate and inexact. Compiling their data from several patients, Penfield and Roberts found that when Broca's area was stimulated, negative effects such as speech arrest, hesitation, slurring, repetition, confusion of numbers while counting, inability to name pictures with retained ability to speak, and difficulty in reading and writing resulted. They define Broca's area as consisting of the three gyri in front of the lower precentral gyrus. This includes BA 46/47, now separated from Broca's area proper by functional imaging literature. By including this area, Penfield and Roberts decreased the specificity of the area they were mapping. They also found that stimulation of sites outside Broca's area and other traditional language areas, such as Wernicke's area in the temporal lobe and the supplementary motor area, caused similar interruption of language. This suggested expansion of the traditional areas. The majority of their patients did have interference only in left Broca's stimulation, as opposed to its right hemisphere counterpart, which they concluded supported the idea of left hemisphere dominance for language. In observing cases of excision of part or all of Broca's area, Penfield and Roberts concluded that permanent aphasia did not necessarily result, but speech was recovered postoperatively in most cases. This suggests that Broca's area is not necessary for normal language function, and it complicates the picture created by their electrical stimulation studies, which show that Broca's area is a key language area.

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