The neurobiology of language and verbal memory: observations from awake neurosurgery

Neurosurgical operations under local anesthesia provide a unique opportunity to investigate the neurobiology of human cognition. We have studied the cortical organization of language and verbal memory in this setting, using two different techniques: electrical stimulation mapping and extracellular microelectrode recording of activity of individual neurons. The two techniques provide very different perspectives. Stimulation mapping identifies brain areas that are essential for a behavior, while changes in neuronal activity can occur in non-essential regions. Stimulation mapping identifies multiple discrete areas in perisylvian cortex of the dominant hemisphere as essential for a function, with separation of areas for different aspects of language including naming in two languages, different semantic classes, naming compared to reading, and language from verbal memory. There is substantial individual variation in the location of these essential areas, variability that in part relates to subjects age, gender and verbal abilities. Neurons changing activity with language or verbal memory are widely distributed, in both hemispheres. However, individual neurons usually change activity with only one function, including naming in only one of two languages, only naming or reading, or with recent verbal memory encoding but not identification of similar items. A few lateralized changes in neuronal activity have been identified, including a predominance of inhibition in dominant hemisphere with naming, and polymodal memory responses in dominant hemisphere, unimodal in nondominant. Specific neuronal populations have been identified that are related to different aspects of memory, that differentiate correct from incorrect identification or memory performance and differentiate learned from unlearned associations, with some evidence of differences in neuronal activity related to subjects' ability.