Abstract
Findings from recordings of human temporal cortical single neuron activity during several measures of language, including object naming and word reading are reviewed and related to changes in activity in the same neurons during recent verbal memory and verbal associative learning measures, in studies conducted during awake neurosurgery for the treatment of epilepsy. The proportion of neurons changing activity with language tasks was similar in either hemisphere. Dominant hemisphere activity was characterized by relative inhibition, some of which occurred during overt speech, possibly to block perception of one’s own voice. However, the majority seems to represent a dynamic network becoming active with verbal memory encoding and especially verbal learning, but inhibited during performance of overlearned language tasks. Individual neurons are involved in different networks for different aspects of language, including naming or reading and naming in different languages. The majority of the changes in activity were tonic sustained shifts in firing. Patterned phasic activity for specific language items was very infrequently recorded. Human single neuron recordings provide a unique perspective on the biologic substrate for language, for these findings are in contrast to many of the findings from other techniques for investigating this.
Highlights
There are multiple methods for investigating the human brain organization for language
Brain Sci. 2013, 3 participate in a function but are not necessarily essential for it. Among the former are the effects of lesions, electrical stimulation mapping and intravascular neuroparalytic drugs. The latter group includes the metabolic correlates of a function as measured by functional magnetic resonance imaging, and “optical imaging” [1] and the electrophysiologic correlates in the scalp electroencephalogram (EEG), electrocorticogram (ECoG) and microelectrode recordings of single neuron activity and local field potentials (LFP)
The relation between these different techniques has been an active area of recent research, for example between techniques identifying essential regions compared to those regions participating in a function [2] or between different techniques for identifying participatory regions [3]
Summary
There are multiple methods for investigating the human brain organization for language. In the studies reviewed here, the clinical setting is cortical resections for the treatment of medically refractory epilepsy with a technique where the patient is awake under local anesthesia for a portion of the procedure [4] This technique is used so that the individual patient’s resection can be tailored to epileptic activity in the ECoG unperturbed by general anesthesia and the location of crucial areas for language based on intraoperative electrical stimulation mapping [5]. Recordings avoid tissue and neuronal activity that shows physiologic changes that have been related to the epileptic process, the extent to which findings can be generalized to other populations, including “normals” is generally not known, as there are no current techniques for obtaining these or similar data in “normals” These studies are limited to that portion of the patient population who consent to participate in them, and, because microelectrode recording is invasive, to tissue that will be subsequently resected to treat that patient’s epilepsy. In the author’s studies control measures are defined contrasting behaviors rather than “rest”
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