Cerebral lateralization of neuronal activity during naming, reading and line-matching

Changes in human right or left temporal cortical neuronal activity during language and visuospatial tasks were investigated during craniotomy under local anesthesia for medically intractable epilepsy in patients known to be left dominant for language based on preoperative intracarotid amobarbital perfusion testing. Extracellular recordings were obtained from 57 neuronal populations (26 from the left hemisphere) in the superior and middle temporal gyri of 34 patients. Frequency of activity was compared during over and silent object naming, word reading and line-matching. Although all recordings were from the cortex not essential for language, statistically significant changes in activity during these tasks were identified in 49% of the populations. Most populations showed significant changes to only one of the tasks, indicating discrete neural networks for reading and naming. Nearby neuronal populations recorded by the same microelectrode usually had different behavioral correlates. There were no significant differences in the proportion of neurons changing activity with language or spatial measures between right (non-dominant) or left (dominant) temporal lobes. Left superior and middle temporal gyrus populations, however, demonstrated significant early reductions in activity during overt or silent naming, while right middle temporal gyrus recordings showed significant early increased activity only during overt naming. Although reading measures elicited more activity from the non-dominant temporal lobe, early reductions in activity were recorded exclusively from the dominant side. Visuospatial tasks evoked changes in neuronal frequency predominantly in the middle temporal gyrus, and during one of these tasks, significant later increases in activity were present bilaterally. Our results indicate that the functional lateralization of cognitive behaviors may depend less on the anatomic location of the associated neuronal activity than on the neurophysiologic characteristics of that activity.