A brain electrical signature of left-lateralized semantic activation from single words

• Introduction of a novel ERP asymmetry index for left-lateralized semantic activation.
• Posterior semantic asymmetry (PSA) occurs considerably earlier than N400.
• Application of a very simple single-word reading paradigm.
• Use of an extremely conservative control condition.
• Promising low-cost tool with high temporal precision for use in language diagnostics.

Lesion and imaging studies consistently indicate a left-lateralization of semantic language processing in human temporo-parietal cortex. Surprisingly, electrocortical measures, which allow a direct assessment of brain activity and the tracking of cognitive functions with millisecond precision, have not yet been used to capture this hemispheric lateralization, at least with respect to posterior portions of this effect. Using event-related potentials, we employed a simple single-word reading paradigm to compare neural activity during three tasks requiring different degrees of semantic processing. As expected, we were able to derive a simple temporo-parietal left-right asymmetry index peaking around 300 ms into word processing that neatly tracks the degree of semantic activation. The validity of this measure in specifically capturing verbal semantic activation was further supported by a significant relation to verbal intelligence. We thus posit that it represents a promising tool to monitor verbal semantic processing in the brain with little technological effort and in a minimal experimental setup.