A method for cognitive function assessment in a patient uses the ERP waveform features and parameters that maximize the correlation between the ERP components and behavioral performance on a neuropsychological test of language comprehension (PPVT-R). The method provides an electrophysiological diagnostic technique that can be used in the assessment of aphasic patients. ERPs are recorded during a computerized version of the Peabody Picture Vocabulary Test-Revised (PPVT-R, Form M). In the computerized version, a picture is presented followed by a congruent or incongruent spoken word. A derived measure is calculated from the ERP differentiation between congruent and incongruent words. The N400 is the primary component elicited to incongruent spoken words.