Event-related potential correlates of verbal and pictorial feature comparison in aphasics and controls

Event-related potentials (ERPs) were recorded from 19 aphasic patients and 18 controls in four versions of a feature comparison task, in which the verbal or pictorial representation of a first stimulus (S1) had to be compared with the verbal or pictorial representation of a second stimulus (S2) presented 2 s later. These tasks were designed to cover some of the discriminatory variance of the token test (TT) including the analytical isolation, encoding and short-term storage of individual features of objects, independent of auditory verbal comprehension. Aphasics made more errors and had longer response latencies than controls in all four tasks, performance being poorest when verbal stimuli had to be processed. ERP analyses - restricted to subjects performing well above chance and to trials with correct responses - were confined to the slow wave (SW) (250-750 ms post-S1-onset) and the contingent negative variation (CNV) preceding the S2. There was no overall group difference that would have suggested that the patients activated different cortical areas than controls on correct performance. A left-hemispheric predominance of the negative SW was found in all four tasks and in both groups, although it was more pronounced in aphasics, and more pronounced in non-fluent than in fluent aphasics. The CNV was characterized by a left-hemispheric accentuation which was more pronounced in controls than in aphasics, particularly in tasks with a verbal S2. Results indicate that successful feature comparisons in the present tasks activate primarily left-anterior cortical areas. During encoding and short-term storage this activation is more pronounced in aphasics than in controls.