Time course of evoked-potential changes in different forms of anomia in aphasia

Impaired word production after brain damage can be due to impairment at lexical-semantic or at lexical-phonological levels of word encoding. These processes are thought to involve different brain regions and to have different time courses. The present study investigated the time course of electrophysiological correlates of anomia in 16 aphasic speakers, divided in two subgroups according to their anomic pattern (8 with lexical-semantic impairment and 8 with lexical-phonological impairment), in comparison to 16 healthy control subjects performing the same picture naming task. Differences in amplitudes and in topographic maps between groups were differently distributed when the whole heterogeneous group of aphasic patients was compared to the control group and when the two more homogeneous subgroups of anomic patients were analyzed. The entire aphasic group expressed different waveforms and topographic patterns than the control group starting about 100 msec after picture presentation. When two subgroups of aphasic patients are considered according to the underlying cognitive impairment, early event-related potential (ERP) abnormalities (100-250 msec) appeared only in the lexical-semantic subgroup, whereas later ERP abnormalities (300-450 msec) occurred only in the lexical-phonological subgroup. These results indicate that the time windows of ERP abnormalities vary depending on the underlying anomic impairment. Moreover, the findings give support to current hypotheses on the time course of processes involved in word production during picture naming.