Effects of distinctiveness and memory on lateralized and unlateralized brain-electrical activity during visual word encoding.

The present study investigated a recently introduced left-lateralized component in the event-related potential (ERP), the posterior semantic asymmetry (PSA), in the context of an isolation paradigm. The PSA is a relative negativity that is most pronounced at temporoparietal electrodes, peaks around 300 ms, and is assumed to reflect early semantic processing of visual words. A free-recall, word-list-learning paradigm was conducted. The learning list comprised two stimuli which were physically isolated from the other stimuli (by different font size or different typeface). The typical behavioral isolation effect with higher recall for isolated stimuli was observed. Furthermore, ERP effects of stimulus type and subsequent memory were analyzed. A left-lateralized negativity that matched the topography of the PSA but occurred somewhat later showed an effect of stimulus distinctiveness, with increased amplitudes for isolates, thus suggesting their deeper semantic processing. However, PSA amplitude did not predict subsequent recall. Unlateralized ERPs replicated previous findings of a greater late frontal positivity during elaborated encoding of both isolated stimuli and subsequently recalled stimuli. This recall effect was greater for isolated than standard stimuli. We argue that physical distinctiveness during encoding facilitates recall to the extent that it promotes the frontally-mediated processes that predict better recall in general.