Time-resolved neuroimaging of visual short term memory consolidation by post-perceptual attention shifts

- Sensory stimulus characteristics were available without capacity limits up to 850 ms.
- Post perceptual event-related slow potentials image visual short term memory encoding and maintenance.
- N700 was reduced by an interference stimulus, while contralateral delay activity (CDA) still developed after post cues.
- Thus N700 was not a passively decaying visual trace but attention attraction was necessary for visual post-processing.
- CDA reflected an active search for required stimulus characteristics regardless of whether they could be maintained or not.

Post-perceptual cues can enhance visual short term memory encoding even after the offset of the visual stimulus. However, both the mechanisms by which the sensory stimulus characteristics are buffered as well as the mechanisms by which post-perceptual selective attention enhances short term memory encoding remain unclear.We analyzed late post-perceptual event-related potentials (ERPs) in visual change detection tasks (100 ms stimulus duration) by high-resolution ERP analysis to elucidate these mechanisms. The effects of early and late auditory post-cues (300 ms or 850 ms after visual stimulus onset) as well as the effects of a visual interference stimulus were examined in 27 healthy right-handed adults.Focusing attention with post-perceptual cues at both latencies significantly improved memory performance, i.e. sensory stimulus characteristics were available for up to 850 ms after stimulus presentation. Passive watching of the visual stimuli without auditory cue presentation evoked a slow negative wave (N700) over occipito-temporal visual areas. N700 was strongly reduced by a visual interference stimulus which impeded memory maintenance. In contrast, contralateral delay activity (CDA) still developed in this condition after the application of auditory post-cues and was thereby dissociated from N700.CDA and N700 seem to represent two different processes involved in short term memory encoding. While N700 could reflect visual post processing by automatic attention attraction, CDA may reflect the top-down process of searching selectively for the required information through post-perceptual attention.