An event-related potential examination of attended and unattended stimuli in visual selection using bilateral stimulus presentation

Facilitation and disruption of lateralized syllable processing by unattended stimuli in the opposite visual field

The influence of lateralized unattended stimuli on the processing of attended stimuli in the opposite visual field can shed light on the nature of information that is transferred between hemispheres. On a cued bilateral task, participants tried to identify a syllable in the attended visual field, which elicits a left hemisphere (LH) advantage and different processing strategies by the two hemispheres. The same or a different syllable or a neutral stimulus appeared in the unattended field. Transmission of unattended syllable codes between hemispheres is symmetric, as revealed by equal interference for the two visual fields. The LH is more accurate than the RH in encoding unattended syllables, as indicated by facilitation in the left but not right visual field and a greater frequency of identifiable intrusions into the left than right field. However, asymmetric encoding strategies are different for attended and unattended syllables.

Testing memory for unseen visual stimuli in patients with extinction and spatial neglect

Visual extinction after right parietal damage involves a loss of awareness for stimuli in the contralesional field when presented concurrently with ipsilesional stimuli, although contralesional stimuli are still perceived if presented alone. However, extinguished stimuli can still receive some residual on-line processing, without awareness. Here we examined whether such residual processing of extinguished stimuli can produce implicit and/or explicit memory traces lasting many minutes. We tested four patients with right parietal damage and left extinction on two sessions, each including distinct study and subsequent test phases. At study, pictures of objects were shown briefly in the right, left, or both fields. Patients were asked to name them without memory instructions (Session 1) or to make an indoor/outdoor categorization and memorize them (Session 2). They extinguished most left stimuli on bilateral presentation. During the test (up to 48 min later), fragmented pictures of the previously exposed objects (or novel objects) were presented alone in either field. Patients had to identify each object and then judge whether it had previously been exposed. Identification of fragmented pictures was better for previously exposed objects that had been consciously seen and critically also for objects that had been extinguished (as compared with novel objects), with no influence of the depth of processing during study. By contrast, explicit recollection occurred only for stimuli that were consciously seen at study and increased with depth of processing. These results suggest implicit but not explicit memory for extinguished visual stimuli in parietal patients.

Event-related evoked potential study of repetition priming to attended and unattended words

The filtering stage of ignored stimuli is one of main issues in the selective attention mechanism. We investigated whether unattended ignored stimuli show repetition priming effects on event-related brain potentials (ERPs), using a lexical decision task involving Japanese Kanji words. The repeated words in the unattended field generated a negative shift of the ERP over the frontal scalp sites. This suggested that unattended stimuli were processed implicitly to some degree and that different neural mechanisms contribute to repetition priming effects to attended and unattended stimuli.

Event-related potential response to attended and unattended locations in an interference task

Event-related potentials were measured in response to an interference task in which unattended stimulus items were compatible, incompatible a neutral with regard to the attended stimulus items. Two stimulus items were presented simultaneously and bilaterally--one in each visual field. This allowed examination of the event-related potential waveform according to whether recording sites were contralateral to the attended or unattended location. The first experiment used sustained cueing with 3.5 degrees separation between attended and unattended locations. Attentional modulation of the N1 was observed but not for the P1. In the second experiment, separation between attended and unattended locations was increased to 11.5 degrees. In both experiments, the hemisphere contralateral to the unattended material (unattended hemisphere) showed a greater negativity in the N2 latency range in the temporal regions to compatible and incompatible conditions compared to a neutral condition. These data are inconsistent with findings suggesting the filtering of material early in visual processing.

Event-related potential correlates of color selection and lexical decision: hierarchical processing or late selection?

In a series of experiments, a color discrimination task was combined with a lexical decision task. Words and non-words that appeared in the relevant color elicited attention-related event-related potential (ERP) components (anterior positivity, posterior N2 and N2b). The results of the recognition tasks, that investigated the retention of the words, were not closely related to the emergence of these ERP components. These findings show that instead of being the correlates of a filtering mechanism confining lexical processing to stimuli in relevant color, the attention-related ERP components are correlates of late selection processes, ensuring task-related stimulus evaluation.