In a previous study, Egly, Driver, and Rafal (1994) observed both space-and object-based components of visual selective attention. However, the mechanisms underlying these two components and the relationship between them are not well understood. In the present research, with a similar paradigm, these issues were addressed by manipulating cue validity. Behavioral results indicated the presence of both space-and object-based components under high cue validity, similar to the results of Egly et al.'s study. In addition, under low cue validity, the space-based component was absent, whereas the object-based component was maintained. Further event-related potential results demonstrated an object-based effect at a sensory level over the posterior areas of brain, and a space-based effect over the anterior region. The present data suggest that the space-and object-based components reflect mainly voluntary and reflexive mechanisms, respectively.

The global precedence hypothesis (Navon, 1977) assumes that the processing of the global level of a hierarchical pattern precedes that of the local level. To explore further the nature of global and local processing of compound stimuli, we recorded the event-related brain potentials (ERPs) associated with ident@ing the global and local levels of nonlinguistic compound stimuli in a selective attention task. Wile subjects' behavioral responses were similar to those observed by Navon (1977), the analyses of ERP data showed that identification of the local level elicited longer N2 and P3 peak latencies with enhanced N2 and decreased P3 amplitudes relative to the identification of the global level. The inconsistency between the global and local levels made N2 and P3 amplitudes more negative with longer peak latencies. This interference effect on N2 and P3 amplitude and P3 latency was stronger on the local level than on the global level. The modulation of N2 by the consistency of the global and local levels observed in this and the previous (Heinze, Muente, et al., 1994) study suggests that the interference effect may be mediated by the early perceptual processing. Moreover, we found that the amplitude of an early posterior P1 component was modulated by attention to the global and local levels, being larger to the local target than to the global one. This PI effect gives no support to the notion that the variation of attentional spotlight determines the global precedence effect.

When subjects are explicitly cued to focus attention on a particular location in visual space, targets presented at that location have been shown to elicit enhanced sensory-evoked activity in recordings of event-related brain potentials (ERPs). The present study sought to determine if this type of sensory facilitation also occurs during visual search tasks in which a feature conjunction target must be identified, presumably by means of focal attention, within an array of distractor items. In this experiment, subjects were required to discriminate the shape of a distinctively colored target item within an array containing 15 distractor items, and ERPs were elicited by task-irrelevant probe stimuli that were presented at the location of the target item or at the location of a distractor item on the opposite side of the array. When the delay between search-array onset and probe onset was 250 msec, the sensory-evoked responses in the latency range 75-200 msec were larger for probes presented at the location of the target than for probes presented at the location of the irrelevant distractor. These results indicate that sensory processing is modulated in a spatially restricted manner during visual search, and that focusing attention on a feature conjunction target engages neural systems that are shared with other forms of visual-spatial attention.