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Colloquium on Neuroimaging of Human Brain Function
FIG. 3. (A) Example of the stimuli used in experiment 2. (B) ERPs from experiment 2, recorded at lateral occipital scalp sites ipsilateral and contralateral to the location of the target stimulus. A large N2pc wave (shaded area) can be observed for the conjunction task when it was performed alone (larger than for the feature task in experiment 1). When the conjunction task was combined with the central letter discrimination task, the N2pc was delayed, suggesting that the observers shifted attention to the iconic image of the target after completing the central letter task and could not perform this task without the use of spatially focused attention.
It is useful to contrast these results with the results of an analogous psychophysical study that was recently reported by Joseph et al. (41). As in the present experiments, this study combined a visual search task with a central attention-demanding task and assessed the extent to which the allocation of attention to the central task interfered with the visual search task. However, the central task in the Joseph et al. study was more complex than the task used in the present study and involved the detection of a target item within a rapid stream of nontarget items (all presented at the fovea). In contrast to the present study, Joseph et al. (41) found that observers could not accurately detect visual search targets defined by a simple feature when performing the concurrent central task. From these results, the authors concluded that there is no direct route from feature coding to awareness and that even simple features must pass through a limited-capacity attentional stage to reach awareness. Although this conclusion seems very sound, it would be easy to draw an unwarranted additional conclusion from these results; namely that attention is necessary for the accurate identification of simple features (as opposed to being necessary for making overt responses on the basis of the feature identities). We have previously shown that the type of central task used by Joseph et al. (41) leads to postperceptual impairments in which items that are fully identified fail to be stored in working memory (28, 29), and it is very likely that the impairment in feature detection performance observed by Joseph et al. (41) reflects the operation of a postperceptual attentional mechanism. Thus, the present experiments indicate that feature identification can be accomplished without the use of perceptual-level attentional mechanisms, whereas the results of Joseph et al. indicate that attention may be required to make the identified features available to awareness.
We would like to acknowledge the important role that several individuals have played in formulating these ideas and in conducting the experiments, including Steven A.Hillyard, Leonardo Chelazzi, Robert Desimone, Edward K.Vogel, Kimron L.Shapiro, Massimo Girelli, and Michele T.McDermott. Preparation of this manuscript and several of the studies described here were supported by Grant 95–38 from the McDonnell-Pew Program in Cognitive Neuroscience and by Grant 1 R29 MH56877–01 from the National Institute of Mental Health.
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