Abstract
A salient object such as a feature singleton captures visual attention only when it matches the observer's attentional set. Here we study whether the neural activities for processing of feature singletons are also contingent on the observers' attentional. We conducted an event-related fMRI study in which observers were required to perform two types of detection task (singleton detection and luminance-change detection task) using the identical stimulus set. The stimulus comprised four drifting sinusoidal gratings surrounded by gray rings. In the three quarters of the trials, one of the four gratings was differentiated in one feature dimension (color, shape, or drifting speed). In one tenth of the trials, luminance of the gray rings were decreased for 16.7 ms. The feature singletons would match to observers' attentional set in the singleton detection task but would not match in the luminance-change detection task. Although the activated brain areas were variable across the feature dimensions and the subjects, a fixed effect analysis from six subjects revealed the following results. Firstly, in the singleton detection task, the singleton stimuli induced brain activities in the bilateral intraparietal areas and in the bilateral extrastriate cortices. In contrast, the control stimuli, which did not contain any feature singleton, did not induce the bilateral intraparietal activation. This result suggests that the bilateral intraparietal areas participate in the processing of the feature singletons. Secondly, in the luminance-change detection task, as in the singleton detection task, the bilateral parietal areas were activated only by the singleton stimulus though the singletons would not match to observers' attentional set. This result suggests that the intraparietal activities induced by the feature singletons were not contingent on the observers' attentional set.