Abstract: Introduction
During the past years, the storage mechanism of visual working memory (VWM) has been extensively studied. Luck & Vogel (1997) indicated that the object stored in the visual working memory in an integrated manner, and that only 4 objects could be retained in VWM, regardless of the number of features containing in individual objects, which was called “strong object” hypothesis. However, more recent research revealed that object-based advantage was weakened when heterogeneous features were integrated in different parts of an object (Xu, 2002). The advantage was further diminished when the conjunction of homogenous features served as the memory materials (Olson & Jiang, 2002; Wheeler & Treisman, 2002). Based upon these results, Olson & Jiang (2002) put forward the “weak object” hypothesis, suggesting that VWM is limited by both the number of objects and the composition of those object features. However, this hypothesis does not explain clearly what kind of information can be integrated into an object and what kind of information can not. Using two different sets of materials, we explored the storage mechanism of VWM. Inspired by the fact that the pattern of perceptual processing can be divided into parallel processing with spread attention and serial processing with focal attention, we hypothesized that attention would serve as the key factor in determining mechanism of storage, and that only information processed in parallel would be stored in an integrated-object retained in VWM.
Method
Using change detection paradigm, we ameliorated Wheeler & Treisman’s research (2002) by changing block design to random design so that the influence of memory strategy could be avoided. Two experiments were conducted, in which the subjects were required to judge whether the test item displayed in the memory item. In experiment 1, the objects were defined by color and shape, which could be processed in parallel. In experiment 2, the objects were defined by landolt rings with color and orientation, and orientation should be processed in serial. The memory performance was checked by varying the test items in three conditions: color change, shape or orientation change, and binding change. Twenty-seven subjects participated in the two experiments (15 subjects in experiment 1, 12 subjects in experiment 2). Paired t test analyses were performed in both experiments.
Results
The data analyses showed that: 1) the accuracy difference between shape change condition and binding change condition in experiment 1 did not reach significance; and 2) the accuracy difference between orientation change condition and binding change condition in experiment 2 reached significance.
Conclusions
The results revealed that: 1) color and shape were stored as an integrated-object in the visual working memory; 2) landolt ring’s color and orientation were difficult to store as an integrated-object. According to the results, it is suggested that objects stored in the visual working memory were the information obtained in the parallel perceptual processing stage; objects including information that need focal attention to process were difficult to store as an integrated-object

Key words: visual working memory, object file, parallel process, serial process