Abstract:

PURPOSE:The research of visual working memory (VWM) has been a hot topic in the cognitive and cognitive neuroscience fields during the past decade years. It focused on the storage mechanism and capacity especially. Three different theories have been proposed to explain the storage mechanism of visual working memory. Luck & Vogel (1997) indicated that the object stored in the VWM in an integrated unit, regardless of the number of features containing in individual objects, and regardless of the features belonging to one or more dimensions. And that was the object-based storage theory, which was also called "strong object" hypothesis. However, many subsequent researches found that VWM for conjoined objects was worse than for objects made of single features when feature heterogeneity was controlled(Olson & Jiang, 2002; Wheeler & Treisman, 2002). Based upon these results, Olson & Jiang put forward the "weak object" hypothesis, suggesting that VWM is limited by both the number of objects and the composition of those object features. And Wheeler & Treisman put forth dual-storage mechanism, assumed features from the same dimension compete for capacity and cannot be bound together, whereas features from different dimensions can be stored in parallel. More researchers identified the dual-storage theory, but there is controversy, especially on the multi-feature stimuli. So we aimed to study the storage mechanism of VWM with 3-D graphs containing two kinds of features, and to test which hypothesis of above are more reasonable.
METHODS: Using single change-detection paradigm, we carried out two experiments to investigate the units of storage of visual object working memory and visual spatial working memory for the 3-D graphs with features of double dimensions. The stimulus graphs in experiment 1 consist of basic features of color and shape, and the memory materials in experiment 2 were presented by different colors and textures. Participants were asked to judge whether the test item varied to the item in sample display. The memory performance was checked by varying the test items in four conditions: color change, shape or texture change, feature exchange(binding change), and location. The ?rst three conditions belong to object WM task and the last belongs to spatial WM task.
RESULTS: The results of 2 (memory set size: 3 vs. 4) × 4(change condition of test item) within-subject repeated measures ANOVA on mean percent correct from two experiments both showed: (1) There was no signi?cant di?erence between the memory performance of feature exchange condition and the worst accuracy of single feature change conditions. (2) There was a signi?cant main e?ect on memory set size, which the memory performance of 3 graphs was higher than that of 4 graphs. (3) The performance of visual object working memory task was signi?cantly higher than that of visual spatial working memory task.
CONCLUSIONS: These findings of two experiments revealed that:（1）The 3-D graphs with basic features of color and shape were stored as an integrated manner in VWM. (2) The 3-D graphs with one basic feature and one detailed feature also could be stored as a binding form. In conclusion, the 3-D graphs with features from different dimensions were stored as integrated units in object working memory and spatial working memory.

Key words: object working memory, spatial working memory, storage mechanism