Capacity limitations in the way humans store and process information in working memory have been extensively studied, and several memory systems have been distinguished. In line with previous capacity estimates for verbal memory and memory for spatial information, recent studies suggest that it is possible to retain up to four objects in visual working memory. The objects used have typically been categorically different colors and shapes. Because knowledge about categories is stored in long-term memory, these estimations of working memory capacity have been contaminated by long-term memory support. We show that when using clearly distinguishable intracategorical items, visual working memory has a maximum capacity of only one object. Because attention is closely involved in the working memory process, our results add to other studies demonstrating capacity limitations of human attention such as inattentional blindness and change blindness. capacity ͉ vision ͉ working memory D emonstrations of limitations in the way humans process and store information have generated much controversy among researchers in the cognitive sciences. A major theme concerns the capacity of working memory. Separate verbal, spatial, and visual object working memory systems can be distinguished (1-3), but similar estimates of their capacities have been established. These estimates have sometimes been made in terms of ''magical numbers,'' which have ranged from seven to about four words, numbers, or locations (4). In line with these results, recent studies suggest that it is possible to retain information of up to four objects in visual working memory (5-9). Studies of visual working memory have routinely used a change detection paradigm (5-7, 10) where a number of distinct objects or colors are presented briefly in a sample array. After a short blank interval, a test array is presented that is identical to the sample array, except for one object that may have changed. The task is to indicate whether all objects in the sample and test arrays are identical or whether one of them has changed. The objects and colors typically used in visual working memory studies have been few, repeatedly presented, and without difficulty classified into distinct categories easily given verbal labels such as red, green, blue, square, disk, etc. Whether the categorical boundaries are rule-, prototype-, or exemplar-based, the categorization process needs support by long-term memory storage. Repeated presentations of a few items, initially not separated by category boundaries, may lead people to develop boundaries and consequently use long-term memory support. Also, because items separated by category boundaries are easily assigned verbal labels, verbal working memory may be activated, leading to overestimation of visual working memory capacity (11). Additional potential problems with the traditional changedetection paradigm are that relational coding influences the results (12) and that spatial memory may be used to assist performance (1). A task that is supposed to give a pure measure of visual working memory capacity of objects should not give the possibility of relationally coding several objects into chunks or be influenced by other memory systems such as spatial memory, verbal working memory, or long-term memory. We investigated visual working memory capacity for items created with continuous feature dimensions making the items difficult to categorize. The influences of relational coding and spatial coding were reduced by using sample arrays consisting of one to four objects followed after 1,000 ms by a single centrally located test object that was a member of the sample array on 50% of the trials and not a member on the remaining 50% (Fig. 1) . Although all objects within all arrays were highly discriminable, some arrays of objects were easily separated by category boundaries and others were difficult to categorize. The goal was to decide whether the test object was present in the sample array. Three different stimulus types were used to create the sample arrays and the test objects in three separate conditions that represented a progression from objects with a few discrete stimulus dimensions that are easily categorized (e.g., red square) to objects with continuous stimulus dimensions not easy to put in separate categories (e.g., ovals with varying aspect ratios and color mixtures crossing the natural boundaries for the perception of color) and with new values on the dimensions on every trial to prevent development of categories. The study presented here is similar to the work on discrimination in visual memory because it also emphasizes continuous dimensions (13, 14) . The main difference is that the stimuli used in the discrimination studies are continuous low-level features such as spatial frequency and orientation, whereas the study presented here investigates object classes in a different way. Two object-discriminating features were used in our discrete color͞shape condition (discrete colors and discrete shapes; Fig. 2a ), continuous color͞shape condition (color mixtures and oval shapes; Fig. 2b) , and the continuous size-ratio͞shape condition (small ovals inside larger ovals; Fig. 2c ). Eight undergraduate students and the authors participated in each condition in the main experiment. All participants received 50 trials in each condition where each condition was a combination of set size with one to four objects and with one of the three stimulus types. The authors and six other participants also completed three discrimination conditions with stimuli from the discrete color͞shape, continuous color͞shape, and continuous size-ratio͞shape conditions. Eight other participants completed a categorization task that investigated the extent to which the participants saw the objects in each of the three conditions as belonging to the same category or to different categories. In the discrete color͞shape condition, discrete shapes and colors easily put in distinct categories were randomly selected from a set of five prespecified shapes (square, circle, bar, cross, and triangle) and eight colors (red, green, blue, yellow, black, white, magenta, and cyan). The combinations of shapes and colors were selected so that the same shape or the same color never appeared in the same trial. In the continuous color͞shape condition, oval aspect ratios were selected from a Euclidian two-dimensional shape-space consisting of orthogonal length and width axes (i.e., aspect ratio space). For each trial, the objects were located at equal distances on a randomly located circle in this shape space. Similarly, the colors were selected from
Using a change-detection paradigm, REF found evidence that visual WM capacities are substantially higher when stimuli are easily categorized.
Visual memory needs categories
Proceedings of the National Academy of Sciences of the United States of America
Proceedings of the National Academy of Sciences of the United States of America
Computer Science Medicine