Behavior can be guided by visual working memory as well as vision. For example, visual exploratory behavior is most efficient if subjects can accurately retain items they have previously fixated. This visual working memory capacity is limited; human studies have estimated the visual working memory capacity as 3 items on average, with values as low as 1.5 in some individuals. To date, no study has determined the capacity limit in animals and as such, no animal model has been established to investigate the neural basis of the capacity of visual working memory. We employed an adaptation of the sequential color-change detection task used in human studies to determine the visual working memory capacity in the macaque monkey (see supplementary figure 1). Each trial began with the presentation of a fixation spot on a blank screen. The monkey was required to fixate on this central fixation spot before a memory array was presented. The memory array consisted of a set of two to five highly discriminable colored stimuli, presented for 500ms. The memory array, except for the fixation spot, was removed for a retention interval of 1000ms, during which the monkey was required to maintain fixation. The test array was then presented with one of the stimuli having changed color. The monkey was required to indicate this change by making a single saccadic eye movement to its location. Consistent with the use of mnemonic processes, the performance decreased with increasing set size (see supplementary figure 2). Using the relationship between performance and set size, monkey visual working memory capacity was at least 2 memoranda, a value within the range of human capacity estimates. This similarity between the monkey and human visual working memory capacity suggests a shared common neural process, which can now be investigated with invasive techniques.
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