VISUAL-SEARCH FOR GLOBAL LOCAL STIMULUS FEATURES IN HUMANS AND BABOONS/

Fagot and Deruelle (1997) demonstrated that, when tested with identica l visual stimuli, baboons exhibit an advantage in processing local fea tures, whereas humans show the ''global precedence'' effect initially reported by Navon (1977). In the present experiments, we investigated the cause of this species difference. Humans and baboons performed a v isual search task in which the target differed from the distracters at either the global or the local level. Humans responded more quickly t o global than to local targets, whereas baboons did the opposite (Expe riment I). Human response times (RTs) were independent of display size , for both local and global processing. Baboon RTs increased linearly with display size, more so for global than for local processing. The s earch slope for baboons disappeared for continuous targets (Experiment 2). That effect was not due to variations in stimulus luminance (Expe riment 3). Finally, variations in stimulus density affected global sea rch slopes in baboons but not in humans (Experiment 4). Overall, resul ts suggest that perceptual grouping operations involved during the pro cessing of hierarchical stimuli are attention demanding for baboons, b ut not for humans.