What do you mean with “direction”? Local and global cues to biological motion perception in pigeons

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Abstract

Highlights

★ Each bird quickly learned to discriminate left and right facing point-light walkers. ★ This contrasts their reportedly poor ability to discriminate mirror-flipped shapes. ★ When put into conflict, the majority of birds used local rather than global cues. ★ The others clearly used global motion-mediated shape. ★ Pigeons have access to both cues, but make a clear decision about which one they use.

Biological motion point-light displays are a rich and versatile instrument to study perceptual organization. Humans are able to retrieve information from biological motion through at least two different channels: The global articulated structure as revealed by the non-rigid, yet highly constrained deformation of the dot pattern, and the characteristics of local motion trajectories of individual dots. Here, we tested eight pigeons on a task in which they had to discriminate a left-facing from a right-facing biological motion point-light figure. Since the two stimuli were mirror-flipped versions of each other, we were not sure if the birds would be able to solve the task at all. However, all birds learned the discrimination quickly and performed at high accuracy. We then challenged them with a number of test trials introduced into the sequence of the normal training trials. Tested on backwards moving walkers, the majority of the birds indicated that they used local motion cues to solve the training task, while the remaining birds obviously used global, configural cues. Testing the pigeons on different versions of scrambled biological motion confirmed that each individual bird had made a clear decision for one of the two potentially available strategies. While we confirm a previously described local precedence in processing visual patterns, the fact that some birds used global features suggests that even the birds who relied on local cues probably dispose of the perceptual abilities to use global structure, but “chose” to not use them.

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